Glossary Plant Breeding

awns, while together produce long awns and thus giving an F 2 ratio of 9A-B-(long
awns): 6aaB-/A-bb (medium awns): 1aabb (awnless).
Additive Gene Action. The way the linear quantitative effect (plus or minus) associated
with the substitution of one allele for the other at a given locus is produced on the
phenotype. This change occurs on the phenotypic scale of measurement regardless of
presence of genes at other loci. For example,
1. 1AABB (7), 2AaBB (5), 1aaBB (3) → replacement of a by A leads to a change of 2 units,
2. 1AABB (7), 2AABb (6), 1aabb (5) → replacement of b by B leads to a change of 1 unit.
The replacement effect is the same whether it occurs in a homozygote or heterozygote
and is also constant regardless of the phase at other locus. It is interesting to note that
the mean values in these examples are 5 and 6, respectively. So replacement of a for
A is by two units and of b for B by one unit; this is called the average effect of an
allele. Thus it can also be defined as the type of gene action in which the change is
associated with the average effect of substituting one allele for the other at a given
locus.
Additive Genetic Variance. That portion of genetic variance which describes difference
between homozygotes at any given locus. In the examples quoted under additive gene
action, the numerical values of additive variance amounted to 2.0 and 0.5 unit 2 ,
respectively which were produced as a consequence of additive effects of 2.0 and 1.0
units, respectively. Thus, one can also define it as that portion of genetic variance
which is produced by additive gene action. As additive gene action itself is dependent
upon the average effect of substitution of one allele for the other, thus it can be
appropriately defined as that portion of the genetic variance which is associated with
the average effect of substituting one allele for another at one, two, or more loci.
Adenine. One of the two purine bases that normally pairs with thymine in the DNA
double helix.
Adenosine. The nucleoside containing adenine as its base.
Adjacent-1 Segregation. A kind of segregation in a translocation heterozygote in which
each of the structurally normal chromosomes moves to the opposite poles along with
8