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(c) 6T and 6t (the phenotype corresponding to genotypes comprised
of 6 contributing and 6 noncontributing alleles),
(d) 2T and 10t (the phenotype corresponding to genotypes composed
of 2 contributing and 10 noncontributing alleles).
If we examine the binomial distribution as shown in Figure 3-9, we see
that the most common manifestation of the trait occurs when k
¼
m (that is,
when the alleles are half of each type). This is the mathematical
explanation for why most people seem to be near the average of a
population trait.
0.40
0.30
0.35
0.25
0.30
0.20
0.25
0.20
0.15
0.15
0.10
0.10
0.05
0.05
0.00
A
0.00
B
0
1
2
3
4
5
6
7
8
0
1
2
3
4
0.20
0.12
0.18
0.10
0.16
0.14
0.08
0.12
0.10
0.06
0.08
0.04
0.06
0.04
0.02
0.02
0.00
0.00
D
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
0
5
10
15
20
25
30
35
40
45
50
C
FIGURE 3-9.
The distribution of colors in F 2 for a different number of controlling genes m when the alleles R and r are equally likely. The number of different phenotypes is
5, 9, 21, and 51, respectively. Notice that (1) the distributions are symmetric; (2) the shade corresponding to exactly m contributing alleles (the shade of the
F 1 ) is the most common; (3) the parental colors (corresponding to 0 and 2m contributing alleles) are the least common; (4) the closer the number of
contributing alleles is to m, the more prevalent the color is; and (5) as m increases, the histograms involve a larger number of classes that correspond to
increased variability of the characteristic in the F 2 . Panel A: m ¼ 2; panel B: m ¼ 4; panel C: m ¼ 10; panel D: m ¼ 25.
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