Biology Reference
In-Depth Information
most common. A helpful analogy is mixing red and white paint. If we
are to choose two drops of red (R) or white (W) paint at random, there
are three different colors arising from the following mixtures: WW, pure
white; WR and RW, pink; and RR, pure red. If we were to choose four
drops of paint, the possible colors are presented in Table 3-2. It seems
apparent that as the number of drops increases, so does the number
of possible colors. If there are m genes that contribute equally to the
expression of a characteristic, then there will be N
¼
2m alleles and
2m
1 different manifestations of the characteristic. According to
Eq. (3-9), the probability that the manifestation corresponds to exactly k
þ
:
N
. Notice the 15:1 ratio between red-
1
2
2m
k
of a particular allele is
and white-kernelled corn in Table 3-2 and that the phenotypic ratios are
1:4:6:4:1, exactly as observed by Nilsson-Ehle. The model, therefore,
describes the experimental results accurately.
E
XERCISE
3-5
for k
4
k
Compute
¼
0, 1, 2, 3, 4. Compare the results with column 3 of
Table 3-2.
E
XERCISE
3-6
Suppose a quantitative genetic trait is determined by six genes (m
6),
each of which has two alleles, T and t. Assuming each allele has an equal
chance of appearing, calculate the proportions of the phenotypes in F
2
corresponding to:
¼
(a) 12T (that is, the genotype comprised only of contributing alleles),
(b) 5T and 7t (the phenotype corresponding to genotypes comprised
of 5 contributing and 7 noncontributing alleles),
Phenotype
(grain color)
Number of
Sequences
Genotypes
Proportion
WWWW
White
1
1/16
RWWW, WRWW, WWRW,
WWWR
Light Pink
4
1/4
RRWW, RWRW, RWWR,
WRRW,WRWR, WWRR
Pink
6
3/8
RRRW, RRWR, RWRR, WRRR
Dark Pink
4
1/4
RRRR
Red
1
1/16
TABLE 3-2.
Genotypes and phenotypes predicted by the polygenic hypothesis.
