Page 81 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.3 Extensions to Mendel for Multifactorial Inheritance 73

Figure 3.28 A Mendelian explanation of continuous variation. The more genes or alleles, the more possible phenotypic classes,
and the greater the similarity to continuous variation. In these examples, several pairs of incompletely dominant alleles have additive effects.
Percentages shown at the bottom denote frequencies of each genotype expressed as fractions of the total population.
A A A A A A
1
1
1
2 2 A B 2 0 A B A B 0 0
A B 2 B 1 2 B 0 B 2 1 0 B 1 B 0
1
1
1 A B 1 A B A 0 A 0 A 0 A 0 2
1
1
1 A B C 0 A B C 0 B C B 0 B C B 0 B 2
1
1
1 1 C 1 0 C 0 1 C 1 0 C 0 A 2 8 7 7 6 6 6 5 5 4
A B C
1
1 B 1
A B
0 6 5 5 4 5 4 4 3 1 7 6 6 5 5 5 4 4 3
C 1 A B
1 2
1 5 4 4 3 4 3 3 2 A 7 6 6 5 5 5 4 4 3
2
A B C
0 B
1 0
A B
0 5 4 4 3 4 3 3 2 A 0 6 5 5 4 4 4 3 3 2
C
1 A B A 0 A 0 A 0 0 B 2
1
1 0 B B 4 3 3 2 3 2 2 1 6 5 5 4 4 4 3 3 2
A B
1 1 0 B C A B
1
1
A B
1 A 0 1 1
4 3 3 2 B 5 4 4 3 4 3 3 2 6 5 5 4 4 4 3 3 2
1
1 0 A B
A B
0 A C 0
0 1
A 1 A 0 A 0 3 2 2 1 B C 4 3 3 2 3 2 2 1 A 0 5 4 4 3 3 3 2 2 1
1
B 1 A 0 0 B 1
A 1 2 1 A 0 3 2 2 1 B 0 4 3 3 2 3 2 2 1 A 0 5 4 4 3 3 3 2 2 1
C B
B 0 0 0
A 0 1 0 2 1 1 0 3 2 2 1 2 1 1 0 4 3 3 2 2 2 1 1 0
(a) 1 gene with (b) 2 genes with 2 (c) 3 genes with 2 alleles yield (d) 2 genes with 3 alleles apiece yield
2 alleles alleles apiece 7 phenotypic classes. 9 phenotypic classes.
yields 3 yield 5
phenotypic phenotypic
classes. classes.
50%
38%
31% 24%
25% 25% 25% 25% 24% 24% 20% 20%
9% 9% 12% 12%
6% 6% 1.5% 1.5% 5% 5%
1% 1%
lie prostrate on the ground); heterozygous A A plants Continuous (or quantitative) traits vary over a range of
0 1
with a height of 1; and homozygous A A plants with a values and can usually be measured: the length of a tobacco
1 1
height of 2 (Fig. 3.28a). This distribution of heights over flower in millimeters, the amount of milk produced by a
three phenotypic classes does not make a continuous cow per day in liters, or the height of a person in meters.
curve. But for two genes, five phenotypic classes will ap- Continuous traits are usually polygenic—controlled by
pear in the F 2 generation (Fig. 3.28b); for three genes, multiple genes—and show the additive effects of a large
seven classes (Fig. 3.28c); and for four genes, nine classes number of alleles, which creates an enormous potential for
(not shown). variation within a population. Differences in the environ-
The distributions produced by three and four genes ments encountered by particular individuals contribute
thus begin to approach continuous variation, and if we add even more variation. We discuss quantitative, multifacto-
a small contribution from environmental variation, a rial traits in detail in Chapter 22.
smoother curve will appear. After all, we would expect
bean plants to grow better in good soil, with ample sun-
light and water. The environmental component effectively A Comprehensive Example: Multiple
converts the stepped bar graph to a continuous curve by Alleles of Several Genes Determine
producing some variation in expressivity within each ge- Dog Coat Color
notypic class. Moreover, additional variation might arise
from more than two alleles at some genes (Fig. 3.28d), Domestic dogs have a wide variety of coat colors and
unequal contribution to the phenotype by the various markings. The E and B genes described previously in Lab-
genes involved (review Fig. 3.21), interactions with modi- rador retrievers (recall Fig. 3.13) are only two of at least
fier genes, and chance. Thus, from what we now know 12 genes that control dog coat color and pattern. The roles
about the relation between genotype and phenotype, it is of seven of these genes—E, B, A, K, D, S, and M—are the
possible to see how only a handful of genes that behave best understood. Table 3.3 lists the proteins these
according to known Mendelian principles can easily gen- genes specify and the nature of various alleles found in
erate continuous variation. domesticated dogs.