Essentials of Anatomy and Physiology

11. The gene for the beta chain of hemoglobin is on
    chromosome 11; an allele for normal hemoglobin is
    dominant, and an allele for sickle-cell hemoglobin is
    recessive. An individual who is heterozygous is said to
    have sickle-cell trait; an individual who is homozygous
    recessive will have sickle-cell anemia. The Punnett
    square in Fig. 21–8 shows that if both parents are het-
    erozygous, each child has a 25% chance (1 of 4) of
    inheriting the two recessive genes. Table 21–4 lists
    some other human genetic diseases and their patterns
    of inheritance.

It is important to remember that in the terminol-

ogy of genetics, dominant does not mean “common”

or “frequent.” It simply means that one gene is suffi-

cient for the characteristic to appear in the phenotype.

Think back to the ABO blood types. Which type is the

most common in a population? Type O is most com-

mon, followed by type A, then type B. Type AB is the

least common. Here is an instance in which the reces-

sive trait, type O, is most frequent in a group of peo-

ple. Why is type AB the least common, even though A

and B alleles are dominant over an O allele? Because

488 Human Development and Genetics

Gene for brown

eye color

Gene for blue

eye color

Ova

Sperm

BB Genotype: homozygous brown (2 dominant genes)

Phenotype: brown

Bb Genotype: heterozygous (1 dominant and 1 recessive gene)

Phenotype: brown (since brown is dominant)

bb Genotype: homozygous blue (2 recessive genes)

Phenotype: blue

Key

Figure 21–7. Inheritance of eye color. Both mother and father are heterozygous for

brown eyes. The Punnett square shows the possible combinations of genes for eye

color for each child of these parents. See text for further description.

QUESTION:If Dad is heterozygous for brown eyes and Mom has blue eyes, what will

be the four possible combinations?