This prediction applies to a single locus, and to the average level of homozygosity afterX
generations of selfing. Thus, after just four generations of selfing, the average level of
homozygosity and the probability that a given locus is homozygous are 94%. However,
whenNloci are considered simultaneously, and if all loci assort independently, then the
probability that allNloci will be homozygous is equal to (Phomo)N. Thus, if a large
number of loci are segregating, then, even after many generations of selfing, there
remains a high probability that at least one of those loci remains heterozygous. This predic-
tion has important consequences in the breeding and release of plant varieties that are
regarded as homozygous—it means that there are always a few loci that may segregate
within the variety. Such phenomena sometimes turn up when a variety is grown in a
novel environment where no one has ever tested it before.
Prior to the development of modern breeding methods (and even afterward, for various
reasons), plant varieties in self-pollinating species were propagated as mixtures of homozy-
gous lines calledlandraces. Each landrace typically arose from generations of bulked selec-
tions from a farmer’s field. Gradually, through selection of desirable types (e.g., large ears of
corn) or elimination of undesirable types (e.g., those with seed that fell off during harvest:
shattering), a farmer might develop a particularly useful landrace and share it with friends.
Landraces often took on the name of a farmer, a region of origin, a defining characteristic, or
a combination thereof (e.g., “Swedish giant”). It was probably rare for a landrace to originate
from a single plant, because there was no knowledge that this might be beneficial, and
because this would have required careful multiplication of seed in isolation from other
crops before there was adequate seed to plant a crop for harvest. In a landrace that had
been grown for many generations, most plants would be homozygous, but the landrace
would remain as aheterogeneousmixture of different genotypes.
Figure 3.2.In repeated self-pollination with no selection, the level of heterozygosity is reduced by
one-half with each selfing generation. This is because only half of the progeny from the heterozygous
genotypes will still be heterozygous, while all the progeny from homozygous genotypes will be
homozygous. Thus, the population gradually approaches complete homozygosity, but with a
mixture of homozygous genotypes.
3.2. CENTRAL CONCEPTS IN PLANT BREEDING 53