Evolution__3rd_Edition

230 PART 2 / Evolutionary Genetics
The additive part of the genetic
effect is most important ...
. . . and is used to predict the
character value in the next
generation
G = A + D
The additive effect is the important one. The parent deviates from the population
mean by a certain amount; its additive genotypic effect is the part of that deviation that
can be passed on. However, when an individual reproduces, only half its genes are
inherited by its offspring. The offspring inherit only half the additive effect of each parent.
Thus the additive effect A for an individual is equal to twice the amount by which
its offspring deviate from the population mean, if mating is random. For the AA parent,
therefore, the additive effect is +0.25. (The full quantitative genetics of the AA individuals
is G =+0.125, A =+0.25, and D =−0.125.) The offspring of Aa birds deviate by
zero: their additive effect is twice zero, which is zero; the amount by which Aa heterozygotes
deviate from the population mean is entirely due to dominance, and is not
inherited by their offspring. (For Aa individuals, G =+0.125, A = 0, and D =+0.125.)
The division of the genotypic effect into additive and dominance components tells
us what proportion of the parent’s deviation from the mean is inherited, and reveals
how the non-inheritance of the Aa individuals’ genotypic effect is due to dominance. In
practice quantitative geneticists do not know the genotypes underlying the characters
they study; they only know the phenotype. They might, for instance, focus on the class
of birds with 1 cm (P =+0.125) beaks. The additive component of their phenotypic
value depends on the frequencies of the AA and Aa genotypes in this example: if all the
birds with 1 cm beaks are Aa heterozygotes, then none of the offspring will inherit their
parents deviation; if they are all AA, then half the offspring will.
Why is the additive effect of a phenotype so important? The answer is that once the
additive effect for a character has been estimated, that estimate has much the same
role in quantitative genetics as the exact knowledge of Mendelian genetics in a one- or
two-locus case (Chapters 5 and 8). It is what we use to predict the frequency distribution
of a character in the offspring, given a knowledge of the parents. In a one-locus
genetic model, we know the genotypes corresponding to each phenotype, and can predict
the phenotypes of offspring from the genotypes of their parents. In the case of
selection, the gene frequency in the next generation is easy to predict if we know
selection allows only AA individuals to breed. In two-locus genetics, the procedure
is the same. If the next generation is formed from a certain mixture of Ab/AB and
AB/AB individuals, we can calculate its haplotype frequencies if we know the exact
mixture of parental genotypes.
In quantitative genetics, we do not know the genotypes. All we have are measurements
of phenotypes, like beak size. But if we can estimate the additive genetic component
of the phenotype, then we can predict the offspring in a manner analogous to
the procedure when the real genetics are known. When we know the genetics, Mendel’s
laws of inheritance tell us how the parental genes are passed on to the offspring. When
we do not know the genetics, the additive effect tells us what component of the parental
phenotype is passed on. Estimating the additive effect is thus the key to understanding
the evolution of quantitative characters. The estimates are practically made by breeding
experiments. In the case of finches with 1 cm beaks in a population of average beak size
0.875 cm, the additive effect can be measured by mating 1 cm-beaked finches to random
members of the population. The additive effect is then two times the offspring’s
deviation from the population mean.
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