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In this article we will discuss about the chromosome theory of heredity with the help of suitable diagram.
By the year 1900, due to improvements in cytological techniques and in the optical properties of microscopes, the behaviour of chromosomes in the cells was studied. William Sutton noticed striking similarities in the behaviour of chromosomes during gamete formation and fertilisation, and the transmission of Mendel’s hereditary factors.
The similarities are as follows:
a. The diploid cell has pairs of homologous chromosomes. In the same way, each cell has a pair of genes for each character.
b. The homologous pair separate during meiosis just the way genes separate during meiosis.
c. The diploid state of the chromosomes is restored during fertilisation. The alleles also come together during fertilisation.
d. One chromosome of each pair is contributed by each parent. In a similar way, of a pair of genes in a zygote, one comes from each parent.
On the basis of these similarities, Sutton and Boveri proposed that chromosomes were the earners of Mendel’s factors and put forth the chromosome theory of heredity. According to the chromosome theory of heredity, ‘the two alleles of a gene pair are located on homologous sites on homologous chromosomes’.
Since an organism has many phenotypic traits in its body and each phenotypic trait is determined by at least a pair of ‘factors’ or alleles, an organism will possess many genes, which may exceed the number of its chromosomes. Thus, each chromosome must carry many factors. Man possesses only 23 pairs of chromosomes, but has many traits which are controlled by many thousands of genes. Thus, each chromosome must carry a large number of genes.
Genes situated on the same chromosome are said to be linked. The genes on a single chromosome form a linkage group. A linkage group usually passes into a gamete and are inherited together. The linkage groups in an organism are the same as the haploid number of chromosomes or the number of pairs of chromosomes. Thus, a fruit fly has 4 linkage groups, human beings 23 linkage groups (23 in females and 24 in males), Pea and Neurospora have 7 linkage groups, Maize 10 linkage groups.
Genes belonging to the same linkage group do not show independent assortment. Therefore, they fail to produce the usual 9:3:3:1 ratio. By conducting test cross it is possible to find out if a di-hybrid contains either linked genes or independently assorted genes. The independently assorted genes give a test cross ratio of 1:1:1:1 as shown in Fig. 1.
The linked genes do not assort independently but stay together as they are in the parents. In Fig. 2 the genes on the left of the slash ( / ) are on one chromosome and those on the right are on its homologous chromosome. The linked genes gives a test cross ratio of 1:1 as shown in Fig. 2.
Mendel did not notice the phenomenon of linkage because he chose characteristics controlled by genes located on different chromosomes. This phenomenon was noticed by post-Mendelian geneticists. Two English geneticists, Bateson and Punnett, demonstrated the first case of linkage in Sweet pea (Lathyrus odoratus) in the year 1906. But it was understood clearly only after the work of Morgan on the fruit fly (Drosophila melanogaster).