Biology Reference
In-Depth Information
We next examine the case of multiple genes, considering an example
with two genes in detail. The ideas are easily generalized to three and
more genes (see Exercise 2 below). Suppose we have two different genes
on two different chromosomes, such as the seed shape gene on Pisum
chromosome 7 and the plant height gene on Pisum chromosome 4. Let us
further suppose we have two parents, one homozygous for dominant
alleles of both genes, and the other recessive for both genes, as in
Figure 3-7.
A
a
A
AA
Aa
a
Aa
aa
When reproduction occurs, one allele from each gene from each parent
is randomly selected to contribute to the offspring. Because of the
parents' genetic purity, the mother must contribute dominant alleles [A,
B] and the father recessive alleles [a, b]. Therefore, this first filial
generation (or F
1
) will have the genotype AaBb and the dominant
phenotype for both genes.
FIGURE 3-6.
Punnett square of Mendel's results diagrammed in
Figure 3-5.
Now, suppose we mate two parents from the F
1
generation (AaBb
AaBb). They will each be able to produce four different types of
gametes, because the first parent may deliver either an A or a allele for
the first gene (two possibilities) and either a B or b allele for the second
gene (two possibilities). So there are 2
4 possibilities from Parent 1.
These possible gametes are AB, Ab, aB, and ab. Parent 2 has the same
four possibilities, so there are a total of 4
2
¼
16 possibilities for the
offspring in the F
2
generation. Because in the F
1
there is a 1:1 ratio of
recessive and dominant alleles, and because we assume random mating
from the F
1
, each of these possibilities is equally likely to occur in the F
2
.
4
¼
We want to know what the phenotypes of the F
2
generation are, and in
what proportions they occur. We calculate this in two ways. First, we list
the 16 possibilities using a Punnett square (see Figure 3-8).
Parents
AABB
aabb
Gametes
AB
ab
F1 generation
AaBb
FIGURE 3-7.
Chromosomes of a cross of double homozygotes.
A
or
a
, seed shape;
B
or
b
, plant height.
