{"id":280,"date":"2024-03-23T09:39:44","date_gmt":"2024-03-23T09:39:44","guid":{"rendered":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/?post_type=chapter&#038;p=280"},"modified":"2024-11-02T16:41:31","modified_gmt":"2024-11-02T16:41:31","slug":"5-5-multiple-allelism-recombination-sex-linkage","status":"publish","type":"chapter","link":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/chapter\/5-5-multiple-allelism-recombination-sex-linkage\/","title":{"raw":"5.5 Multiple allelism, Recombination, Sex linkage","rendered":"5.5 Multiple allelism, Recombination, Sex linkage"},"content":{"raw":"<h2>Multiple allelism<\/h2>\r\nAccording to Mendel, <span>only two alleles, one dominant and one recessive, could exist for a given gene.<\/span>\r\n\r\nMost diploid organisms including humans\u00a0<span>can only have two alleles for a given gene<\/span>\r\n\r\n<span>However multiple alleles may exist at the population level .<\/span>\r\n\r\n<span>In such cases many combinations of two alleles are observed. <\/span>\r\n\r\n<span>When many alleles exist for the same gene, it is convention to denote the most common phenotype or genotype as the <\/span><strong>wild type<\/strong><span>\u00a0(often abbreviated \u201c+\u201d); <\/span>\r\n\r\n<span>\u00a0All other phenotypes or genotypes are considered <\/span><strong>variants<\/strong><span>\u00a0of this standard, meaning that they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.<\/span>\r\n\r\n&nbsp;\r\n\r\nExample :\u00a0 C<span>oat color in rabbits . <\/span>\r\n\r\n<span>Here, four alleles exist for the <\/span><em>c<\/em><span>\u00a0gene. <\/span>\r\n<ol>\r\n \t<li><span>The wild-type version,\u00a0 has Genotype : <\/span><em>C<sup data-redactor-tag=\"sup\">+<\/sup>C<sup>+<\/sup><\/em><span>, is expressed as brown fur. <\/span><\/li>\r\n \t<li><span>The chinchilla , genotype : has\u00a0 <\/span><em>c<i data-redactor-tag=\"i\"><sup>ch<\/sup><\/i>c<i><sup data-redactor-tag=\"sup\">ch<\/sup><\/i><\/em><span>, is expressed as black-tipped white fur. <\/span><\/li>\r\n \t<li><span>The Himalayan phenotype, has genotype :\u00a0 <\/span><em>c<i data-redactor-tag=\"i\"><sup>h<\/sup><\/i>c<i><sup data-redactor-tag=\"sup\">h<\/sup><\/i><\/em><span>, has black fur on the extremities and white fur elsewhere. <\/span><\/li>\r\n \t<li><span>\u00a0The albino, or \u201ccolorless\u201d phenotype,\u00a0 genotype <\/span><em>cc<\/em><span>, is expressed as white fur. <\/span><\/li>\r\n<\/ol>\r\n<span>In cases of multiple alleles, dominance hierarchies can exist.<\/span>\r\n\r\n<span> In above case, the wild-type allele is dominant over all the others, chinchilla is incompletely dominant over Himalayan and albino, and Himalayan is dominant over albino.\u00a0<\/span>\r\n\r\n<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02182457\/Figure_12_02_05.jpg\" alt=\"This illustration shows the four different variants for coat color in rabbits at the c allele. The genotype CC produces the wild type phenotype, which is brown. The genotype c^{ch}c^{ch} produces the chinchilla phenotype, which is black-tipped white fur. The genotype c^{h}c^{h} produces the Himalayan phenotype, which is white on the body and black on the extremities. The genotype cc produces the recessive phenotype, which is white\" \/>\r\n\r\n<a href=\"https:\/\/courses.lumenlearning.com\/wm-biology1\/chapter\/reading-multiple-alleles\/\" target=\"_blank\" rel=\"noopener\">\"Four different alleles of rabbit\"<\/a><span>\u00a0by\u00a0<\/span><a>Lumen learning<\/a><a><\/a><a><\/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<strong>Another example for multiple allelism is ABO blood grouping in humans<\/strong>\r\n\r\nThe ABO blood groups is determined according to the type of the antigen protein expressed on the surface of the Red Blood Cells .The three alleles: I<sup>A<\/sup>, I<sup>B<\/sup>, and i determine the blood group\r\n<ul>\r\n \t<li>The A blood group blood RBCs express the Antigen A on their\u00a0 surface ; Expressed by the alleles I<sup>A<\/sup> I<sup>A\u00a0<\/sup><sup>\u00a0 <\/sup>and\u00a0 I<sup>A\u00a0 \u00a0<\/sup>\u00a0i<\/li>\r\n \t<li>The B blood group blood RBCs express the Antigen B on their\u00a0 surface ; Expressed by the alleles I<sup>B<\/sup> I<sup>B\u00a0\u00a0 <\/sup>and\u00a0 I<sup>B\u00a0\u00a0<\/sup>\u00a0i<\/li>\r\n \t<li>The A Bblood group blood RBCs express both the Antigen A and B on their\u00a0 surface ; Expressed by the alleles\u00a0 \u00a0I<sup>A<\/sup> I<sup>B\u00a0<\/sup><sup> \u00a0<\/sup><\/li>\r\n \t<li>The O blood group blood RBCs do not express any Antigen A on their\u00a0 surface ; These possess the alleles<sup>\u00a0\u00a0<\/sup> ii<\/li>\r\n<\/ul>\r\nAlleles I<sup>A<\/sup><span>\u00a0<\/span>and I<sup>B<\/sup><span>\u00a0<\/span>are codominant with respect to one another, and both are dominant to i. These t<span>hree alleles in the population result in six genotypes, and four phenotypes.<\/span>\r\n<figure id=\"attachment_1214\" class=\"wp-caption aligncenter\" style=\"width: 651px\" aria-describedby=\"caption-attachment-1214\"><img loading=\"lazy\" class=\" wp-image-1214\" src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/20\/2021\/02\/Blood-types-1-1024x385.png\" alt=\"\" width=\"651\" height=\"245\" \/><\/figure>\r\n<p style=\"text-align: center\"><a href=\"https:\/\/slcc.pressbooks.pub\/collegebiology1\/chapter\/incomplete-dominance-and-codominance\/\" target=\"_blank\" rel=\"noopener\">\" ABO blood system in humans \"<\/a><span>\u00a0by\u00a0<\/span><a>Melissa Hardy<\/a><a><\/a><a><\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY-NC 4.0<\/a><\/p>\r\nWatch the video from Amoeba Sisters on Multiple alleles\r\n\r\n&nbsp;\r\n\r\n[embed]https:\/\/youtu.be\/9O5JQqlngFY?si=BnJmNQ4oL2LbGhDM[\/embed]\r\n<h3>Genetic Recombination<\/h3>\r\nGenetic recombination involves the exhange of genetic material between organisms. This leads to the production of offspring with combinations of traits that differ from either parent.\r\n\r\nIn eukaryotes ,genetic recombination occurs during meiosis .\r\n\r\nRecombination can be classified in to two types\r\n<ol>\r\n \t<li>Interchromosomal recombination : This occurs through independent assortment of alleles, which are on different but on homologous chromosomes.<\/li>\r\n \t<li>Intrachromosomal recombination : Occurs through crossing over<\/li>\r\n<\/ol>\r\n<h3>Crossing over<\/h3>\r\nEach parent cell has pairs of homologous chromosomes,\r\n\u2022 One homolog from the Paternal and one from the maternal origin.\r\n\u2022 These chromosomes are replicated before cell division\r\n\u2022 In prophase I of meiosis, the replicated homologous pair of chromosomes comes together and this process is called Synapsis.\r\n\u2022 Sections of the paired chromosomes are exchanged.\r\n\u2022 This exchange occurs by a process called crossing over.\r\n\u2022 After crossing over, the resultant chromosomes are neither resemble entirely maternal nor entirely paternal, but contain genes from both parents.\r\n\u2022 This ensures genetic variation in sexually reproducing organisms\r\n\r\n<strong>The main features of crossing over are given below:<\/strong>\r\n1.Crossing over takes place during meiotic prophase, i.e., during pachytene. Each pair of chromosome has four chromatids at that time.<strong>(Tetrad)<\/strong>\r\n2. Crossing over occurs between non-sister chromatids.\r\n3. One chromatid from each of the two homologus chromosomes is involved in crossing over.\r\n4. Each crossing over involves only two of the four chromatids of two homologus chromosomes.\r\n5.Rarely double or multiple crossing over may involve all four, three or two of the four chromatids.\r\n6. Crossing over leads to<strong> re-combinations or new combinations.<\/strong>\r\n7.Crossing over generally yields two recombinant types or crossover types and two parental types or non-crossover types.\r\n6. Crossing over generally leads to exchange of equal segments or genes and recombination is always reciprocal. However, unequal crossing over has also been reported.\r\n7. The value of crossover or recombinants may vary from 0-50%.\r\n<h2>Chiasma and Crossing Over<\/h2>\r\n\u2022 The point of exchange of segments between non-sister chromatids of homologous chromosomes during meiotic prophase is called<strong> chiasma (pleural chiasmata)<\/strong>.\r\n\u2022 It is the place where crossing over takes place.\r\n\u2022 Depending on the position, chiasma is of two types, viz., terminal and interstitial.\r\n\u2022 When the chiasma is located at the end of the pairing chromatids, it is known as <strong>terminal chiasma<\/strong>\r\n\u2022 when it is located in the middle part of non-sister chromatids, it is referred to as <strong>interstitial chiasma.<\/strong>\r\n\u2022 Later on interstitial chiasma is changed to terminal position by the process of <strong>chiasma terminalization.<\/strong>\r\n\u2022 The number of chiasma per bivalent may vary from one to more than one depending upon the length of chromatids.\r\n<h2>Chiasma Terminalization<\/h2>\r\n\u2022 The movement of chiasma away from the centromere and towards the end of tetrads is called terminalization.\r\n\u2022 The total number of chiasmata terminalized at any given stage or time is known as coefficient of terminalization.\r\n\u2022 Generally, chiasma terminalization occurs between diplotene and metaphase I\r\n<h2>Mechanism of Crossing Over<\/h2>\r\n\u2022 Crossing over begins with a double strand break in one of the DNA molecules.\r\n\u2022 This leads to two hanging single-stranded regions\r\n\u2022 These single strand regions get coated with proteins called recombinase that catalyze recombination.\r\n\u2022 The first invading strand behaves like a primer and \u00a0this synthesizes a double stranded region for itself using one strand of its non-sister chromatid as a template.\r\n\u2022 This leads to its complementary strand getting displaced and base pairing with the second single stranded region that was initially generated by the exonuclease.\r\n\u2022 Ultimately, this results in two strands being exchanged with the formation of a cross-like structure called the <strong>Holliday junction<\/strong>\r\n\r\n<img src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/6\/6d\/Homologous_Recombination.jpg?20131023161439\" alt=\"File:Homologous Recombination.jpg\" class=\"aligncenter\" \/>\r\n\r\n<span>Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left. Most recombination events appear to be the SDSA type.<\/span>\r\n\r\n<a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Homologous_Recombination.jpg\" target=\"_blank\" rel=\"noopener\">\"Homologous Recombination\"<\/a><span>\u00a0by\u00a0<\/span><a>Harris Bernstein, Carol Bernstein and Richard E. Michod<\/a><a><\/a><a><\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/3.0\" target=\"_blank\" rel=\"noopener\">CC BY 3.0<\/a>\r\n<h1>Linkage<\/h1>\r\nThere are more genes than the chromosome\r\n\u2022 Therefore each chromosome contains more than one gene.\r\n\u2022 The genes for different characters may be either situated in the same chromosome or in different chromosome\r\n\u2022 When the genes are situated in different chromosomes the characters they control appear in the next generation either together or apart. i.e they assort independently according to Mendel\u2019s Law of Independent Assortment.\r\n<ul>\r\n \t<li>But if genes are situated in the same chromosome and if they are fairly close to each other ,they tend to be inherited together. This is called Linkage\r\n\u2022 The linked genes do not assort independently but tend to stay together in the same combination as they were in the parents.<\/li>\r\n \t<li>Genetic linkage describes the tendency of allele that are located close together on a chromosome to\r\nbe inherited together during the meiosis phase of sexual reproduction.\r\n\u2022 Genes whose loci are nearer to each other are less likely to be separated onto different chromatids during chromosomal crossover.\r\n\u2022 Such genes are said to be genetically linked i.e., the nearer two genes are on a chromosome, lesser is the chance of a crossing occurring between them, and therefore they are more likely they are to be\r\ninherited together.<\/li>\r\n \t<li>Genetic linkage was first discovered by the British geneticists William Bateson, Edith Rebecca Saunders and Reginald Punnett shortly after Mendel's laws were rediscovered.\r\n\u2022 The study about genetic linkage was expanded by the work of Thomas Hunt Morgan.\r\n\u2022 Morgan observed that the amount of crossing over between linked genes differs\r\n\u2022 This observation led to the idea that crossover frequency might indicate the distance separating genes on the chromosome.<\/li>\r\n<\/ul>\r\n<h2>Chromosomes Theory of Linkage<\/h2>\r\nMorgan &amp; castle formulated the chromosome theory of linkage which states that :-\r\n1. The genes which show the phenomenon of linkage are situated in the same chromosomes and inherited together\r\nduring the process of inheritance.\r\n2. The distance between the linked genes determines the strength of linkage. The closely located genes show strong linkage than the widely located genes which show the weak linkage.\r\n3. The genes are arranged in linear fashion in the chromosomes.\r\n\r\n<strong>Genetic maps or Linkage maps\u00a0<\/strong>\r\n\r\nAlfred Sturtevant (student of Morgan\u2019s) first developed genetic maps.These are also known as linkage maps.\r\n\u2022 He proposed that the greater the distance between linked genes, the greater the chance that non-sister\r\nchromatids would cross over in the region between the genes.\r\n\u2022 By calculating the number of recombinants it is possible to measure the distance between the genes.\r\n\u2022 This distance is expressed in terms of a <strong>genetic map unit (m.u.), or a centimorgan<\/strong>\r\n\u2022 It is defined as the distance between genes for which one product of meiosis in 100 is recombinant.\r\n\r\nA recombinant frequency (RF) of 1% is equivalent to 1 m.u.\r\n\u2022 This equivalence approximation holds good only for small percentages;\r\n\u2022 The largest percentage of recombinants cannot exceed 50% ,which would be the situation where the two\r\ngenes are at the extreme opposite ends of the same chromosomes\r\n\r\n<strong>Types of Linkage:<\/strong>\r\nLinkage is of two types, complete and incomplete.\r\n1.<strong> Complete Linkage (Morgan, 1919):<\/strong>\r\nThe genes located on the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over.\r\nComplete linkage allows the combination of parental traits to be inherited as such. It is rare but has been reported in male Drosophila and some other heterogametic organisms\r\n\r\nExample:\r\n\u2022 In Drosophila, genes of grey body and long wings are dominant over black body and vestigial (short)\r\nwings.\r\n\u2022 If pure breeding grey bodied long winged Drosophila (GL\/ GL) flies are crossed with black bodied vestigial winged flies (gl\/gl), the F2shows a 3 : 1 ratio of parental phenotypes (3 grey body long winged and one black body vestigial winged).\r\n\u2022 This is explained by assuming that genes of body colour and wing length are found on the same chromosome and are completely linked.\r\n\r\n<strong>Incomplete Linkage:<\/strong>\r\n\u2022 Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type.\r\n\u2022 The number of recombinant individuals is usually less than the number expected in independent assortment.\r\n\u2022 In independent assortment all the four types (two parental types and two recombinant types) are each 25%.\r\n\u2022 In case of linkage, each of the two parental types is more than 25% while each of the recombinant types is less than 25%\r\n\r\nExample\r\n\r\nA red eyed normal winged or wild type dominant homozygous female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny or F1 individuals are heterozygous red eyed and normal winged. F1 female flies are test crossed with homozygous recessive males. It does not yield the ratio of 1: 1: 1: 1. Instead the ratio comes out to be 9: 1: 1: 8. This shows that the two genes did not segregate independently of each other.\r\n\r\nOnly 10.7% recombinant types were observed which is quite different from 50% recombinants in case of independent assortment. This shows that in the oocytes of the F1 , generation only some of the chromatids undergo crossover while the majority is preserved intact. \u00a0This produces 90.3% parental types in the progeny\r\n\r\n<a href=\"http:\/\/Watch%20the%20video,%20(AP%20Biology)%20Linked%20Genes,%20Unlinked%20Genes,%20Incomplete%20Linkage,%20and%20Gene%20Mapping,%20by%20Mr.%20Cronin\u2019s%20Videos%20(2019)%20on%20YouTube\">Watch the video, (AP Biology) Linked Genes, Unlinked Genes, Incomplete Linkage, and Gene Mapping, by Mr. Cronin\u2019s Videos (2019) on YouTube<\/a>\r\n\r\n<strong>Test your Understanding<\/strong>\r\n\r\n<span>[h5p id=\"85\"]<\/span>\r\n\r\n&nbsp;","rendered":"<h2>Multiple allelism<\/h2>\n<p>According to Mendel, <span>only two alleles, one dominant and one recessive, could exist for a given gene.<\/span><\/p>\n<p>Most diploid organisms including humans\u00a0<span>can only have two alleles for a given gene<\/span><\/p>\n<p><span>However multiple alleles may exist at the population level .<\/span><\/p>\n<p><span>In such cases many combinations of two alleles are observed. <\/span><\/p>\n<p><span>When many alleles exist for the same gene, it is convention to denote the most common phenotype or genotype as the <\/span><strong>wild type<\/strong><span>\u00a0(often abbreviated \u201c+\u201d); <\/span><\/p>\n<p><span>\u00a0All other phenotypes or genotypes are considered <\/span><strong>variants<\/strong><span>\u00a0of this standard, meaning that they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.<\/span><\/p>\n<p>&nbsp;<\/p>\n<p>Example :\u00a0 C<span>oat color in rabbits . <\/span><\/p>\n<p><span>Here, four alleles exist for the <\/span><em>c<\/em><span>\u00a0gene. <\/span><\/p>\n<ol>\n<li><span>The wild-type version,\u00a0 has Genotype : <\/span><em>C<sup data-redactor-tag=\"sup\">+<\/sup>C<sup>+<\/sup><\/em><span>, is expressed as brown fur. <\/span><\/li>\n<li><span>The chinchilla , genotype : has\u00a0 <\/span><em>c<i data-redactor-tag=\"i\"><sup>ch<\/sup><\/i>c<i><sup data-redactor-tag=\"sup\">ch<\/sup><\/i><\/em><span>, is expressed as black-tipped white fur. <\/span><\/li>\n<li><span>The Himalayan phenotype, has genotype :\u00a0 <\/span><em>c<i data-redactor-tag=\"i\"><sup>h<\/sup><\/i>c<i><sup data-redactor-tag=\"sup\">h<\/sup><\/i><\/em><span>, has black fur on the extremities and white fur elsewhere. <\/span><\/li>\n<li><span>\u00a0The albino, or \u201ccolorless\u201d phenotype,\u00a0 genotype <\/span><em>cc<\/em><span>, is expressed as white fur. <\/span><\/li>\n<\/ol>\n<p><span>In cases of multiple alleles, dominance hierarchies can exist.<\/span><\/p>\n<p><span> In above case, the wild-type allele is dominant over all the others, chinchilla is incompletely dominant over Himalayan and albino, and Himalayan is dominant over albino.\u00a0<\/span><\/p>\n<p><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/110\/2016\/05\/02182457\/Figure_12_02_05.jpg\" alt=\"This illustration shows the four different variants for coat color in rabbits at the c allele. The genotype CC produces the wild type phenotype, which is brown. The genotype c^{ch}c^{ch} produces the chinchilla phenotype, which is black-tipped white fur. The genotype c^{h}c^{h} produces the Himalayan phenotype, which is white on the body and black on the extremities. The genotype cc produces the recessive phenotype, which is white\" \/><\/p>\n<p><a href=\"https:\/\/courses.lumenlearning.com\/wm-biology1\/chapter\/reading-multiple-alleles\/\" target=\"_blank\" rel=\"noopener\">&#8220;Four different alleles of rabbit&#8221;<\/a><span>\u00a0by\u00a0<\/span><a>Lumen learning<\/a><a><\/a><a><\/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<p><strong>Another example for multiple allelism is ABO blood grouping in humans<\/strong><\/p>\n<p>The ABO blood groups is determined according to the type of the antigen protein expressed on the surface of the Red Blood Cells .The three alleles: I<sup>A<\/sup>, I<sup>B<\/sup>, and i determine the blood group<\/p>\n<ul>\n<li>The A blood group blood RBCs express the Antigen A on their\u00a0 surface ; Expressed by the alleles I<sup>A<\/sup> I<sup>A\u00a0<\/sup><sup>\u00a0 <\/sup>and\u00a0 I<sup>A\u00a0 \u00a0<\/sup>\u00a0i<\/li>\n<li>The B blood group blood RBCs express the Antigen B on their\u00a0 surface ; Expressed by the alleles I<sup>B<\/sup> I<sup>B\u00a0\u00a0 <\/sup>and\u00a0 I<sup>B\u00a0\u00a0<\/sup>\u00a0i<\/li>\n<li>The A Bblood group blood RBCs express both the Antigen A and B on their\u00a0 surface ; Expressed by the alleles\u00a0 \u00a0I<sup>A<\/sup> I<sup>B\u00a0<\/sup><sup> \u00a0<\/sup><\/li>\n<li>The O blood group blood RBCs do not express any Antigen A on their\u00a0 surface ; These possess the alleles<sup>\u00a0\u00a0<\/sup> ii<\/li>\n<\/ul>\n<p>Alleles I<sup>A<\/sup><span>\u00a0<\/span>and I<sup>B<\/sup><span>\u00a0<\/span>are codominant with respect to one another, and both are dominant to i. These t<span>hree alleles in the population result in six genotypes, and four phenotypes.<\/span><\/p>\n<figure id=\"attachment_1214\" class=\"wp-caption aligncenter\" style=\"width: 651px\" aria-describedby=\"caption-attachment-1214\"><img decoding=\"async\" class=\"wp-image-1214\" src=\"https:\/\/slcc.pressbooks.pub\/app\/uploads\/sites\/20\/2021\/02\/Blood-types-1-1024x385.png\" alt=\"\" width=\"651\" height=\"245\" \/><\/figure>\n<p style=\"text-align: center\"><a href=\"https:\/\/slcc.pressbooks.pub\/collegebiology1\/chapter\/incomplete-dominance-and-codominance\/\" target=\"_blank\" rel=\"noopener\">&#8221; ABO blood system in humans &#8220;<\/a><span>\u00a0by\u00a0<\/span><a>Melissa Hardy<\/a><a><\/a><a><\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by-nc\/4.0\" target=\"_blank\" rel=\"noopener\">CC BY-NC 4.0<\/a><\/p>\n<p>Watch the video from Amoeba Sisters on Multiple alleles<\/p>\n<p>&nbsp;<\/p>\n<p><iframe id=\"oembed-1\" title=\"Multiple Alleles (ABO Blood Types) and Punnett Squares\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/9O5JQqlngFY?feature=oembed&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<h3>Genetic Recombination<\/h3>\n<p>Genetic recombination involves the exhange of genetic material between organisms. This leads to the production of offspring with combinations of traits that differ from either parent.<\/p>\n<p>In eukaryotes ,genetic recombination occurs during meiosis .<\/p>\n<p>Recombination can be classified in to two types<\/p>\n<ol>\n<li>Interchromosomal recombination : This occurs through independent assortment of alleles, which are on different but on homologous chromosomes.<\/li>\n<li>Intrachromosomal recombination : Occurs through crossing over<\/li>\n<\/ol>\n<h3>Crossing over<\/h3>\n<p>Each parent cell has pairs of homologous chromosomes,<br \/>\n\u2022 One homolog from the Paternal and one from the maternal origin.<br \/>\n\u2022 These chromosomes are replicated before cell division<br \/>\n\u2022 In prophase I of meiosis, the replicated homologous pair of chromosomes comes together and this process is called Synapsis.<br \/>\n\u2022 Sections of the paired chromosomes are exchanged.<br \/>\n\u2022 This exchange occurs by a process called crossing over.<br \/>\n\u2022 After crossing over, the resultant chromosomes are neither resemble entirely maternal nor entirely paternal, but contain genes from both parents.<br \/>\n\u2022 This ensures genetic variation in sexually reproducing organisms<\/p>\n<p><strong>The main features of crossing over are given below:<\/strong><br \/>\n1.Crossing over takes place during meiotic prophase, i.e., during pachytene. Each pair of chromosome has four chromatids at that time.<strong>(Tetrad)<\/strong><br \/>\n2. Crossing over occurs between non-sister chromatids.<br \/>\n3. One chromatid from each of the two homologus chromosomes is involved in crossing over.<br \/>\n4. Each crossing over involves only two of the four chromatids of two homologus chromosomes.<br \/>\n5.Rarely double or multiple crossing over may involve all four, three or two of the four chromatids.<br \/>\n6. Crossing over leads to<strong> re-combinations or new combinations.<\/strong><br \/>\n7.Crossing over generally yields two recombinant types or crossover types and two parental types or non-crossover types.<br \/>\n6. Crossing over generally leads to exchange of equal segments or genes and recombination is always reciprocal. However, unequal crossing over has also been reported.<br \/>\n7. The value of crossover or recombinants may vary from 0-50%.<\/p>\n<h2>Chiasma and Crossing Over<\/h2>\n<p>\u2022 The point of exchange of segments between non-sister chromatids of homologous chromosomes during meiotic prophase is called<strong> chiasma (pleural chiasmata)<\/strong>.<br \/>\n\u2022 It is the place where crossing over takes place.<br \/>\n\u2022 Depending on the position, chiasma is of two types, viz., terminal and interstitial.<br \/>\n\u2022 When the chiasma is located at the end of the pairing chromatids, it is known as <strong>terminal chiasma<\/strong><br \/>\n\u2022 when it is located in the middle part of non-sister chromatids, it is referred to as <strong>interstitial chiasma.<\/strong><br \/>\n\u2022 Later on interstitial chiasma is changed to terminal position by the process of <strong>chiasma terminalization.<\/strong><br \/>\n\u2022 The number of chiasma per bivalent may vary from one to more than one depending upon the length of chromatids.<\/p>\n<h2>Chiasma Terminalization<\/h2>\n<p>\u2022 The movement of chiasma away from the centromere and towards the end of tetrads is called terminalization.<br \/>\n\u2022 The total number of chiasmata terminalized at any given stage or time is known as coefficient of terminalization.<br \/>\n\u2022 Generally, chiasma terminalization occurs between diplotene and metaphase I<\/p>\n<h2>Mechanism of Crossing Over<\/h2>\n<p>\u2022 Crossing over begins with a double strand break in one of the DNA molecules.<br \/>\n\u2022 This leads to two hanging single-stranded regions<br \/>\n\u2022 These single strand regions get coated with proteins called recombinase that catalyze recombination.<br \/>\n\u2022 The first invading strand behaves like a primer and \u00a0this synthesizes a double stranded region for itself using one strand of its non-sister chromatid as a template.<br \/>\n\u2022 This leads to its complementary strand getting displaced and base pairing with the second single stranded region that was initially generated by the exonuclease.<br \/>\n\u2022 Ultimately, this results in two strands being exchanged with the formation of a cross-like structure called the <strong>Holliday junction<\/strong><\/p>\n<p><img decoding=\"async\" src=\"https:\/\/upload.wikimedia.org\/wikipedia\/commons\/6\/6d\/Homologous_Recombination.jpg?20131023161439\" alt=\"File:Homologous Recombination.jpg\" class=\"aligncenter\" \/><\/p>\n<p><span>Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left. Most recombination events appear to be the SDSA type.<\/span><\/p>\n<p><a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Homologous_Recombination.jpg\" target=\"_blank\" rel=\"noopener\">&#8220;Homologous Recombination&#8221;<\/a><span>\u00a0by\u00a0<\/span><a>Harris Bernstein, Carol Bernstein and Richard E. Michod<\/a><a><\/a><a><\/a><span>\u00a0is licensed under\u00a0<\/span><a href=\"http:\/\/creativecommons.org\/licenses\/by\/3.0\" target=\"_blank\" rel=\"noopener\">CC BY 3.0<\/a><\/p>\n<h1>Linkage<\/h1>\n<p>There are more genes than the chromosome<br \/>\n\u2022 Therefore each chromosome contains more than one gene.<br \/>\n\u2022 The genes for different characters may be either situated in the same chromosome or in different chromosome<br \/>\n\u2022 When the genes are situated in different chromosomes the characters they control appear in the next generation either together or apart. i.e they assort independently according to Mendel\u2019s Law of Independent Assortment.<\/p>\n<ul>\n<li>But if genes are situated in the same chromosome and if they are fairly close to each other ,they tend to be inherited together. This is called Linkage<br \/>\n\u2022 The linked genes do not assort independently but tend to stay together in the same combination as they were in the parents.<\/li>\n<li>Genetic linkage describes the tendency of allele that are located close together on a chromosome to<br \/>\nbe inherited together during the meiosis phase of sexual reproduction.<br \/>\n\u2022 Genes whose loci are nearer to each other are less likely to be separated onto different chromatids during chromosomal crossover.<br \/>\n\u2022 Such genes are said to be genetically linked i.e., the nearer two genes are on a chromosome, lesser is the chance of a crossing occurring between them, and therefore they are more likely they are to be<br \/>\ninherited together.<\/li>\n<li>Genetic linkage was first discovered by the British geneticists William Bateson, Edith Rebecca Saunders and Reginald Punnett shortly after Mendel&#8217;s laws were rediscovered.<br \/>\n\u2022 The study about genetic linkage was expanded by the work of Thomas Hunt Morgan.<br \/>\n\u2022 Morgan observed that the amount of crossing over between linked genes differs<br \/>\n\u2022 This observation led to the idea that crossover frequency might indicate the distance separating genes on the chromosome.<\/li>\n<\/ul>\n<h2>Chromosomes Theory of Linkage<\/h2>\n<p>Morgan &amp; castle formulated the chromosome theory of linkage which states that :-<br \/>\n1. The genes which show the phenomenon of linkage are situated in the same chromosomes and inherited together<br \/>\nduring the process of inheritance.<br \/>\n2. The distance between the linked genes determines the strength of linkage. The closely located genes show strong linkage than the widely located genes which show the weak linkage.<br \/>\n3. The genes are arranged in linear fashion in the chromosomes.<\/p>\n<p><strong>Genetic maps or Linkage maps\u00a0<\/strong><\/p>\n<p>Alfred Sturtevant (student of Morgan\u2019s) first developed genetic maps.These are also known as linkage maps.<br \/>\n\u2022 He proposed that the greater the distance between linked genes, the greater the chance that non-sister<br \/>\nchromatids would cross over in the region between the genes.<br \/>\n\u2022 By calculating the number of recombinants it is possible to measure the distance between the genes.<br \/>\n\u2022 This distance is expressed in terms of a <strong>genetic map unit (m.u.), or a centimorgan<\/strong><br \/>\n\u2022 It is defined as the distance between genes for which one product of meiosis in 100 is recombinant.<\/p>\n<p>A recombinant frequency (RF) of 1% is equivalent to 1 m.u.<br \/>\n\u2022 This equivalence approximation holds good only for small percentages;<br \/>\n\u2022 The largest percentage of recombinants cannot exceed 50% ,which would be the situation where the two<br \/>\ngenes are at the extreme opposite ends of the same chromosomes<\/p>\n<p><strong>Types of Linkage:<\/strong><br \/>\nLinkage is of two types, complete and incomplete.<br \/>\n1.<strong> Complete Linkage (Morgan, 1919):<\/strong><br \/>\nThe genes located on the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over.<br \/>\nComplete linkage allows the combination of parental traits to be inherited as such. It is rare but has been reported in male Drosophila and some other heterogametic organisms<\/p>\n<p>Example:<br \/>\n\u2022 In Drosophila, genes of grey body and long wings are dominant over black body and vestigial (short)<br \/>\nwings.<br \/>\n\u2022 If pure breeding grey bodied long winged Drosophila (GL\/ GL) flies are crossed with black bodied vestigial winged flies (gl\/gl), the F2shows a 3 : 1 ratio of parental phenotypes (3 grey body long winged and one black body vestigial winged).<br \/>\n\u2022 This is explained by assuming that genes of body colour and wing length are found on the same chromosome and are completely linked.<\/p>\n<p><strong>Incomplete Linkage:<\/strong><br \/>\n\u2022 Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type.<br \/>\n\u2022 The number of recombinant individuals is usually less than the number expected in independent assortment.<br \/>\n\u2022 In independent assortment all the four types (two parental types and two recombinant types) are each 25%.<br \/>\n\u2022 In case of linkage, each of the two parental types is more than 25% while each of the recombinant types is less than 25%<\/p>\n<p>Example<\/p>\n<p>A red eyed normal winged or wild type dominant homozygous female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny or F1 individuals are heterozygous red eyed and normal winged. F1 female flies are test crossed with homozygous recessive males. It does not yield the ratio of 1: 1: 1: 1. Instead the ratio comes out to be 9: 1: 1: 8. This shows that the two genes did not segregate independently of each other.<\/p>\n<p>Only 10.7% recombinant types were observed which is quite different from 50% recombinants in case of independent assortment. This shows that in the oocytes of the F1 , generation only some of the chromatids undergo crossover while the majority is preserved intact. \u00a0This produces 90.3% parental types in the progeny<\/p>\n<p><a href=\"http:\/\/Watch%20the%20video,%20(AP%20Biology)%20Linked%20Genes,%20Unlinked%20Genes,%20Incomplete%20Linkage,%20and%20Gene%20Mapping,%20by%20Mr.%20Cronin\u2019s%20Videos%20(2019)%20on%20YouTube\">Watch the video, (AP Biology) Linked Genes, Unlinked Genes, Incomplete Linkage, and Gene Mapping, by Mr. Cronin\u2019s Videos (2019) on YouTube<\/a><\/p>\n<p><strong>Test your Understanding<\/strong><\/p>\n<p><span><\/p>\n<div id=\"h5p-85\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-85\" class=\"h5p-iframe\" data-content-id=\"85\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Extensions of the Laws of Inheritance Ch 8.3 Exercises\"><\/iframe><\/div>\n<\/div>\n<p><\/span><\/p>\n<p>&nbsp;<\/p>\n","protected":false},"author":1,"menu_order":5,"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":"Mutliple alleles, Recombination, Linkage","pb_subtitle":"Mutliple alleles, Recombination, Linkage","pb_authors":["malathi"],"pb_section_license":"cc-by-nc-sa"},"chapter-type":[],"contributor":[62],"license":[57],"class_list":["post-280","chapter","type-chapter","status-publish","hentry","contributor-malathi","license-cc-by-nc-sa"],"aioseo_notices":[],"part":57,"_links":{"self":[{"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapters\/280","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":41,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapters\/280\/revisions"}],"predecessor-version":[{"id":1344,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapters\/280\/revisions\/1344"}],"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\/280\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/wp\/v2\/media?parent=280"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/pressbooks\/v2\/chapter-type?post=280"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/wp\/v2\/contributor?post=280"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.justwrite.in\/interactive-biology-secondary\/wp-json\/wp\/v2\/license?post=280"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}