Essentials of Anatomy and Physiology

there are not many B alleles to be found in a popula-
tion; not many people have a B allele. If you look at
Table 21–6 later in this chapter, you will certainly be
able to find other dominant traits that are rare.

INHERITANCE: MULTIPLE ALLELES

The best example of this pattern of inheritance is the
human blood type of the ABO group. For each gene
of this blood type, there are three possible alleles: A,
B, or O. A person will have only two of these alleles,
which may be the same or different. O is the recessive
allele; A and B are co-dominant alleles, that is, domi-
nant over O but not over each other.
You already know that in this blood group there are
four possible blood types: O, AB, A, and B. Table 21–5
shows the combinations of alleles for each type.
Notice that for types O and AB there is only one pos-
sible genotype. For types A and B, however, there are
two possible genotypes, because both A and B alleles
are dominant over an O allele.
Let us now use a problem to illustrate the inheri-
tance of blood type. The Punnett square in Fig. 21–9
shows that Mom has type O blood and Dad has type
AB blood. The boxes of the square show the possible
blood types for each child. Each child has a 50%

chance of having type A blood, and a 50% chance of

having type B blood. The genotype, however, will

always be heterozygous. Notice that in this example,

the blood types of the children will not be the same as

those of the parents. (The Rh factor is inherited inde-

pendently of the ABO type, and is of the dominant–

recessive pattern. Rh positive is dominant and Rh

negative is recessive.)

Table 21–6 lists some other human genetic traits,

with the dominant and recessive phenotype for each.

INHERITANCE: SEX-LINKED TRAITS

Sex-linked traits may also be called X-linked traits

because the genes for them are located only on the X

chromosome. The Y does not have corresponding

genes for many of the genes on the X chromosome.

(The Y chromosome is very small and has only about

20 genes. Some of these genes are active only in the

testes and contribute to spermatogenesis. Others are

active in many body cells, but their functions are not

yet known with certainty.)

The genes for sex-linked traits are recessive, but

because there are no corresponding genes on the Y

chromosome to mask them, a man needs only one

gene to express one of these traits in his phenotype. A

Human Development and Genetics 489

Hh

H HH Hh

hHhhh

Hh

H

h

Mom

Has sickle-cell trait

Key: H

h

Gene for normal hemoglobin

Gene for sickle-cell hemoglobin

Dad

Has sickle-cell

trait

Figure 21–8. Inheritance of sickle-cell anemia
(dominant–recessive pattern). See text for
description.
QUESTION:If Dad has sickle-cell trait and Mom is
homozygous for normal hemoglobin, can a child
of theirs have sickle-cell anemia?