genetics much simpler than would be the case if he’d picked plants that were normally
(or even partially)outcrossers. He used plant lines that would only generate plants of
a single type when the plants were allowed to self-fertilize. These plants werehomozygous
for that trait, which meant that the two homologous chromosomes had the same
allele. When homozygous plants are selfed, the resulting progeny are always homozygous.
Mendel’s method of tracking segregation was based on crossing plants that were
homozygous and differed for the phenotypic trait of interest. For example, he would
cross (instead of selfing) plants that were homozygous yellow and homozygous
green for seed color, and then record the phenotypic ratio in progeny of each
subsequent generation.
By crossing different homozygotes, Mendel generated plants whose two homologous
chromosomes each had a different allele of the gene (Fig. 2.5a). The condition of having
two different alleles in a single gene is calledheterozygous. All the plants generated
from the initial cross (F 1 hybridsor F 1 generation) would have the same genotype, but
could have either one of two different parental phenotypes. In the heterozygous plants,
Mendel discovered that certain variants of a trait appeared to mask or cover the expression
of other variants. A variant that would cover the other type was termeddominant, while the
phenotype that would disappear was calledrecessive. When we write allele names, we often
use uppercase letters for dominant alleles and lowercase letters for recessive alleles. Today
we understand that dominant alleles have a sequence of DNA that encodes for a functional
protein, while many recessive alleles have changes in the DNA sequence, calledmutations,
which render the encoded protein nonfunctional. Therefore, in a heterozygous plant, func-
tional and nonfunctional proteins are produced, and the plant has the phenotype of the
dominant allele from the functional protein. In Mendel’s experiments, he would see that
the dominant trait would mask the expression of the recessive trait.
Figure 2.5.A monohybrid crossing system involving a single-gene model where the two alleles seg-
regate from one another in the production of gametes: (a) monohybrid cross; (b) F1 self-fertilization.
2.2. MENDELIAN GENETICS 27