Biology Now, 2e

142 ■ CHAPTER 08 Chromosomes and Human Genetics

GENETICS

disorders. Aldrich correctly concluded that the

gene leading to the family’s bleeding disease

was located on the X chromosome, so it was

X-li n ked.

X Marks the Spot

Thanks to advances in molecular biology tools,

in 1994, researchers determined that the gene

causing WAS is on the X chromosome and

called it, unsurprisingly, WA S (gene names are

typically italicized, while disorder names, in

this case “WAS,” are not). WA S, they discov-

ered, is the genetic code for a protein crucial for

the formation and function of blood cells and

immune system cells. Without a healthy copy of

this gene, individuals acquire blood and immune

system disorders, are susceptible to infections,

and have increased risk of lymphoma, cancer of

the lymph nodes.

Felix inherited a mutated version of WA S

from his mother and no allele from his father.

We can use a Punnett square to illustrate how

the X-linked recessive mutation for WAS is

inherited. We label the recessive mutated WA S

allele a, and in the Punnett square we write

this allele as Xa to emphasize the fact that it is

on the X chromosome. Then we label the domi-

nant, healthy allele A and write this allele as XA

in the Punnett square (Figure 8.9). Individuals

like Felix’s mother, who have only one copy of a

recessive allele, are said to be genetic carriers

of the disorder. Carriers can pass on the disorder

allele, but they do not have the disease.

If a carrier female like Felix’s mother, with geno-

type XAXa, has children with a normal male (with

genotype XAY), each of their sons will have a 50

percent chance of getting the disorder. Felix had a

50 percent chance of getting WAS, and he did.

Males of genotype XaY suffer from the condi-

tion because the Y chromosome does not have a

Figure 8.9

X-linked recessive conditions are more

common in males

The recessive disorder allele (a) is located on the

X chromosome and is denoted by Xa. The dominant

normal allele (A) on the X chromosome is denoted

by XA. M

Q1: Which of the children specified in this

Punnett square represents Felix? What is

his genotype?

Q2: Explain why Felix is neither homozygous

nor heterozygous for the WAS gene.

Q3: Create a Punnett square to illustrate

the offspring that could result if Felix had

children with a noncarrier woman. What is

the probability that a son would have WAS?

What is the probability that a daughter

would be a carrier of WAS?

XAXa XAY

Eggs

Sperm

Affected

son

Noncarrier

daughter

Carrier

daughter

XA

XA

XAXA XAY

XAXa XaY

Xa

Xa XA

XA

Y

Y

×

Noncarrier

son

Carrier mother Noncarrier father

Robert Aldrich was an American pediatrician

who lived from 1917 to 1998. By creating a

family pedigree, Aldrich demonstrated that the

mysterious bleeding syndrome afflicting infant

boys was a sex-linked, recessive disorder.

ROBERT ALDRICH