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| Linkage |
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| Every individual organism bears several heritable characters. Which are represented by the innumerable genes present on the chromosomes. During meiosis, the chromosomes move into the gametes as units, all the genes present on any given chromosome will segregate as a group and move together from generation to generation. This tendency of the genes located on the same chromosome, to stay together in hereditary transmission, is known as linkage. The genes located on the same chromosome are called linked genes. |
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| The principle of linkage was discovered by Bateson and Punnet in 1906 in the sweat pea, plant, Lathyrus odoratus. However, linkage, as a concept was put forth by Thomas Hunt Morgan in 1910 based on his experiment on Drosophila melanogaster. |
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| Morgan, along with Castle formulated the chromosome theory of linkage. It has the following postulates; |
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| 1. Genes are found arranged in a linear manner in the chromosomes. |
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| 2. Genes which exhibit linkage are located on the same chromosome. |
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| 3. Genes generally tend to stay in parental combination, except in cases of crossing over. |
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| 4. The distance between linked genes in a chromosome determines the strength of linkage. Genes located close to each other show stronger linkage than that are located far from each other, since the former are less likely to enter into crossing over. |
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| All the genes located on a particular chromosome, form a linkage group. Since the genes present on a particular chromosome have their alleles located on its homologous chromosome, genes on a pair of homologous chromosomes. Hence, the number of linkage groups corresponds to the number of haploid chromosomes found in a species. |
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| Drosophila melanogaster has four linkage groups which can be distinguished into three large and one small linkage groups corresponding to the four pairs of chromosomes. Twenty-three linkage groups are present in humans corresponding to 23 pairs of chromosomes. Pea plant has seven linkage groups, corresponding to the seven pairs of chromosomes. |
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| Complete Linkage |
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| The genes closely located in the chromosome show complete linkage as they have no chance of separating by crossing over and are always transmitted together to the same gamete and the same offspring. Thus, the parental combination of traits is inherited as such by the young one. |
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| Incomplete Linkage |
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| The genes distantly located in the chromosome show incomplete linkage because they have a chance of separation by crossing over and of going into different gametes and offspring. |
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