Linkage is that phenomenon in which genes tend to remain together in heritance. Chromosomes are regarded as the bearers of hereditary characters. Each of the chromosomes is made up of several genes, each being generally responsible for a single character.
These genes are arranged in a linear fashion in the chromosome. At a certain stage in meiosis we find that homologous chromosomes come in close association in two’s and interchange their parts, and while doing so some of the genes of one chromosome go over to the other chromosome.
But some genes of a particular chromosome, whether paternal or maternal, tend to remain together from generation to generation, i.e. these genes are linked together in inheritance even after crossing over.
Such a phenomenon where genes tend to remain together in inheritance is known as linkage, as first noted by Morgan in 1910. For example, in sweet peas, as first noted by Bateson and Punned in 1906, there are two flower-colours—purple and red, and two types of pollen—long and round. Purple colour and long pollen behave as dominants to red and round respectively, each character being represented by a single gene in inheritance.
In a cross between purple long and red round the F, individuals are all purple long. In the F2 generation it is found that the parental combinations, i.e. purple long and red round, are much more numerous than the expected Mendel a ratio of 9: 3: 3: 1. The unexpected number is explained as due to the fact that the genes for purple long as well as red round remain together, i.e. linked in inheritance, and hence more of these parental combinations are produced.
In tomatoes it has been found that there are two factor pairs Rr and Yy governing the fruit-colour. R factor produces red flesh and is dominant over factor r which produces yellow flesh.
The dominant factor Y produces yellow colour and the recessive factor y represents colorless fruit. It has been found that the factor pair Yy is linked with the size of the fruit. There is a close association of y genes with large fruit.
In maize or Indian corn ten linkage groups have been noted, e.g. coloured and full endosperm, colorless and shrunken endosperm, etc. Generally the number of linkage groups in any plant or animal is the same as the number of chromosome pairs. Linkage is not, however, always very complete, i.e. a few linked genes break apart and recombination’s take place.