Evolution And History

Heredity 39

or phenotype, as the homozygous (AA), even though the

two have a somewhat different genetic composition, or

genotype. Only the homozygous recessive genotype (OO)

will show the phenotype of type O blood.

The dominance of one allele does not mean that the

recessive one is lost or in some way blended. A type A

heterozygous parent (AO) will produce sex cells contain-

ing both A and O alleles. (This is an example of Mendel’s

law of segregation—that alleles retain their separate iden-

tities.) Recessive alleles can be handed down for genera-

tions before they are matched with another recessive in

the process of sexual reproduction and show up in the

phenotype. The presence of the dominant allele simply

masks the expression of the recessive allele.

All of the traits Mendel studied in garden peas showed

this dominant–recessive relationship, and so for some years

it was believed that this was the only relationship possible.

Later studies, however, have indicated that patterns of in-

heritance are not always so simple. In some cases, neither

allele is dominant; they are both co-dominant. An example

of co-dominance in human heredity can be seen in the in-

heritance of blood types. Type A is produced by one allele;

type B by another. A heterozygous individual will have a

phenotype of AB, because neither allele can dominate the

other.

The inheritance of blood types points out another

complexity of heredity. Although each of us has at most

two alleles for any given gene, the number of possible al-

leles is by no means limited to two. Certain traits have

three or more allelic forms. For example, over a hundred

alleles exist for hemoglobin, the blood protein that car-

ries oxygen. Only one allele can appear on each of the two

homologous chromosomes, so each individual is limited

to two genetic alleles.

Polygenetic Inheritance

So far, we have spoken as if all the traits of organ-

isms are determined by just one gene. However, most

physical traits—such as height, skin color, or liability

to disease—are controlled by multiple genes. In such

cases, we speak of polygenetic inheritance, where the

What happens when a child inherits the allele for
type O blood from one parent and that for type A from
the other? Will the child have blood of type A, O, or some
mixture of the two? Figure 2.6 illustrates some of the pos-
sible outcomes. Many of these questions were answered by
Mendel’s original experiments.
Mendel discovered that certain alleles are able to mask
the presence of others; one allele is dominant, whereas the
other is recessive. Actually, it is the traits that are domi-
nant or recessive, rather than the alleles themselves; genet-
icists merely speak of dominant and recessive alleles for
the sake of convenience. Among your biological relatives
you can trace classic examples of visible traits governed by
simple dominance such as a widow’s peak (dominant), at-
tached earlobes (recessive), or the presence of hair on the
back of the middle section of each finger (dominant). A
person with a widow’s peak may be either homozygous or
heterozygous because the presence of one allele will mask
the allele for an un-peaked hairline. Similarly, one might
speak of the allele for type A blood as being dominant to
the one for type O. An individual whose blood-type genes
are heterozygous, with one A and one O allele, will have
type A blood. In other words, the heterozygous condition
(AO) will show exactly the same physical characteristic,

phenotype The observable characteristic of an organism that

may or may not reflect a particular genotype due to the variable

expression of dominant and recessive alleles.

dominance The ability of one allele for a trait to mask the

presence of another allele.

recessive An allele for a trait whose expression is masked by

the presence of a dominant allele.

hemoglobin The protein that carries oxygen in red blood cells.

polygenetic inheritance When two or more genes contrib-

ute to the phenotypic expression of a single characteristic.

Figure 2.6 These four Punnett squares (named for British
geneticist Reginald Punnett) illustrate some of the possible
phenotypes and genotypes of offspring within the A-B-O
system. Each individual possesses two alleles within this
system, and together these two alleles constitute the
individual’s genotype. “Phenotype” refers to the physical
characteristics expressed by the individual. The alleles of one
parent are listed on the left-hand side of the square while the
other parent’s alleles are listed across the top. The potential
genotypes of offspring are listed in the colored squares by
letter. Phenotypes are indicated by color: Blue indicates
the type A phenotype; orange indicates the B phenotype.
Individuals with one A and one B allele have the AB phenotype
and make both blood antigens. Individuals with the O
phenotype have two O alleles.

OO

AO

A AOAO

A

OO

AO AO

B BOBO

A

AB

AA AB

B BBBA

A

OO

AO AO

O OOOO

AO A