HUMAN BIOLOGY

iNtroDUCtioN to GeNetiCs 379

wHy do many genetic traits not Have clear

dominant and recessive forms?

• Pleiotropy is common in humans. In pleiotropy, one gene affects
  more than one trait. The effects may not be simultaneous but
  may have repercussions over time as one altered trait changes
  another trait and so on, as in sickle-cell anemia.


• Some genes have codominant alleles—that is, both are
  expressed.


• Some genes have more than two alleles. These multiple allele
  systems include the alleles for the ABO blood group.

taKe-Home messaGe

meaning “more,” and -tropic, meaning “to change”). Much
of what researchers have learned about how such genes
function has come from studies of genetic diseases. An
example is sickle-cell anemia, which was introduced in
the discussion of malaria in Chapter 8. This disabling and
painful disease is most common in people of African and
Mediterranean descent. It arises when a person is homo-
zygous for a recessive allele. The normal allele, HbA, has
instructions for building normal hemoglobin, the oxygen-
transporting protein in red blood cells. When a person
inherits two copies of the recessive mutant allele, HbS, he or
she develops sickle-cell anemia (Figure 19.8).
Red blood cells, which normally are biconcave disks,
take on a sickle shape when the oxygen content of blood
falls below a certain level (Section 8.9 and Figure 19.8A).
The sickled cells clump in blood capillaries and can
rupture. The flow of blood can be so disrupted that the
person’s oxygen-deprived tissues are severely damaged

Genotypes:

Phenotypes
(Blood type):

AA

AO

or

A

BB

BO

or

B

AB

AB

OO

O

A

(Figure 19.8B, right). Homozygotes for the mutated hemo-

globin gene (HbS/HbS) often die relatively young. Hetero-

zygotes (HbA/HbS), on the other hand, have sickle-cell trait.

They generally have few symptoms

because the one HbA allele provides

enough normal hemoglobin to pre-

vent red blood cells from sickling.

During a crisis, sickle-cell anemia

patients may receive blood trans-

fusions, oxygen, antibiotics, and

painkilling drugs. There is evidence

that the food additive butyrate can

reactivate “dormant” genes respon-

sible for fetal hemoglobin, an effi-

cient oxygen carrier that normally is produced only before

birth. For this reason, some states require hospitals to

screen newborn infants for sickle-cell anemia so that

appropriate action can begin right away.

in codominance, more than one allele

of a gene is expressed

As you now know, people who are heterozygous for a

trait have two contrasting alleles for that trait. Usually,

one is dominant and one is recessive. In some cases, how-

ever, both alleles are expressed. We see an example of this

codominance in people who are hetero zygotes for alleles

that confer A and B blood types (Figure 19.9). Remember

from Section 8.4 that the alleles you carry for the ABO gene

determines your blood type. The gene provides instruc-

tions for making an enzyme that helps build a polysaccha-

ride (a sugar) on the surface of red blood cells. Each ABO

allele provides slightly different instructions for building

the sugar. The sugar in turn gives each person’s red blood

cells their particular chemical identity—which we call

blood type.

Two ABO alleles, IA and IB, are codomi nant when

paired with each other. Someone who inherits them has

type AB blood. A third allele, O, is recessive. When paired

with either IA or IB, the O’s effect is masked. A person who

has it plus an IA allele has type A blood, while someone

who has it paired with the IB allele has type B blood. Some-

one who inherits two Os has type O blood. A gene that has

three or more alleles is called a multiple allele system.

There are many such human genes.

codominance The expres-

sion of both contrasting

alleles of a gene in

heterozygotes.

multiple allele system

Gene that has three or

more alleles.

pleiotropy The expression

of a gene that affects more

than one trait.

Figure 19.9 There are several possible allele combinations for
ABO blood types. A An overview of the genotypes that produce
the phenotypes of the human ABO blood group—an example of
codominance. B Figuring out ABO phenotypes using a Punnett
square. (© Cengage Learning)

IA

IA

IB

i

IB i

or

or

or or

IAIA

Possible alleles

in gametes from

mother:

Possible alleles in

gametes from father:

IAIB IAi

IAIB IBIB IBi

IAiIBiii

AAAB

AOB

AB B B

B

David Scharf/Peter Arnold, Inc.

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).