Evolution__3rd_Edition

34 PART 1 / Introduction
Dominance complicates the relation
between genotype and phenotype
Mendelism explains the ratios of
genotypes in the offspring of
particular parents
property may or may not be easily visible. Suppose they influence color. The A gene
might encode a black pigment and AA individuals would be black; aa individuals, lacking
the pigment, would be (let us say) white. The coloration is then the phenotype controlled
by the genotype at that locus: an individual’s phenotype is its body and behavior
as we observe them.
If we consider only the AA and aa genotypes and phenotypes in this example, there is
a one-to-one relation between genotype and phenotype. But there does not have to be
a one-to-one relation, as can be illustrated by considering two possibilities for the
phenotype of the Aa genotype. One possibility is that the color of Aa individuals is
intermediate between the two homozygotes a they are gray. In this case, there are three
phenotypes for the three genotypes and there is still a one-to-one relation between
them. The second possibility is that the Aa heterozygotes resemble one of the homozygotes;
they might be black, for instance. The A allele is then called dominant and the a
allele recessive. (An allele is dominant if the phenotype of the heterozygote looks like the
homozygote of that allele; the other allele in the heterozygote is called recessive.) If
there is dominance, there will be only two phenotypes for the three genotypes and there
is no longer a one-to-one relation between them. If all you know is that an organism has
a black phenotype, you do not know its genotype.
At different genetic loci, there can be any degree of dominance. Full dominance, in
which the heterozygote resembles one of the homozygotes, and no dominance, in
which it is intermediate between the homozygotes, are extreme cases. The phenotype of
the heterozygote could be anywhere between the two homozygotes. Instead of being
either black or gray, it could have had any degree of grayness. Dominance is only one
of a number of factors that complicate the relation between genotype and phenotype.
The most important such factor is the environment in which an individuals grows
up (Chapter 9).
2.8 Genes are inherited in characteristic Mendelian ratios
Mendelian ratios express the proportions of different genotypes in the offspring of parents
of particular combinations of genotypes. The easiest case is a cross between an AA
male and an AA female (Figure 2.8a). After meiosis, all the male gametes contain the A
allele and all the female gametes also contain the A allele. They combine to produce AA
offspring. The Mendelian ratio is therefore 100% AA offspring.
Now consider a mating between an AA homozygote and an Aa heterozygote (Figure
2.8b). Again, all the AA individual’s gametes contain a single A gene. When a heterozygote
reproduces, half its gametes contain an A gene, and half an a. The pair will
produce AA : Aa offspring in a 50 : 50 ratio.
Finally, consider a cross between two heterozygotes (Figure 2.8c). Both male and
female produces half a gametes and half A gametes. If we consider the female gametes
(eggs or ovules), half of them are a, and half of them will be fertilized by a sperm, and
half by A sperm; the other half are A, and half of them will be fertilized by a sperm and
half by A sperm. The resulting ratio of offspring is 25% AA : 50% Aa : 25% aa.
The separation of an individual’s two genes at a locus into its offspring is called
segregation. The ratios of offspring types produced by different kinds of matings are
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