Q.Differentiate linkage and crossing over.

Linkage keeps genes on the same chromosome inherited together; crossing over is the physical process that breaks that linkage by exchanging chromosome segments, producing new gene combinations.

Linkage:

1. Refers to the tendency of two or more genes located on the same chromosome to be inherited together, as a single unit, because of their physical proximity on that chromosome.
2. It tends to preserve the original (parental) combinations of alleles, since linked genes do not assort independently.
3. It reduces genetic variation among offspring compared with genes on different chromosomes (which follow independent assortment).
4. It was discovered by T.H. Morgan and colleagues through breeding experiments in Drosophila.
5. The strength of linkage between two genes is inversely related to the distance between them on the chromosome -- genes located close together show stronger (tighter) linkage.

Crossing over:

1. Refers to the actual physical exchange of segments (breakage and reunion) between non-sister chromatids of homologous chromosomes.
2. It occurs during the pachytene sub-stage of prophase I of meiosis, after synapsis (pairing of homologous chromosomes) has formed a tetrad/bivalent.
3. It breaks up existing (parental) linked gene combinations and creates new combinations of alleles -- called recombinants.
4. It increases genetic variation among the gametes/offspring produced. …