Principles of Genetics and Breeding

7.1.1 Linkage and crossing over

It is now known that the number of genes in an organism far exceeds the number of chromosome, e.g.,about 850 genes are known in the fruit fly, Drosophila melanogaster which has only 4 pairs of chromosomes. Since the genes are present in the chromosomes, and it has been cytologically confirmed, that a single chromosome bears several genes.

• Genes present at the same locus have the tendency to be linked together from one generation to the other and are not disturbed by the meiotic recombination in contrast to genes present on different homologous chromosomes that are inherited independent of the another (Mendel's Law of Independent Assortment).
• Such genes are therefore called linked genes.
• As the linked genes are inherited in a block, the traits they produce are also inherited together.
• Linkage is now defined as the tendency of genes to remain together during the process of inheritance.
• When two loci were linked genetically on the same chromosome they did not segregate independently as predicted by Mendel’s second law.
• The extent of linkage is correlated with the distance between the genes in the chromosomes – closer the genes, stronger the linkage and vice versa.
• The linkage of genes, according to their theory, is linear. An important implication of the feature is that genes are located in the linear fashion in the chromosomes.
• All the genes which are linked with one another constitute group.
• Since several genes are present in the same chromosome, it follows that the number of linkage groups in an animal or plant is equal to the number of pairs of chromosomes present in its cells.
• This hypothesis was given by Sutton and was verified by experiments on D. melanogaster by T.H. Morgan and his associates.
• They discovered four linkage groups in this insect.
• Each linkage group may contain several genes affecting a single trait, e.g., 13 genes influence age, colour and 33 genes influence the wings in Drosophila.