Question

Hybrid vigour is due to
a. Chiasma
b. Linkage
c. Crossing over
d. Heterozygosity

Answer 1

Hybrid vigour or Heterosis, is the increase in characteristics of a hybrid organism such as size, growth rate, fertility, and yield over the parent generation.

Complete answer :
The increase in the traits of hybrid strength due to crossing over is called hybrid vigour or heterosis. The term hybrid vigour can be observed as phenotypes more than pure breeding. The word heterosis is derived from heterozygosity. It shows the origin of the effect. Crossing follows the inverse pattern in comparison to inbreeding.
In heterozygotes, a positive gene provides a dominant deviation. In complete dominance, the heterozygote has the same value as the dominant homozygote. Hybrid vigour is directly proportional to the strength of heterozygosity.
So, the correct answer is (D) Heterozygosity

Note: Hybrid vigour, heterosis, or outbreeding enhancement is the improved function of a biological quality in a hybrid offspring. Heterosis describes the phenomenon in which hybrids are more robust or vigorous than their parents. So, the terms heterosis and hybrid vigour are often interchangeable.
The hypotheses for the heterosis mechanisms are:

Dominance
overdominance
The dominance hypothesis, says harmful recessive alleles accumulate i.e. inbreeding depression takes place, resulting in the reduction in vigour. When two pure line individuals are mated, the offspring is heterozygous for that loci. The detrimental alleles are covered by dominant alleles, remove inbreeding depression and restores vigour.
In the overdominance hypothesis, the heterozygous loci are more robust than those of either parent homozygous locus. So, increased homozygosity reduces vigour, and leads to enhanced robustness.
These two hypotheses make similar predictions at the level of genetics but are distinctive due to combined molecular and genetic analysis. Both mechanisms are important in heterosis for different organisms under different conditions.