Biology Now, 2e

146 ■ CHAPTER 08 Chromosomes and Human Genetics

GENETICS

parents, are genetic carriers of the disorder;

they carry the disorder allele (a) but do not

have the disease.

If two carriers of a recessive genetic disorder

have children, the patterns of inheritance are

the same as for any other recessive trait. Each

child, male or female, has a 25 percent chance of

not carrying the disorder allele (genotype AA),

a 50 percent chance of being a carrier (geno-

type Aa), and a 25 percent chance of inheriting

the disorder (genotype aa). Zoe did not beat

the odds.

These percentages reveal one way in which

lethal recessive disorders such as cystic fibrosis

can persist in the human population. Although

homozygous recessive individuals (with geno-

type aa) often die before they are old enough

to have children, carriers (with genotype Aa)

are not harmed by the disorder. In a sense, the

a alleles can hide in heterozygous carriers, and

those carriers are likely to pass the disorder

allele to half of their children. An estimated one

in 29 European Americans has a mutated CFTR

gene. Recessive genetic disorders can also arise

in the human population because new mutations

can produce new copies of the recessive alleles.

Deadly with One Allele

Cystic fibrosis is an example of a recessive genetic

disorder, in which a child, like Zoe, inherits two

recessive copies of a disorder allele. A more rare

type of inherited disease is a dominant genetic

disorder, caused by an autosomal dominant allele

(A). In this case, the allele that causes a disorder

cannot “hide” in the same way that a recessive allele

can: AA and Aa individuals get the disorder; only

aa individuals are symptom-free (Figure 8.13).

Dominant genetic disorders are more rare than

recessive disorders because a dominant disorder

often produces serious negative effects immedi-

ately upon birth, and individuals with the A allele

may not live long enough to reproduce. Hence, few

people with a dominant genetic disorder pass the

allele on to their children.

For this reason, most cases of a dominant

genetic disorder are produced by a new muta-

tion in a generation. For example, achondropla-

sia, a form of dwarfism, is caused by a mutation

Aa Aa

Eggs

Sperm

Affected

child

Carrier

child

A

A

AA Aa

Aa aa

a

a A

A

a

a

×

Noncarrier

child

Carrier

child

Parents (carriers)

Gamete

production

Figure 8.12

Inheritance of cystic fibrosis, an

autosomal recessive disorder

The patterns of inheritance for a human autosomal

recessive genetic disorder are the same as for

any recessive trait (compare this figure with

the pattern shown by Mendel’s pea plants in

Figure 7.5). Recessive disorder alleles are denoted

a. Dominant, normal alleles are denoted A. Here,

the parents are a carrier female (genotype Aa)

and a carrier male (genotype Aa). M

Q1: Which of the children in this Punnett

square represents Zoe? What is her genotype?

Q2: If Zoe’s parents had another child, what is

the probability that the child would have cystic

fibrosis? That the child would be a CF carrier?

Q3: If Zoe is able to have a child of her own

someday, and the other parent is not a carrier

of cystic fibrosis (he would likely be tested

before they chose to have children), what is

the probability that the child would have cystic

fibrosis? That the child would be a carrier?