{"id":274,"date":"2024-03-23T09:37:18","date_gmt":"2024-03-23T09:37:18","guid":{"rendered":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/?post_type=chapter&#038;p=274"},"modified":"2024-10-06T10:45:55","modified_gmt":"2024-10-06T10:45:55","slug":"5-2-mendelian-inheritance","status":"publish","type":"chapter","link":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/chapter\/5-2-mendelian-inheritance\/","title":{"raw":"5.2 Mendelian inheritance","rendered":"5.2 Mendelian inheritance"},"content":{"raw":"<div class=\"content user-select-text\" aria-label=\"Sent by Copilot: Sure! Let's delve deeper into the concepts of heredity and variation. ### Heredity Heredity is the process through which traits are passed from parents to offspring. This transmission occurs through genes, which are segments of DNA located on chromosomes. Each gene can exist in different forms called alleles, which contribute to the diversity of traits observed in a population. #### Key Concepts in Heredity: - **Genes and Alleles**: Genes are the basic units of heredity, and alleles are different versions of a gene. For example, the gene for eye color may have an allele for blue eyes and another for brown eyes. - **Dominant and Recessive Traits**: Traits can be dominant or recessive. Dominant traits are expressed if at least one dominant allele is present, while recessive traits are expressed only when two recessive alleles are present[^4^]. - **Genotype and Phenotype**: The genotype is the genetic makeup of an organism, while the phenotype is the observable characteristics. The phenotype results from the interaction of the genotype with the environment[^4^]. ### Variation Variation refers to the differences in traits among individuals within a population. These differences can be genetic or environmental in origin. #### Sources of Genetic Variation: 1. **Mutations**: Changes in the DNA sequence that can introduce new genetic variations. Mutations can be beneficial, neutral, or harmful[^11^]. 2. **Genetic Recombination**: During sexual reproduction, the process of meiosis leads to the shuffling of genes, creating new combinations of alleles[^12^]. 3. **Gene Flow**: The movement of genes between populations, which can introduce new genetic material into a population[^12^]. #### Types of Variation: - **Continuous Variation**: Traits that show a range of phenotypes, such as height or skin color. These traits are usually influenced by multiple genes (polygenic inheritance) and environmental factors[^12^]. - **Discontinuous Variation**: Traits that have distinct categories, such as blood type or flower color. These traits are typically controlled by a single gene or a few genes[^12^]. ### Importance of Variation Variation is crucial for the survival and evolution of species. It provides the raw material for natural selection, allowing populations to adapt to changing environments. Without variation, a population may not be able to cope with new challenges, such as diseases or climate change[^12^]. Would you like to explore any specific examples or dive into another related topic?\">\r\n<div class=\"ac-container ac-adaptiveCard\" style=\"text-align: center\">\r\n<div class=\"ac-textBlock\">\r\n<h1 class=\"import-Normal\">Mendelian Genetics<\/h1>\r\n<\/div>\r\n<div class=\"ac-textBlock\">\r\n<div style=\"text-align: left\"><span>Johann <\/span>Gregor Mendel is regarded as \u201cFather of modern genetics,\"<\/div>\r\n<div style=\"text-align: left\">\u2022Genetics is the study of heredity.<\/div>\r\n<div style=\"text-align: left\">\u2022Mendel was born in Austria in 1822. He was a monk and he discovered the basic principles of heredity<\/div>\r\n<div style=\"text-align: left\">\u2022 He conducted experiments on Pea plant in his monastery's garden.<\/div>\r\n<div style=\"text-align: left\">\u2022His experiments laid the foundation of modern Genetics<\/div>\r\n<div style=\"text-align: left\">\u2022 The postulates put forth by Mendel form the basis of classical, or Mendelian, genetics.<\/div>\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n<img src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image11.png\" alt=\"image\" class=\"aligncenter\" \/>\r\n\r\nPea plants - his primary model system to study a specific biological phenomenon to be applied to other systems.\r\n\r\nIn 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society.\r\n\r\nHe demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns.\r\n\r\nIn 1866, he published his work, Experiments in Plant Hybridization, in the proceedings of the Natural History Society of Br\u00fcnn in 1866\r\n\r\nThe scientific world largely ignored Mendel's discoveries because they mistakenly thought that heredity involved combining parental features to give kids an intermediate physical appearance.\r\n\r\nDuring his lifetime, Mendel did not receive recognition for his outstanding contributions to science. It was not\u00a0 until 1900 that his work was rediscovered, <span style=\"font-size: 1em\">by three European botanists working independently namely, <\/span><span style=\"font-size: 1em\">Hugo de Vries:\u00a0<\/span><span style=\"font-size: 1em\">From Holland , <\/span><span style=\"font-size: 1em\">Carl Correns:\u00a0<\/span><span style=\"font-size: 1em\">From Germany ,<\/span><span style=\"font-size: 1em\">Erich von Tschermak:\u00a0<\/span><span style=\"font-size: 1em\">From Austria<span class=\"UV3uM\"> .<\/span><\/span>\r\n<div class=\"WaaZC\">\r\n<h3 class=\"import-Normal\"><em>Mendel's Experiments with Pisum sativum<\/em> as Model System<\/h3>\r\n<span>Mendel carried out his experiments in\u00a0 the garden pea,\u00a0<\/span><em>Pisum sativum<\/em><span>, to study inheritance.<\/span>\r\n<ul>\r\n \t<li style=\"text-align: left\">Mendel performed hybridizations Experiments which involves mating two true-breeding individuals that have different traits.<\/li>\r\n \t<li style=\"text-align: left\">Pea, is naturally self-pollinating.<\/li>\r\n \t<li style=\"text-align: left\">Mendel pollinated the pea plants by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety.<\/li>\r\n \t<li style=\"text-align: left\">In plants, pollen carries the male gametes to the stigma, a sticky organ that traps pollen and allows the male gamete to move down the pistil to the female gametes (ova)<\/li>\r\n \t<li style=\"text-align: left\">\r\n<div>To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel removed all of the anthers from the plant\u2019s flowers before they had a chance to mature.<\/div><\/li>\r\n \t<li style=\"text-align: left\">\r\n<div>Plants used in first-generation crosses were called P0, or parental generation one, plants .<\/div><\/li>\r\n \t<li style=\"text-align: left\">\r\n<div>Mendel collected the seeds belonging to the P0 plants that resulted from each cross and grew them the following season.<\/div><\/li>\r\n \t<li style=\"text-align: left\">\r\n<div>These offspring were called the F1, or the first filial (filial = offspring, daughter or son), generation.<\/div><\/li>\r\n \t<li style=\"text-align: left\">\r\n<div>Once Mendel examined the characteristics in the F1 generation of plants, he allowed them to self-fertilize naturally.<\/div><\/li>\r\n \t<li style=\"text-align: left\">\r\n<div>He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation.<\/div><\/li>\r\n \t<li style=\"text-align: left\">\r\n<div>Mendel\u2019s experiments extended beyond the F2 generation to the F3 and F4generations, and so on.<\/div><\/li>\r\n \t<li>\r\n<div style=\"text-align: left\">Interestingly it was the ratio of characteristics in the P0\u2212F1\u2212F2 generations that were became the basis for Mendel\u2019s postulates.<\/div>\r\n<div><\/div><\/li>\r\n<\/ul>\r\n<img src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image27.png\" alt=\"image\" class=\"alignleft\" \/>\r\n<h3>Why did Mendel choose pea plant for his experiments?<\/h3>\r\n<ul>\r\n \t<li style=\"text-align: left\"><span>This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. <\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or \u201ctrue-breeding,\u201d pea plants and produce offspring that look like the parent. <\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>By carrying out his experiments with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding.<\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time.<\/span><\/li>\r\n \t<li>\r\n<div class=\"WaaZC\">\r\n<p style=\"text-align: left\"><span>Large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.<\/span><\/p>\r\n\r\n<\/div><\/li>\r\n<\/ul>\r\nMendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits .<strong>A trait is defined as a variation in the physical appearance of a heritable <em>characteristic<\/em><\/strong><em>.<\/em>\r\n\r\nThe contrasting characteristics\u00a0 studied by Mendel in\u00a0 pea plant included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position.\r\n\r\nFor the characteristic of flower color, for example, the two contrasting traits were white versus violet.\r\n<pre><a href=\"https:\/\/slcc.pressbooks.pub\/humanbiology\/chapter\/__unknown__-6\/\" target=\"_blank\" rel=\"noopener\">\"Mendel hybridization experiment\"<\/a><span>\u00a0by\u00a0<\/span><a>Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a><\/pre>\r\nMendel generated large numbers of F1 and F2 plants in order to fully examine each characteristic. His findings were consistent.\r\n\r\nReginald Punnett, who developed a simple tool, now known as the <strong>Punnett Square<\/strong>, to predict the probability of genotypes and phenotypes from controlled crosses.\r\n\r\n<\/div>\r\n<img src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image13.png\" alt=\"image\" \/>\r\n<pre><a href=\"https:\/\/slcc.pressbooks.pub\/humanbiology\/chapter\/__unknown__-6\/\" target=\"_blank\" rel=\"noopener\">\"Mendel hybridization experiment\"<\/a><span>\u00a0by\u00a0<\/span><a>Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a><\/pre>\r\n<span style=\"font-family: 'Cormorant Garamond', serif;font-size: 1.602em\">Monohybrid Cross<\/span>\r\n\r\n<\/div>\r\n\u2022 A monohybrid cross \u2013 is a cross between two homozygous individuals resulting in the hybrid of two individuals\r\n\r\n\u2022 It can be easily shown through a Punnett Square.\r\n\r\n<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02180907\/Figure_12_02_02.png\" alt=\"This illustration shows a monohybrid cross. In the P generation, one parent has a dominant yellow phenotype and the genotype YY, and the other parent has the recessive green phenotype and the genotype yy. Each parent produces one kind of gamete, resulting in an F_{1} generation with a dominant yellow phenotype and the genotype Yy. Self-pollination of the F_{1} generation results in an F_{2} generation with a 3 to 1 ratio of yellow to green peas. One out of three of the yellow pea plants has a dominant genotype of YY, and 2 out of 3 have the heterozygous phenotype Yy. The homozygous recessive plant has the green phenotype and the genotype yy.\" class=\"aligncenter\" \/>\r\n<pre><a href=\"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology1\/chapter\/the-father-of-genetics\/\" target=\"_blank\" rel=\"noopener\">\"Monohybrid cross\"<\/a><span>\u00a0by\u00a0<\/span><a>Shelli Carter and Lumen Learning.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a><\/pre>\r\n<div style=\"text-align: left\">\u2022Based on this experimental findings Mendel concluded that the characteristics could be divided into expressed and latent traits.<\/div>\r\n<div style=\"text-align: left\">\u2022He called these, respectively, <strong>dominant and recessive traits.<\/strong><\/div>\r\n<div style=\"text-align: left\">\u2022<strong>Dominant traits<\/strong>\u00a0are those that are inherited unchanged in a hybridization.<\/div>\r\n<div style=\"text-align: left\">\u2022<strong>Recessive traits\u00a0<\/strong>become latent, or disappear, in the offspring of a hybridization.<\/div>\r\n<div style=\"text-align: left\">\u2022The recessive trait does, however, reappear in the progeny of the hybrid offspring.<\/div>\r\n<div style=\"text-align: left\">\u2022For example <span>pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype. This cross produces F1 heterozygotes with a yellow phenotype.<\/span><\/div>\r\n<div style=\"text-align: left\">\u2022 <span>Punnett square analysis can be used to predict the genotypes of the F2 generation.<\/span><\/div>\r\n<ul>\r\n \t<li style=\"text-align: left\"><span>A self-cross of one of the Yy heterozygous offspring can be represented in a\u00a0 Punnett square because each parent can donate one of two different alleles.<\/span><\/li>\r\n \t<li style=\"text-align: left\"><span> Therefore, the offspring can potentially have one of four allele combinations: YY,Yy, yY, or yy <\/span><span>. <\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>There are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm ( Reciprocal cross)<\/span><\/li>\r\n \t<li style=\"text-align: left\"><span> Both of these possibilities must be counted. <\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>The result of these heterozygous combinations are genotypically and phenotypically identical offsprings despite their dominant and recessive alleles deriving from different parents. They are grouped together.<\/span><\/li>\r\n \t<li style=\"text-align: left\"><span> Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy <strong>genotypes<\/strong> of 1:2:1 <\/span><span>. <strong>( genotypic ratio)<\/strong><\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a <strong>phenotypic ratio<\/strong> of 3 yellow:1 green. <\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>\u00a0Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits.<\/span><\/li>\r\n \t<li style=\"text-align: left\"><span> Mendel validated these results by performing an F3 cross\u00a0<\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>In F3 he self-crossed the dominant- and recessive-expressing F2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy. <\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds. <\/span><\/li>\r\n \t<li style=\"text-align: left\"><span>In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous(Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross.<\/span><\/li>\r\n<\/ul>\r\nBased on these experimental results Mendel postulated the Laws of Heredity , which are popularly called as Mendel's Laws of Inheritance.\r\n<h2>Law of dominance:<\/h2>\r\nThe law of dominance states that <em><strong>\u201cIn a cross between a pair of organisms with pure contrasting characteristics, only the dominant of the pair expresses itself phenotypically while the other remains hidden in the F1 generation\u201d. The character that expresses in F1 is called Dominant character While that is hidden is called Recessive character\"<\/strong><\/em>\r\n\r\n<span style=\"font-family: 'Cormorant Garamond', serif;font-size: 1.602em;font-weight: bold\">Test cross<\/span>\r\n\r\n<\/div>\r\n<div class=\"ac-container ac-adaptiveCard\" style=\"text-align: center\">\r\n<div class=\"ac-textBlock\">\r\n<div style=\"text-align: left\">\u2022In genetics, a test cross, first introduced by Gregor Mendel,<\/div>\r\n<div style=\"text-align: left\">\u2022Involves the breeding of an individual with a phenotypically recessive individual,<\/div>\r\n<div style=\"text-align: left\">\u2022To determine the zygosity of the former<\/div>\r\n<div style=\"text-align: left\">\u2022By analyzing proportions of offspring phenotypes.<\/div>\r\n<div style=\"text-align: left\">\u2022 Zygosity can either be heterozygous or homozygous.<\/div>\r\n<\/div>\r\n<\/div>\r\n<div class=\"ac-container ac-adaptiveCard\" style=\"text-align: center\">\r\n<div class=\"ac-textBlock\">\r\n<h3><img src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image18.png\" alt=\"image\" class=\"aligncenter\" \/><\/h3>\r\n<pre><a href=\"https:\/\/slcc.pressbooks.pub\/humanbiology\/chapter\/__unknown__-6\/\" target=\"_blank\" rel=\"noopener\">\"Mendel hybridization experiment\"<\/a><span>\u00a0by\u00a0<\/span><a>Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a>\r\n\r\n<\/pre>\r\n<h2>Back Cross<\/h2>\r\n\u2022 Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent,\r\n\u2022 In order to achieve offspring with a genetic identity which is closer to that of the parent.\r\n\u2022 It is used in horticulture, animal breeding and in production of gene knockout organisms.\r\n\r\n<img src=\"https:\/\/www.wikilectures.eu\/thumb.php?f=DominantBc.png&amp;width=400\" alt=\"DominantBc.png\" class=\"aligncenter\" \/>\r\n\r\n<a href=\"https:\/\/www.wikilectures.eu\/w\/Backcross\" target=\"_blank\" rel=\"noopener\">\"Back cross\"<\/a><span>\u00a0by\u00a0<\/span><a>WikiLectures, project of the First Faculty of Medicine, Charles University<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by-sa\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY-SA 4.0<\/a>\r\n<h2>Law of Segregation<\/h2>\r\n\u2022 This law is also known as Mendel's Law of purity of gametes\r\n\r\n\u2022 The law states that \u201c<em><strong>each characteristic of an organism is controlled by two alleles. During gametes formation in meiosis I, the alleles from each gene will segregate from each other and each gamete will only carry one of the alleles\u201d<\/strong><\/em>\r\n<ul>\r\n \t<li style=\"text-align: left\">When a pair of alleles are brought together in the F1 generation, they remain together without mixing or contaminating each other and separate completely during the gametogenesis.<\/li>\r\n \t<li style=\"text-align: left\">Also called Law of purity of gametes because each gamete remains pure in itself i.e., having T gene for Tallness and t gene for dwarfness.<\/li>\r\n<\/ul>\r\n<h1>Dihybrid cross<\/h1>\r\nDihybrid cross is a cross between two different genes that differ in two observed traits\r\n\r\n<span>A dihybrid cross is a cross between two true-breeding parents that express different traits for two characteristics. <\/span>\r\n\r\n<span>for example consider the characteristics of seed color and seed texture for two pea plants, one that has green, wrinkled seeds (yyrr) and another that has yellow, round seeds (YYRR). <\/span>\r\n\r\n<span>Because each parent is homozygous, the law of segregation indicates that the gametes for the green\/wrinkled plant all are yr, and the gametes for the yellow\/round plant are all YR. Therefore, the F1 generation of offspring all are YyRr i.e.,<\/span>\u00a0each gamete receives either an R allele or an r allele along with either a Y allele or a y allele.\r\n\r\nThe cross is based on Mendel\u2019s<strong> Law of Independent Assortment <\/strong>which states that<em><strong> \u201cWhen two or more characteristics are <\/strong><\/em>\r\n<em><strong>inherited, individual hereditary factors assort independently during gamete production and the inheritance of one trait <\/strong><\/em>\r\n<em><strong>does not affect the inheritance of another\u201d<\/strong><\/em>\r\n\r\nThe law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele.\r\n\r\nTherefore when the F1 heterozygote is self-crossed : four equally likely gametes that can be formed lows: YR, Yr, yR, and yr.\r\n\r\nArranging these gametes along the top and left of a 4 by 4 Punnett square gives us 16 equally likely genotypic combinations.\r\n\r\nFrom these genotypes, we infer a phenotypic ratio of 9 round\/yellow:3 round\/green:3 wrinkled\/yellow:1 wrinkled\/green.\r\n\r\nThe 9:3:3:1 dihybrid phenotypic ratio can be divided into two 3:1 ratios due to separate assortment and dominance; these ratios are typical of any monohybrid cross that exhibits both dominant and recessive traits.\r\n\r\nIn the aforementioned dihybrid cross, if we were to ignore seed color and simply take seed texture into account, we would anticipate that three quarters of the F2 generation progeny would be round and one quarter would be wrinkled.\r\n\r\nIf we were to separate out solely the color of the seeds, we would predict that three-quarters of the F2 offspring would be yellow and the remaining one-quarter would be green.\r\n\r\nWe can use the product rule because the sorting of alleles for texture and color is an independent occurrence. Consequently, it is anticipated that the proportion of round and yellow F2 offspring would be (3\/4) \u00c5~ (3\/4) = 9\/16, and the proportion of wrinkled and green offspring is expected to be (1\/4) \u00c5~ (1\/4) = 1\/16.\r\n\r\nThese ratios are the same as what a Punnett square would yield. Because each of these genotypes has a dominant and a recessive phenotype, the product rule can also be used to determine the round, green, wrinkled, yellow offspring.\r\n\r\nThus, the formula for calculating each proportion is\u00a0\u00a0calculated as (3\/4) \u00c5~ (1\/4) = 3\/16.\r\n\r\nThe law of independent assortment also indicates that a cross between yellow, wrinkled (YYrr) and green, round (yyRR) parents would yield the same F1 and F2 offspring as in the YYRR x yyrr cross.\r\n<h3 class=\"import-Normal\"><img src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image14.png\" alt=\"image\" class=\"aligncenter\" \/><\/h3>\r\n<a href=\"https:\/\/slcc.pressbooks.pub\/humanbiology\/chapter\/__unknown__-6\/\" target=\"_blank\" rel=\"noopener\">\"Mendel hybridization experiment\"<\/a><span>\u00a0by\u00a0<\/span><a>Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a>\r\n\r\n<\/div>\r\n<h1>Co dominance<\/h1>\r\nCodominance is a form of inheritance\r\n\u2022 In this case the alleles of a gene pair in a heterozygote are both expressed.\r\n\u2022 As a result, the phenotype of the offspring is a combination of the phenotype of both the parents.\r\n\u2022 Thus, the trait is neither dominant nor recessive\r\n\r\n<\/div>\r\n<h3>ABO Blood Group system as an example for Codominance<\/h3>\r\n<img src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/thumb\/3\/3f\/Blood_Type_Codominance.png\/566px-Blood_Type_Codominance.png?20200528152317\" alt=\"File:Blood Type Codominance.png\" class=\"aligncenter\" width=\"325\" height=\"344\" \/>\r\n<p style=\"text-align: center\"><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Intermediate_inheritance_-_incomplete_dominance.png\" target=\"_blank\" rel=\"noopener\">\"Co Dominance \"<\/a><span>\u00a0by\u00a0<\/span><a>DylanAudette, via Wikimedia Commons<\/a><a><\/a><a><\/a><span>\u00a0is in the\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\" target=\"_blank\" rel=\"noopener\">Public Domain, CC0<\/a><\/p>\r\n\u2022 There are different types of red blood cells such as A, B, AB and O\r\n\u2022 These blood groups can be with or without the Rh factor.\r\n\u2022 The difference is in the antigen present on the RBC surface\r\n\u2022 This determines the specific blood group in an organism.\r\n\u2022 For example: If a person is blood group A, it means the RBC surface consists of antigen-A, coded by the gene I.\r\n\u2022 The gene I have three types of alleles namely, IA, IB and i.\r\n\u2022 The alleles IA and IB produce two different antigens A, B respectively\r\n\u2022 The allele-i do not produce any antigen.\r\n\u2022 Hence, alleles IA and IB are dominant over the allele i.\r\n\u2022 As we know, each diploid organism bears two pairs of alleles.\r\n\u2022 Hence, depending on the allelic combination and dominance of allele,\u00a0 blood type of\u00a0an individual could be determined.\r\n\r\n\u2022So if an individual inherits allele A from one parent and allele B from other parent, they have blood type AB\r\n\r\n<\/div>\r\n<h3>Sickle Cell Anaemia as an example for Co dominance<\/h3>\r\nSickle cell anaemia is a genetic disease which affects the heamoglobin of the red blood cells.\r\n\u2022 Haemoglobin is normally a ball-shaped molecule\r\n\u2022 The sickle cell allele makes it form long strands.\r\n\u2022 As the result the shape of the RBCS is distorted.\r\n\u2022 They assume sickle shape. Hence prone to more degradation\r\n\u2022 As a consequence ,anaemia results, called Sickle cell anemia.\r\n\u2022 The shape of the haemoglobin molecule is controlled by two alleles:\r\n\u2022 Normal Haemoglobin allele\r\n\u2022 Sickle Cell Haemoglobin allele\r\n\r\nThere are three phenotypes\r\n\u2022 Normal : Normal individuals have two normal haemoglobin alleles\r\n\u2022 Sickle cell anaemia : A severe form where all the red blood cells are affected. Sickle cell anaemia patients have two sickle cell alleles in their genotype\r\n\u2022 Sickle cell trait : A mild condition where 50% of the red blood cells are affected. Sickle cell trait individuals are heterozygotes, having one copy of each allele\r\n\r\nThe heterozygotes have their own phenotype\r\n\u2022 Hence this gives rise to different proportions amongst their offspring\r\n\u2022 Unlike with crosses between heterozygotes for dominant and recessive alleles\r\n\r\nSickle cell anemic person has one copy of the sickle cell allele\r\n\u2022 As a result half of their red blood cells will be misshapen.\r\n\u2022 In this way, the allele is codominant, since both normal and sickled shapes are seen in the blood\r\n\r\n<span>Visit this website to learn more about how a mutation in DNA leads to sickle-cell anemia:\u00a0<\/span><a href=\"https:\/\/dnalc.cshl.edu\/resources\/3d\/17-sickle-cell.html\">Biology &amp; 3D Animation Library \u2013<span>\u00a0<\/span><\/a><a href=\"https:\/\/dnalc.cshl.edu\/resources\/3d\/17-sickle-cell.html\">Sickle Cell<\/a><span>,<\/span>\r\n\r\n&nbsp;\r\n<h1>Incomplete Dominance<\/h1>\r\nA heterozygous condition in which both alleles at a gene locus are partially expressed and produces an intermediate phenotype is\r\ncalled Incomplete Dominance\r\n\u2022 Incomplete dominance occurs because neither of the two alleles is completely dominant over the other. As a result the phenotype is a combination of both alleles.\r\n\u2022 Gregor Mendel studied on seven characters with contrasting traits and all of them showed a similar pattern of inheritance in Pisum\r\nsativum . Based on this, he generalized the law of inheritance.\r\n\r\n\u2022 Later, researchers repeated Mendel\u2019s experiment on other plants.\r\n\u2022 Surpisingly, they noted that the F1 Generation showed variation from the usual pattern of inheritance.\r\n\u2022 F1 Progeny of the monohybrid cross didn\u2019t show any resemblance to either of the parents, but instead appeared an intermediate progeny.\r\n\r\n&nbsp;\r\n\r\nExample :\r\n\r\nConsider a Monohybrid cross between the red and white coloured flowers of Snapdragon plant.\r\n\u2022 Thepure breed of the red flower has RR pair of alleles and that for the white flower is rr.\r\n\u2022 Pure-breeding red (RR) and white (rr) coloured flowers of snapdragon were crossed.\r\n\u2022 The F1generation produced a pink coloured flower with Rr pair of alleles.\r\n\u2022 When F1 progeny was self-pollinated \u2022 it resulted in red (RR), pink (Rr) and white (rr) flowers\u00a0in the ratio of 1:2:1.\r\n\u2022 Here the genotype ratio of F2 generation is same as the monohybrid cross Mendel of 1:2:1.\r\n\u2022 However, the phenotype ratio has changed from 3:1 to 1:2:1.\r\n\u2022 The reason for this variation is the incomplete dominance of the allele R over the allele r which led to the blending of colour in flowers.\r\n\r\n<img src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/thumb\/1\/18\/Intermediate_inheritance_-_incomplete_dominance.png\/800px-Intermediate_inheritance_-_incomplete_dominance.png?20191019051448\" alt=\"File:Intermediate inheritance - incomplete dominance.png\" class=\"aligncenter\" \/>\r\n<div class=\"content user-select-text\" aria-label=\"Sent by Copilot: Sure! Let's delve deeper into the concepts of heredity and variation. ### Heredity Heredity is the process through which traits are passed from parents to offspring. This transmission occurs through genes, which are segments of DNA located on chromosomes. Each gene can exist in different forms called alleles, which contribute to the diversity of traits observed in a population. #### Key Concepts in Heredity: - **Genes and Alleles**: Genes are the basic units of heredity, and alleles are different versions of a gene. For example, the gene for eye color may have an allele for blue eyes and another for brown eyes. - **Dominant and Recessive Traits**: Traits can be dominant or recessive. Dominant traits are expressed if at least one dominant allele is present, while recessive traits are expressed only when two recessive alleles are present[^4^]. - **Genotype and Phenotype**: The genotype is the genetic makeup of an organism, while the phenotype is the observable characteristics. The phenotype results from the interaction of the genotype with the environment[^4^]. ### Variation Variation refers to the differences in traits among individuals within a population. These differences can be genetic or environmental in origin. #### Sources of Genetic Variation: 1. **Mutations**: Changes in the DNA sequence that can introduce new genetic variations. Mutations can be beneficial, neutral, or harmful[^11^]. 2. **Genetic Recombination**: During sexual reproduction, the process of meiosis leads to the shuffling of genes, creating new combinations of alleles[^12^]. 3. **Gene Flow**: The movement of genes between populations, which can introduce new genetic material into a population[^12^]. #### Types of Variation: - **Continuous Variation**: Traits that show a range of phenotypes, such as height or skin color. These traits are usually influenced by multiple genes (polygenic inheritance) and environmental factors[^12^]. - **Discontinuous Variation**: Traits that have distinct categories, such as blood type or flower color. These traits are typically controlled by a single gene or a few genes[^12^]. ### Importance of Variation Variation is crucial for the survival and evolution of species. It provides the raw material for natural selection, allowing populations to adapt to changing environments. Without variation, a population may not be able to cope with new challenges, such as diseases or climate change[^12^]. Would you like to explore any specific examples or dive into another related topic?\">\r\n<div class=\"ac-container ac-adaptiveCard\" style=\"text-align: center\">\r\n<div class=\"ac-textBlock\">\r\n<pre><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Intermediate_inheritance_-_incomplete_dominance.png\" target=\"_blank\" rel=\"noopener\">\"Incomplete Dominance \"<\/a><span>\u00a0by\u00a0<\/span><a>Sciencia58, via Wikimedia Commons<\/a><span>\u00a0is in the\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\" target=\"_blank\" rel=\"noopener\">Public Domain, CC0<\/a><\/pre>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nTest your understanding\r\n\r\n<span>[h5p id=\"81\"]<\/span>","rendered":"<div class=\"content user-select-text\" aria-label=\"Sent by Copilot: Sure! Let's delve deeper into the concepts of heredity and variation. ### Heredity Heredity is the process through which traits are passed from parents to offspring. This transmission occurs through genes, which are segments of DNA located on chromosomes. Each gene can exist in different forms called alleles, which contribute to the diversity of traits observed in a population. #### Key Concepts in Heredity: - **Genes and Alleles**: Genes are the basic units of heredity, and alleles are different versions of a gene. For example, the gene for eye color may have an allele for blue eyes and another for brown eyes. - **Dominant and Recessive Traits**: Traits can be dominant or recessive. Dominant traits are expressed if at least one dominant allele is present, while recessive traits are expressed only when two recessive alleles are present[^4^]. - **Genotype and Phenotype**: The genotype is the genetic makeup of an organism, while the phenotype is the observable characteristics. The phenotype results from the interaction of the genotype with the environment[^4^]. ### Variation Variation refers to the differences in traits among individuals within a population. These differences can be genetic or environmental in origin. #### Sources of Genetic Variation: 1. **Mutations**: Changes in the DNA sequence that can introduce new genetic variations. Mutations can be beneficial, neutral, or harmful[^11^]. 2. **Genetic Recombination**: During sexual reproduction, the process of meiosis leads to the shuffling of genes, creating new combinations of alleles[^12^]. 3. **Gene Flow**: The movement of genes between populations, which can introduce new genetic material into a population[^12^]. #### Types of Variation: - **Continuous Variation**: Traits that show a range of phenotypes, such as height or skin color. These traits are usually influenced by multiple genes (polygenic inheritance) and environmental factors[^12^]. - **Discontinuous Variation**: Traits that have distinct categories, such as blood type or flower color. These traits are typically controlled by a single gene or a few genes[^12^]. ### Importance of Variation Variation is crucial for the survival and evolution of species. It provides the raw material for natural selection, allowing populations to adapt to changing environments. Without variation, a population may not be able to cope with new challenges, such as diseases or climate change[^12^]. Would you like to explore any specific examples or dive into another related topic?\">\n<div class=\"ac-container ac-adaptiveCard\" style=\"text-align: center\">\n<div class=\"ac-textBlock\">\n<h1 class=\"import-Normal\">Mendelian Genetics<\/h1>\n<\/div>\n<div class=\"ac-textBlock\">\n<div style=\"text-align: left\"><span>Johann <\/span>Gregor Mendel is regarded as \u201cFather of modern genetics,&#8221;<\/div>\n<div style=\"text-align: left\">\u2022Genetics is the study of heredity.<\/div>\n<div style=\"text-align: left\">\u2022Mendel was born in Austria in 1822. He was a monk and he discovered the basic principles of heredity<\/div>\n<div style=\"text-align: left\">\u2022 He conducted experiments on Pea plant in his monastery&#8217;s garden.<\/div>\n<div style=\"text-align: left\">\u2022His experiments laid the foundation of modern Genetics<\/div>\n<div style=\"text-align: left\">\u2022 The postulates put forth by Mendel form the basis of classical, or Mendelian, genetics.<\/div>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image11.png\" alt=\"image\" class=\"aligncenter\" \/><\/p>\n<p>Pea plants &#8211; his primary model system to study a specific biological phenomenon to be applied to other systems.<\/p>\n<p>In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society.<\/p>\n<p>He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns.<\/p>\n<p>In 1866, he published his work, Experiments in Plant Hybridization, in the proceedings of the Natural History Society of Br\u00fcnn in 1866<\/p>\n<p>The scientific world largely ignored Mendel&#8217;s discoveries because they mistakenly thought that heredity involved combining parental features to give kids an intermediate physical appearance.<\/p>\n<p>During his lifetime, Mendel did not receive recognition for his outstanding contributions to science. It was not\u00a0 until 1900 that his work was rediscovered, <span style=\"font-size: 1em\">by three European botanists working independently namely, <\/span><span style=\"font-size: 1em\">Hugo de Vries:\u00a0<\/span><span style=\"font-size: 1em\">From Holland , <\/span><span style=\"font-size: 1em\">Carl Correns:\u00a0<\/span><span style=\"font-size: 1em\">From Germany ,<\/span><span style=\"font-size: 1em\">Erich von Tschermak:\u00a0<\/span><span style=\"font-size: 1em\">From Austria<span class=\"UV3uM\"> .<\/span><\/span><\/p>\n<div class=\"WaaZC\">\n<h3 class=\"import-Normal\"><em>Mendel&#8217;s Experiments with Pisum sativum<\/em> as Model System<\/h3>\n<p><span>Mendel carried out his experiments in\u00a0 the garden pea,\u00a0<\/span><em>Pisum sativum<\/em><span>, to study inheritance.<\/span><\/p>\n<ul>\n<li style=\"text-align: left\">Mendel performed hybridizations Experiments which involves mating two true-breeding individuals that have different traits.<\/li>\n<li style=\"text-align: left\">Pea, is naturally self-pollinating.<\/li>\n<li style=\"text-align: left\">Mendel pollinated the pea plants by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety.<\/li>\n<li style=\"text-align: left\">In plants, pollen carries the male gametes to the stigma, a sticky organ that traps pollen and allows the male gamete to move down the pistil to the female gametes (ova)<\/li>\n<li style=\"text-align: left\">\n<div>To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel removed all of the anthers from the plant\u2019s flowers before they had a chance to mature.<\/div>\n<\/li>\n<li style=\"text-align: left\">\n<div>Plants used in first-generation crosses were called P0, or parental generation one, plants .<\/div>\n<\/li>\n<li style=\"text-align: left\">\n<div>Mendel collected the seeds belonging to the P0 plants that resulted from each cross and grew them the following season.<\/div>\n<\/li>\n<li style=\"text-align: left\">\n<div>These offspring were called the F1, or the first filial (filial = offspring, daughter or son), generation.<\/div>\n<\/li>\n<li style=\"text-align: left\">\n<div>Once Mendel examined the characteristics in the F1 generation of plants, he allowed them to self-fertilize naturally.<\/div>\n<\/li>\n<li style=\"text-align: left\">\n<div>He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation.<\/div>\n<\/li>\n<li style=\"text-align: left\">\n<div>Mendel\u2019s experiments extended beyond the F2 generation to the F3 and F4generations, and so on.<\/div>\n<\/li>\n<li>\n<div style=\"text-align: left\">Interestingly it was the ratio of characteristics in the P0\u2212F1\u2212F2 generations that were became the basis for Mendel\u2019s postulates.<\/div>\n<div><\/div>\n<\/li>\n<\/ul>\n<p><img decoding=\"async\" src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image27.png\" alt=\"image\" class=\"alignleft\" \/><\/p>\n<h3>Why did Mendel choose pea plant for his experiments?<\/h3>\n<ul>\n<li style=\"text-align: left\"><span>This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. <\/span><\/li>\n<li style=\"text-align: left\"><span>The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or \u201ctrue-breeding,\u201d pea plants and produce offspring that look like the parent. <\/span><\/li>\n<li style=\"text-align: left\"><span>By carrying out his experiments with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding.<\/span><\/li>\n<li style=\"text-align: left\"><span>The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time.<\/span><\/li>\n<li>\n<div class=\"WaaZC\">\n<p style=\"text-align: left\"><span>Large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.<\/span><\/p>\n<\/div>\n<\/li>\n<\/ul>\n<p>Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits .<strong>A trait is defined as a variation in the physical appearance of a heritable <em>characteristic<\/em><\/strong><em>.<\/em><\/p>\n<p>The contrasting characteristics\u00a0 studied by Mendel in\u00a0 pea plant included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position.<\/p>\n<p>For the characteristic of flower color, for example, the two contrasting traits were white versus violet.<\/p>\n<pre><a href=\"https:\/\/slcc.pressbooks.pub\/humanbiology\/chapter\/__unknown__-6\/\" target=\"_blank\" rel=\"noopener\">\"Mendel hybridization experiment\"<\/a><span>\u00a0by\u00a0<\/span><a>Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a><\/pre>\n<p>Mendel generated large numbers of F1 and F2 plants in order to fully examine each characteristic. His findings were consistent.<\/p>\n<p>Reginald Punnett, who developed a simple tool, now known as the <strong>Punnett Square<\/strong>, to predict the probability of genotypes and phenotypes from controlled crosses.<\/p>\n<\/div>\n<p><img decoding=\"async\" src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image13.png\" alt=\"image\" \/><\/p>\n<pre><a href=\"https:\/\/slcc.pressbooks.pub\/humanbiology\/chapter\/__unknown__-6\/\" target=\"_blank\" rel=\"noopener\">\"Mendel hybridization experiment\"<\/a><span>\u00a0by\u00a0<\/span><a>Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a><\/pre>\n<p><span style=\"font-family: 'Cormorant Garamond', serif;font-size: 1.602em\">Monohybrid Cross<\/span><\/p>\n<\/div>\n<p>\u2022 A monohybrid cross \u2013 is a cross between two homozygous individuals resulting in the hybrid of two individuals<\/p>\n<p>\u2022 It can be easily shown through a Punnett Square.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02180907\/Figure_12_02_02.png\" alt=\"This illustration shows a monohybrid cross. In the P generation, one parent has a dominant yellow phenotype and the genotype YY, and the other parent has the recessive green phenotype and the genotype yy. Each parent produces one kind of gamete, resulting in an F_{1} generation with a dominant yellow phenotype and the genotype Yy. Self-pollination of the F_{1} generation results in an F_{2} generation with a 3 to 1 ratio of yellow to green peas. One out of three of the yellow pea plants has a dominant genotype of YY, and 2 out of 3 have the heterozygous phenotype Yy. The homozygous recessive plant has the green phenotype and the genotype yy.\" class=\"aligncenter\" \/><\/p>\n<pre><a href=\"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology1\/chapter\/the-father-of-genetics\/\" target=\"_blank\" rel=\"noopener\">\"Monohybrid cross\"<\/a><span>\u00a0by\u00a0<\/span><a>Shelli Carter and Lumen Learning.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a><\/pre>\n<div style=\"text-align: left\">\u2022Based on this experimental findings Mendel concluded that the characteristics could be divided into expressed and latent traits.<\/div>\n<div style=\"text-align: left\">\u2022He called these, respectively, <strong>dominant and recessive traits.<\/strong><\/div>\n<div style=\"text-align: left\">\u2022<strong>Dominant traits<\/strong>\u00a0are those that are inherited unchanged in a hybridization.<\/div>\n<div style=\"text-align: left\">\u2022<strong>Recessive traits\u00a0<\/strong>become latent, or disappear, in the offspring of a hybridization.<\/div>\n<div style=\"text-align: left\">\u2022The recessive trait does, however, reappear in the progeny of the hybrid offspring.<\/div>\n<div style=\"text-align: left\">\u2022For example <span>pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype. This cross produces F1 heterozygotes with a yellow phenotype.<\/span><\/div>\n<div style=\"text-align: left\">\u2022 <span>Punnett square analysis can be used to predict the genotypes of the F2 generation.<\/span><\/div>\n<ul>\n<li style=\"text-align: left\"><span>A self-cross of one of the Yy heterozygous offspring can be represented in a\u00a0 Punnett square because each parent can donate one of two different alleles.<\/span><\/li>\n<li style=\"text-align: left\"><span> Therefore, the offspring can potentially have one of four allele combinations: YY,Yy, yY, or yy <\/span><span>. <\/span><\/li>\n<li style=\"text-align: left\"><span>There are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm ( Reciprocal cross)<\/span><\/li>\n<li style=\"text-align: left\"><span> Both of these possibilities must be counted. <\/span><\/li>\n<li style=\"text-align: left\"><span>The result of these heterozygous combinations are genotypically and phenotypically identical offsprings despite their dominant and recessive alleles deriving from different parents. They are grouped together.<\/span><\/li>\n<li style=\"text-align: left\"><span> Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy <strong>genotypes<\/strong> of 1:2:1 <\/span><span>. <strong>( genotypic ratio)<\/strong><\/span><\/li>\n<li style=\"text-align: left\"><span>Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a <strong>phenotypic ratio<\/strong> of 3 yellow:1 green. <\/span><\/li>\n<li style=\"text-align: left\"><span>\u00a0Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits.<\/span><\/li>\n<li style=\"text-align: left\"><span> Mendel validated these results by performing an F3 cross\u00a0<\/span><\/li>\n<li style=\"text-align: left\"><span>In F3 he self-crossed the dominant- and recessive-expressing F2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy. <\/span><\/li>\n<li style=\"text-align: left\"><span>When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds. <\/span><\/li>\n<li style=\"text-align: left\"><span>In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous(Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross.<\/span><\/li>\n<\/ul>\n<p>Based on these experimental results Mendel postulated the Laws of Heredity , which are popularly called as Mendel&#8217;s Laws of Inheritance.<\/p>\n<h2>Law of dominance:<\/h2>\n<p>The law of dominance states that <em><strong>\u201cIn a cross between a pair of organisms with pure contrasting characteristics, only the dominant of the pair expresses itself phenotypically while the other remains hidden in the F1 generation\u201d. The character that expresses in F1 is called Dominant character While that is hidden is called Recessive character&#8221;<\/strong><\/em><\/p>\n<p><span style=\"font-family: 'Cormorant Garamond', serif;font-size: 1.602em;font-weight: bold\">Test cross<\/span><\/p>\n<\/div>\n<div class=\"ac-container ac-adaptiveCard\" style=\"text-align: center\">\n<div class=\"ac-textBlock\">\n<div style=\"text-align: left\">\u2022In genetics, a test cross, first introduced by Gregor Mendel,<\/div>\n<div style=\"text-align: left\">\u2022Involves the breeding of an individual with a phenotypically recessive individual,<\/div>\n<div style=\"text-align: left\">\u2022To determine the zygosity of the former<\/div>\n<div style=\"text-align: left\">\u2022By analyzing proportions of offspring phenotypes.<\/div>\n<div style=\"text-align: left\">\u2022 Zygosity can either be heterozygous or homozygous.<\/div>\n<\/div>\n<\/div>\n<div class=\"ac-container ac-adaptiveCard\" style=\"text-align: center\">\n<div class=\"ac-textBlock\">\n<h3><img decoding=\"async\" src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image18.png\" alt=\"image\" class=\"aligncenter\" \/><\/h3>\n<pre><a href=\"https:\/\/slcc.pressbooks.pub\/humanbiology\/chapter\/__unknown__-6\/\" target=\"_blank\" rel=\"noopener\">\"Mendel hybridization experiment\"<\/a><span>\u00a0by\u00a0<\/span><a>Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a>\r\n\r\n<\/pre>\n<h2>Back Cross<\/h2>\n<p>\u2022 Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent,<br \/>\n\u2022 In order to achieve offspring with a genetic identity which is closer to that of the parent.<br \/>\n\u2022 It is used in horticulture, animal breeding and in production of gene knockout organisms.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/www.wikilectures.eu\/thumb.php?f=DominantBc.png&amp;width=400\" alt=\"DominantBc.png\" class=\"aligncenter\" \/><\/p>\n<p><a href=\"https:\/\/www.wikilectures.eu\/w\/Backcross\" target=\"_blank\" rel=\"noopener\">&#8220;Back cross&#8221;<\/a><span>\u00a0by\u00a0<\/span><a>WikiLectures, project of the First Faculty of Medicine, Charles University<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by-sa\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY-SA 4.0<\/a><\/p>\n<h2>Law of Segregation<\/h2>\n<p>\u2022 This law is also known as Mendel&#8217;s Law of purity of gametes<\/p>\n<p>\u2022 The law states that \u201c<em><strong>each characteristic of an organism is controlled by two alleles. During gametes formation in meiosis I, the alleles from each gene will segregate from each other and each gamete will only carry one of the alleles\u201d<\/strong><\/em><\/p>\n<ul>\n<li style=\"text-align: left\">When a pair of alleles are brought together in the F1 generation, they remain together without mixing or contaminating each other and separate completely during the gametogenesis.<\/li>\n<li style=\"text-align: left\">Also called Law of purity of gametes because each gamete remains pure in itself i.e., having T gene for Tallness and t gene for dwarfness.<\/li>\n<\/ul>\n<h1>Dihybrid cross<\/h1>\n<p>Dihybrid cross is a cross between two different genes that differ in two observed traits<\/p>\n<p><span>A dihybrid cross is a cross between two true-breeding parents that express different traits for two characteristics. <\/span><\/p>\n<p><span>for example consider the characteristics of seed color and seed texture for two pea plants, one that has green, wrinkled seeds (yyrr) and another that has yellow, round seeds (YYRR). <\/span><\/p>\n<p><span>Because each parent is homozygous, the law of segregation indicates that the gametes for the green\/wrinkled plant all are yr, and the gametes for the yellow\/round plant are all YR. Therefore, the F1 generation of offspring all are YyRr i.e.,<\/span>\u00a0each gamete receives either an R allele or an r allele along with either a Y allele or a y allele.<\/p>\n<p>The cross is based on Mendel\u2019s<strong> Law of Independent Assortment <\/strong>which states that<em><strong> \u201cWhen two or more characteristics are <\/strong><\/em><br \/>\n<em><strong>inherited, individual hereditary factors assort independently during gamete production and the inheritance of one trait <\/strong><\/em><br \/>\n<em><strong>does not affect the inheritance of another\u201d<\/strong><\/em><\/p>\n<p>The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele.<\/p>\n<p>Therefore when the F1 heterozygote is self-crossed : four equally likely gametes that can be formed lows: YR, Yr, yR, and yr.<\/p>\n<p>Arranging these gametes along the top and left of a 4 by 4 Punnett square gives us 16 equally likely genotypic combinations.<\/p>\n<p>From these genotypes, we infer a phenotypic ratio of 9 round\/yellow:3 round\/green:3 wrinkled\/yellow:1 wrinkled\/green.<\/p>\n<p>The 9:3:3:1 dihybrid phenotypic ratio can be divided into two 3:1 ratios due to separate assortment and dominance; these ratios are typical of any monohybrid cross that exhibits both dominant and recessive traits.<\/p>\n<p>In the aforementioned dihybrid cross, if we were to ignore seed color and simply take seed texture into account, we would anticipate that three quarters of the F2 generation progeny would be round and one quarter would be wrinkled.<\/p>\n<p>If we were to separate out solely the color of the seeds, we would predict that three-quarters of the F2 offspring would be yellow and the remaining one-quarter would be green.<\/p>\n<p>We can use the product rule because the sorting of alleles for texture and color is an independent occurrence. Consequently, it is anticipated that the proportion of round and yellow F2 offspring would be (3\/4) \u00c5~ (3\/4) = 9\/16, and the proportion of wrinkled and green offspring is expected to be (1\/4) \u00c5~ (1\/4) = 1\/16.<\/p>\n<p>These ratios are the same as what a Punnett square would yield. Because each of these genotypes has a dominant and a recessive phenotype, the product rule can also be used to determine the round, green, wrinkled, yellow offspring.<\/p>\n<p>Thus, the formula for calculating each proportion is\u00a0\u00a0calculated as (3\/4) \u00c5~ (1\/4) = 3\/16.<\/p>\n<p>The law of independent assortment also indicates that a cross between yellow, wrinkled (YYrr) and green, round (yyRR) parents would yield the same F1 and F2 offspring as in the YYRR x yyrr cross.<\/p>\n<h3 class=\"import-Normal\"><img decoding=\"async\" src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/17\/2020\/08\/image14.png\" alt=\"image\" class=\"aligncenter\" \/><\/h3>\n<p><a href=\"https:\/\/slcc.pressbooks.pub\/humanbiology\/chapter\/__unknown__-6\/\" target=\"_blank\" rel=\"noopener\">&#8220;Mendel hybridization experiment&#8221;<\/a><span>\u00a0by\u00a0<\/span><a>Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S.<\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY 4.0<\/a><\/p>\n<\/div>\n<h1>Co dominance<\/h1>\n<p>Codominance is a form of inheritance<br \/>\n\u2022 In this case the alleles of a gene pair in a heterozygote are both expressed.<br \/>\n\u2022 As a result, the phenotype of the offspring is a combination of the phenotype of both the parents.<br \/>\n\u2022 Thus, the trait is neither dominant nor recessive<\/p>\n<\/div>\n<h3>ABO Blood Group system as an example for Codominance<\/h3>\n<p><img decoding=\"async\" src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/thumb\/3\/3f\/Blood_Type_Codominance.png\/566px-Blood_Type_Codominance.png?20200528152317\" alt=\"File:Blood Type Codominance.png\" class=\"aligncenter\" width=\"325\" height=\"344\" \/><\/p>\n<p style=\"text-align: center\"><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Intermediate_inheritance_-_incomplete_dominance.png\" target=\"_blank\" rel=\"noopener\">&#8220;Co Dominance &#8220;<\/a><span>\u00a0by\u00a0<\/span><a>DylanAudette, via Wikimedia Commons<\/a><a><\/a><a><\/a><span>\u00a0is in the\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\" target=\"_blank\" rel=\"noopener\">Public Domain, CC0<\/a><\/p>\n<p>\u2022 There are different types of red blood cells such as A, B, AB and O<br \/>\n\u2022 These blood groups can be with or without the Rh factor.<br \/>\n\u2022 The difference is in the antigen present on the RBC surface<br \/>\n\u2022 This determines the specific blood group in an organism.<br \/>\n\u2022 For example: If a person is blood group A, it means the RBC surface consists of antigen-A, coded by the gene I.<br \/>\n\u2022 The gene I have three types of alleles namely, IA, IB and i.<br \/>\n\u2022 The alleles IA and IB produce two different antigens A, B respectively<br \/>\n\u2022 The allele-i do not produce any antigen.<br \/>\n\u2022 Hence, alleles IA and IB are dominant over the allele i.<br \/>\n\u2022 As we know, each diploid organism bears two pairs of alleles.<br \/>\n\u2022 Hence, depending on the allelic combination and dominance of allele,\u00a0 blood type of\u00a0an individual could be determined.<\/p>\n<p>\u2022So if an individual inherits allele A from one parent and allele B from other parent, they have blood type AB<\/p>\n<\/div>\n<h3>Sickle Cell Anaemia as an example for Co dominance<\/h3>\n<p>Sickle cell anaemia is a genetic disease which affects the heamoglobin of the red blood cells.<br \/>\n\u2022 Haemoglobin is normally a ball-shaped molecule<br \/>\n\u2022 The sickle cell allele makes it form long strands.<br \/>\n\u2022 As the result the shape of the RBCS is distorted.<br \/>\n\u2022 They assume sickle shape. Hence prone to more degradation<br \/>\n\u2022 As a consequence ,anaemia results, called Sickle cell anemia.<br \/>\n\u2022 The shape of the haemoglobin molecule is controlled by two alleles:<br \/>\n\u2022 Normal Haemoglobin allele<br \/>\n\u2022 Sickle Cell Haemoglobin allele<\/p>\n<p>There are three phenotypes<br \/>\n\u2022 Normal : Normal individuals have two normal haemoglobin alleles<br \/>\n\u2022 Sickle cell anaemia : A severe form where all the red blood cells are affected. Sickle cell anaemia patients have two sickle cell alleles in their genotype<br \/>\n\u2022 Sickle cell trait : A mild condition where 50% of the red blood cells are affected. Sickle cell trait individuals are heterozygotes, having one copy of each allele<\/p>\n<p>The heterozygotes have their own phenotype<br \/>\n\u2022 Hence this gives rise to different proportions amongst their offspring<br \/>\n\u2022 Unlike with crosses between heterozygotes for dominant and recessive alleles<\/p>\n<p>Sickle cell anemic person has one copy of the sickle cell allele<br \/>\n\u2022 As a result half of their red blood cells will be misshapen.<br \/>\n\u2022 In this way, the allele is codominant, since both normal and sickled shapes are seen in the blood<\/p>\n<p><span>Visit this website to learn more about how a mutation in DNA leads to sickle-cell anemia:\u00a0<\/span><a href=\"https:\/\/dnalc.cshl.edu\/resources\/3d\/17-sickle-cell.html\">Biology &amp; 3D Animation Library \u2013<span>\u00a0<\/span><\/a><a href=\"https:\/\/dnalc.cshl.edu\/resources\/3d\/17-sickle-cell.html\">Sickle Cell<\/a><span>,<\/span><\/p>\n<p>&nbsp;<\/p>\n<h1>Incomplete Dominance<\/h1>\n<p>A heterozygous condition in which both alleles at a gene locus are partially expressed and produces an intermediate phenotype is<br \/>\ncalled Incomplete Dominance<br \/>\n\u2022 Incomplete dominance occurs because neither of the two alleles is completely dominant over the other. As a result the phenotype is a combination of both alleles.<br \/>\n\u2022 Gregor Mendel studied on seven characters with contrasting traits and all of them showed a similar pattern of inheritance in Pisum<br \/>\nsativum . Based on this, he generalized the law of inheritance.<\/p>\n<p>\u2022 Later, researchers repeated Mendel\u2019s experiment on other plants.<br \/>\n\u2022 Surpisingly, they noted that the F1 Generation showed variation from the usual pattern of inheritance.<br \/>\n\u2022 F1 Progeny of the monohybrid cross didn\u2019t show any resemblance to either of the parents, but instead appeared an intermediate progeny.<\/p>\n<p>&nbsp;<\/p>\n<p>Example :<\/p>\n<p>Consider a Monohybrid cross between the red and white coloured flowers of Snapdragon plant.<br \/>\n\u2022 Thepure breed of the red flower has RR pair of alleles and that for the white flower is rr.<br \/>\n\u2022 Pure-breeding red (RR) and white (rr) coloured flowers of snapdragon were crossed.<br \/>\n\u2022 The F1generation produced a pink coloured flower with Rr pair of alleles.<br \/>\n\u2022 When F1 progeny was self-pollinated \u2022 it resulted in red (RR), pink (Rr) and white (rr) flowers\u00a0in the ratio of 1:2:1.<br \/>\n\u2022 Here the genotype ratio of F2 generation is same as the monohybrid cross Mendel of 1:2:1.<br \/>\n\u2022 However, the phenotype ratio has changed from 3:1 to 1:2:1.<br \/>\n\u2022 The reason for this variation is the incomplete dominance of the allele R over the allele r which led to the blending of colour in flowers.<\/p>\n<p><img decoding=\"async\" src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/thumb\/1\/18\/Intermediate_inheritance_-_incomplete_dominance.png\/800px-Intermediate_inheritance_-_incomplete_dominance.png?20191019051448\" alt=\"File:Intermediate inheritance - incomplete dominance.png\" class=\"aligncenter\" \/><\/p>\n<div class=\"content user-select-text\" aria-label=\"Sent by Copilot: Sure! Let's delve deeper into the concepts of heredity and variation. ### Heredity Heredity is the process through which traits are passed from parents to offspring. This transmission occurs through genes, which are segments of DNA located on chromosomes. Each gene can exist in different forms called alleles, which contribute to the diversity of traits observed in a population. #### Key Concepts in Heredity: - **Genes and Alleles**: Genes are the basic units of heredity, and alleles are different versions of a gene. For example, the gene for eye color may have an allele for blue eyes and another for brown eyes. - **Dominant and Recessive Traits**: Traits can be dominant or recessive. Dominant traits are expressed if at least one dominant allele is present, while recessive traits are expressed only when two recessive alleles are present[^4^]. - **Genotype and Phenotype**: The genotype is the genetic makeup of an organism, while the phenotype is the observable characteristics. The phenotype results from the interaction of the genotype with the environment[^4^]. ### Variation Variation refers to the differences in traits among individuals within a population. These differences can be genetic or environmental in origin. #### Sources of Genetic Variation: 1. **Mutations**: Changes in the DNA sequence that can introduce new genetic variations. Mutations can be beneficial, neutral, or harmful[^11^]. 2. **Genetic Recombination**: During sexual reproduction, the process of meiosis leads to the shuffling of genes, creating new combinations of alleles[^12^]. 3. **Gene Flow**: The movement of genes between populations, which can introduce new genetic material into a population[^12^]. #### Types of Variation: - **Continuous Variation**: Traits that show a range of phenotypes, such as height or skin color. These traits are usually influenced by multiple genes (polygenic inheritance) and environmental factors[^12^]. - **Discontinuous Variation**: Traits that have distinct categories, such as blood type or flower color. These traits are typically controlled by a single gene or a few genes[^12^]. ### Importance of Variation Variation is crucial for the survival and evolution of species. It provides the raw material for natural selection, allowing populations to adapt to changing environments. Without variation, a population may not be able to cope with new challenges, such as diseases or climate change[^12^]. Would you like to explore any specific examples or dive into another related topic?\">\n<div class=\"ac-container ac-adaptiveCard\" style=\"text-align: center\">\n<div class=\"ac-textBlock\">\n<pre><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Intermediate_inheritance_-_incomplete_dominance.png\" target=\"_blank\" rel=\"noopener\">\"Incomplete Dominance \"<\/a><span>\u00a0by\u00a0<\/span><a>Sciencia58, via Wikimedia Commons<\/a><span>\u00a0is in the\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/publicdomain\/zero\/1.0\/\" target=\"_blank\" rel=\"noopener\">Public Domain, CC0<\/a><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<p>Test your understanding<\/p>\n<p><span><\/p>\n<div id=\"h5p-81\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-81\" class=\"h5p-iframe\" data-content-id=\"81\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Mendel\u2019s Experiments Ch 8.1 Exercises\"><\/iframe><\/div>\n<\/div>\n<p><\/span><\/p>\n","protected":false},"author":1,"menu_order":2,"template":"","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_uf_show_specific_survey":0,"_uf_disable_surveys":false,"pb_show_title":"on","pb_short_title":"Mendelian Inheritance","pb_subtitle":"Mendelian Inheritance","pb_authors":["malathi"],"pb_section_license":"cc-by-sa"},"chapter-type":[],"contributor":[62],"license":[54],"class_list":["post-274","chapter","type-chapter","status-publish","hentry","contributor-malathi","license-cc-by-sa"],"aioseo_notices":[],"part":57,"_links":{"self":[{"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapters\/274","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/wp\/v2\/users\/1"}],"version-history":[{"count":6,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapters\/274\/revisions"}],"predecessor-version":[{"id":1256,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapters\/274\/revisions\/1256"}],"part":[{"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/parts\/57"}],"metadata":[{"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapters\/274\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/wp\/v2\/media?parent=274"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapter-type?post=274"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/wp\/v2\/contributor?post=274"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/wp\/v2\/license?post=274"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}