Linkage groups and Detection of linkage

 Linkage groups

A linkage group refers to a group of genes that are present in one chromosome. In other words, all those genes which are located in one chromosome constitute one linkage group. The number of linkage groups is limited in each individual. The maximum number of linkage groups is equal to the haploid (n) chromosome number of an organism. However, in the case of species having dissimilar sex chromosomes, linkage groups are one more than the haploid chromosome number (n+1). For example, there are ten linkage groups in corn, 7 in garden pea, 7 in barley, 4 in Drosophila melanogaster and 23 (22 somatic chromosomes + 1 X chromosome) in women, and 24 in man (22 somatic chromosomes + 1 X chromosome + 1 Y chromosome). Genes are assigned to various chromosomes of a species with the help of deletion, monosomic, and nullisomic analyses. Linkage groups are assigned to different chromosomes in a linear fashion and the same sequence as they normally list. Generally, a relative length of various linkage groups in a species exhibits a close agreement with the relative length of chromosomes in which they exist.

Detection of linkage

A test cross is the most common method of detecting the linkage. In this method, the F1 heterozygous at two loci (say AaBb) is crossed to a double recessive parent (aabb) and the phenotypic ratio of testcross progeny is examined. If the phenotypic ratio of test crosses progeny shows a 1:1:1:1 ratio of parental and recombinant genotypes, it indicates the absence of linkage. If the frequency of parental types and recombinant types deviate significantly from the normal test cross-ratio of 1:1:1:1, it reveals the presence of linkage between two genes under study.

There is another way to detect the presence or absence of linkage. The individual heterozygous at two loci (AaBb) is self-pollinated. If there is complete dominance at each locus and no epistasis, the segregation ratio of the progeny will be 9:3:3:1. The presence of linkage either in the coupling or repulsion phase will lead to a significant deviation from the 9:3:3:1 ratio. The deviation of observed values from the expected ratio is tested with the help of χ2 test.