Glossary Plant Breeding

then it takes one generation to equalize the frequency between the sexes, and the
second generation to reach the equilibrium. For the similar reason, the equilibrium is
not attained even after two generations for the sex-linked genes if males and females
start with unequal gene frequencies. One consequence of this equilibrium is that rare
alleles are virtually never in homozygous condition. Most copies of such rare alleles
are found in heterozygotes. The relative frequency in heterozygotes as opposed to
homozygotes is p/q, which for q = 0.001 is a ratio of 999:1. For two genes that are
independent in inheritance, if we start with a population of individuals of which all
are AaBb so that p = q = r = s (that is, all gametes AB, Ab, aB, ab are equally
present), equilibrium is reached in one generation of random mating as occurs for one
gene.
Hardy-Weinberg Law. A law dealing with constancy of gene frequency across
generations in a panmictic population in the absence of disturbing forces. In an
infinitely large random mating population, the frequencies of adaptively neutral
alleles of a gene and the corresponding genotypic frequencies (at equilibrium) remain
constant from one generation to the other unless there are disturbances due to
selection, non-random mating, differential migration and differential mutation.
Harlequin Chromosomes. Sister chromatids that stain differently, so one appears dark
and the other light.
Harvest Index. The proportion of dry matter that is accumulated in economically usable
parts. If the crop is grown primarily for grain purpose, then, harvest index (HI) is the
proportion of the dry matter that is accumulated in grain. As HI is a ratio of economic
yield and biological yield, a genotype having low grain yield could have high HI and
vice-versa. Thus selection entirely on the basis of HI is not reliable.
Heaving. Lifting effect of the soil due to alternate freezing and thawing. It may result in
the lifting up of plants and may tear them loose from the soil, or sheer off roots.
Helicase. An enzyme that breaks hydrogen bonds in DNA and unwinds it during
movement of the replication fork after initiation of process of DNA replication.
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