5 Genetics and Evolution
5.2 Mendelian inheritance
Mendelian Inheritance
Dr V Malathi
Mendelian Genetics
Pea plants – his primary model system to study a specific biological phenomenon to be applied to other systems.
In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society.
He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns.
In 1866, he published his work, Experiments in Plant Hybridization, in the proceedings of the Natural History Society of Brünn in 1866
The scientific world largely ignored Mendel’s discoveries because they mistakenly thought that heredity involved combining parental features to give kids an intermediate physical appearance.
During his lifetime, Mendel did not receive recognition for his outstanding contributions to science. It was not until 1900 that his work was rediscovered, by three European botanists working independently namely, Hugo de Vries: From Holland , Carl Correns: From Germany ,Erich von Tschermak: From Austria .
Mendel’s Experiments with Pisum sativum as Model System
Mendel carried out his experiments in the garden pea, Pisum sativum, to study inheritance.
- Mendel performed hybridizations Experiments which involves mating two true-breeding individuals that have different traits.
- Pea, is naturally self-pollinating.
- 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.
- 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)
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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’s flowers before they had a chance to mature.
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Plants used in first-generation crosses were called P0, or parental generation one, plants .
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Mendel collected the seeds belonging to the P0 plants that resulted from each cross and grew them the following season.
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These offspring were called the F1, or the first filial (filial = offspring, daughter or son), generation.
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Once Mendel examined the characteristics in the F1 generation of plants, he allowed them to self-fertilize naturally.
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He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation.
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Mendel’s experiments extended beyond the F2 generation to the F3 and F4generations, and so on.
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Interestingly it was the ratio of characteristics in the P0−F1−F2 generations that were became the basis for Mendel’s postulates.
Why did Mendel choose pea plant for his experiments?
- This species naturally self-fertilizes, such that pollen encounters ova within individual flowers.
- The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants and produce offspring that look like the parent.
- 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.
- The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time.
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Large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.
Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits .A trait is defined as a variation in the physical appearance of a heritable characteristic.
The contrasting characteristics studied by Mendel in pea plant included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position.
For the characteristic of flower color, for example, the two contrasting traits were white versus violet.
"Mendel hybridization experiment" by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under CC BY 4.0
Mendel generated large numbers of F1 and F2 plants in order to fully examine each characteristic. His findings were consistent.
Reginald Punnett, who developed a simple tool, now known as the Punnett Square, to predict the probability of genotypes and phenotypes from controlled crosses.
"Mendel hybridization experiment" by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under CC BY 4.0
Monohybrid Cross
• A monohybrid cross – is a cross between two homozygous individuals resulting in the hybrid of two individuals
• It can be easily shown through a Punnett Square.
"Monohybrid cross" by Shelli Carter and Lumen Learning. is licensed under CC BY 4.0
- A self-cross of one of the Yy heterozygous offspring can be represented in a Punnett square because each parent can donate one of two different alleles.
- Therefore, the offspring can potentially have one of four allele combinations: YY,Yy, yY, or yy .
- 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)
- Both of these possibilities must be counted.
- 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.
- 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 genotypes of 1:2:1 . ( genotypic ratio)
- 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 phenotypic ratio of 3 yellow:1 green.
- Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits.
- Mendel validated these results by performing an F3 cross
- 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.
- 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.
- 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.
Based on these experimental results Mendel postulated the Laws of Heredity , which are popularly called as Mendel’s Laws of Inheritance.
Law of dominance:
The law of dominance states that “In 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”. The character that expresses in F1 is called Dominant character While that is hidden is called Recessive character”
Test cross
"Mendel hybridization experiment" by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under CC BY 4.0
Back Cross
• Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent,
• In order to achieve offspring with a genetic identity which is closer to that of the parent.
• It is used in horticulture, animal breeding and in production of gene knockout organisms.
“Back cross” by WikiLectures, project of the First Faculty of Medicine, Charles University is licensed under CC BY-SA 4.0
Law of Segregation
• This law is also known as Mendel’s Law of purity of gametes
• The law states that “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”
- 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.
- 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.
Dihybrid cross
Dihybrid cross is a cross between two different genes that differ in two observed traits
A dihybrid cross is a cross between two true-breeding parents that express different traits for two characteristics.
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).
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., each gamete receives either an R allele or an r allele along with either a Y allele or a y allele.
The cross is based on Mendel’s Law of Independent Assortment which states that “When two or more characteristics are
inherited, individual hereditary factors assort independently during gamete production and the inheritance of one trait
does not affect the inheritance of another”
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.
Therefore when the F1 heterozygote is self-crossed : four equally likely gametes that can be formed lows: YR, Yr, yR, and yr.
Arranging these gametes along the top and left of a 4 by 4 Punnett square gives us 16 equally likely genotypic combinations.
From these genotypes, we infer a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green.
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.
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.
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.
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) Å~ (3/4) = 9/16, and the proportion of wrinkled and green offspring is expected to be (1/4) Å~ (1/4) = 1/16.
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.
Thus, the formula for calculating each proportion is calculated as (3/4) Å~ (1/4) = 3/16.
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.
“Mendel hybridization experiment” by Nancy Barrickman; Kathy Bell, DVM, MPH; and Chris Cowan, M.S. is licensed under CC BY 4.0
Co dominance
Codominance is a form of inheritance
• In this case the alleles of a gene pair in a heterozygote are both expressed.
• As a result, the phenotype of the offspring is a combination of the phenotype of both the parents.
• Thus, the trait is neither dominant nor recessive
ABO Blood Group system as an example for Codominance
“Co Dominance “ by DylanAudette, via Wikimedia Commons is in the Public Domain, CC0
• There are different types of red blood cells such as A, B, AB and O
• These blood groups can be with or without the Rh factor.
• The difference is in the antigen present on the RBC surface
• This determines the specific blood group in an organism.
• For example: If a person is blood group A, it means the RBC surface consists of antigen-A, coded by the gene I.
• The gene I have three types of alleles namely, IA, IB and i.
• The alleles IA and IB produce two different antigens A, B respectively
• The allele-i do not produce any antigen.
• Hence, alleles IA and IB are dominant over the allele i.
• As we know, each diploid organism bears two pairs of alleles.
• Hence, depending on the allelic combination and dominance of allele, blood type of an individual could be determined.
•So if an individual inherits allele A from one parent and allele B from other parent, they have blood type AB
Sickle Cell Anaemia as an example for Co dominance
Sickle cell anaemia is a genetic disease which affects the heamoglobin of the red blood cells.
• Haemoglobin is normally a ball-shaped molecule
• The sickle cell allele makes it form long strands.
• As the result the shape of the RBCS is distorted.
• They assume sickle shape. Hence prone to more degradation
• As a consequence ,anaemia results, called Sickle cell anemia.
• The shape of the haemoglobin molecule is controlled by two alleles:
• Normal Haemoglobin allele
• Sickle Cell Haemoglobin allele
There are three phenotypes
• Normal : Normal individuals have two normal haemoglobin alleles
• 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
• 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
The heterozygotes have their own phenotype
• Hence this gives rise to different proportions amongst their offspring
• Unlike with crosses between heterozygotes for dominant and recessive alleles
Sickle cell anemic person has one copy of the sickle cell allele
• As a result half of their red blood cells will be misshapen.
• In this way, the allele is codominant, since both normal and sickled shapes are seen in the blood
Visit this website to learn more about how a mutation in DNA leads to sickle-cell anemia: Biology & 3D Animation Library – Sickle Cell,
Incomplete Dominance
A heterozygous condition in which both alleles at a gene locus are partially expressed and produces an intermediate phenotype is
called Incomplete Dominance
• 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.
• Gregor Mendel studied on seven characters with contrasting traits and all of them showed a similar pattern of inheritance in Pisum
sativum . Based on this, he generalized the law of inheritance.
• Later, researchers repeated Mendel’s experiment on other plants.
• Surpisingly, they noted that the F1 Generation showed variation from the usual pattern of inheritance.
• F1 Progeny of the monohybrid cross didn’t show any resemblance to either of the parents, but instead appeared an intermediate progeny.
Example :
Consider a Monohybrid cross between the red and white coloured flowers of Snapdragon plant.
• Thepure breed of the red flower has RR pair of alleles and that for the white flower is rr.
• Pure-breeding red (RR) and white (rr) coloured flowers of snapdragon were crossed.
• The F1generation produced a pink coloured flower with Rr pair of alleles.
• When F1 progeny was self-pollinated • it resulted in red (RR), pink (Rr) and white (rr) flowers in the ratio of 1:2:1.
• Here the genotype ratio of F2 generation is same as the monohybrid cross Mendel of 1:2:1.
• However, the phenotype ratio has changed from 3:1 to 1:2:1.
• 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.
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