(1) If we perform many identical monohybrid crossings of this type,
one-quarter of the offspring will be homozygous for the dominant gene (AA). One-half
will be heterozygous (Aa), having one dominant and one recessive gene. The
remaining quarter will be homozygous for the recessive gene (aa).
(2) Three-quarters of the offspring (AA or Aa) will have the phenotype trait
produced by the dominant gene (A). One-quarter (aa) will show the phenotype trait
produced by the recessive gene.
(3) As we have seen, the heterozygous organisms (Aa) make up 50% of the
offspring. These are often called carriers. Although their phenotype does not show the
recessive trait, they can still transmit that trait to their offspring.
b. The Dihybrid Crossing (Figure 14-3). Now, consider two genes in one set
of chromosomes and the corresponding pair of genes in the other set. Assume that
each parent is heterozygous for both genes (AaBb), where A and B are dominant and a
and b are recessive. The potential gametes from each parent will then have gene pairs
AB, Ab, aB, or ab.
Figure 14-3. A dihybrid crossing.
(1) If we perform many identical dihybrid crossings of this type, 14 out of 16
(7 out of 8) will have genotypes including both dominant and recessive genes.
One-fourth will be AaBb. AaBB, AABb, Aabb, and aaBb will each account for
one-eighth of the total offspring. AABB, AAbb, aaBB, and aabb will each account for
one-sixteenth of the total offspring. Thus, one-fourth (4 out of 16) are homozygous.
(2) This example helps to illustrate the consequences of large numbers of
gene pairs. Since there are many, many pairs of genes in the 46 chromosomes of
humans, there will be a huge number of different offspring that are possible. Thus,