One of the most useful aspects of the study of the phenomenon of linkage and crossing over is the help it provides in locating different genes on the chromosome.
This is made possible by observing the products of a cross as to “their phenotypic traits. The inheritance pattern of two gene loci and their corresponding phenotypic trait provides a clue as to the distance between them.
A comparison between the parental combination and recombination among the progeny provides an understanding of the possible distance between the two genes in question.
A higher parental combination with a relatively low recombination suggests that the two genes are very close, while an increased recombination value indicates that the genes are apart. The following are some of the salient features which are to be borne in mind as a preparation for the construction of linkage maps of chromosomes.
1. The number of genes far exceeds the number of chromosomes. Hence many genes are present on any single chromosome.
2 Genes are arranged in a linear fashion on the chromosome and they follow the chromosome during inheritance.
3. Generally speaking, all the genes on a chromosome behave as a single large linkage group and are inherited together. The number of linkage groups corresponds to the number of chromosomes.
4. Linked genes however may be assorted separately due to cross over taking place between their loci. The extent of separation is an index of the distance between two genes in question.
5. The strength of linkage is inversely proportional to the distance between two genes.
6. Each gene in a linkage group has a definite sequence or order as the cross over value between any two genes is always constant.
It was Sturtevant (1913), who thought that the constant value of cross over between any two genes can be exploited successfully to determine their location and relative distance on the chromosome.
One can also determine the order of genes in a large linkage group. The first map of a chromosome was constructed for the X chromosome of Drosophila by Sturtevant and Morghan. They plotted the position of five genes.
Chromosome map may be defined as the graphic representation of genes in a linear fashion, with the distances between them (genes) being decided by cross over values.