Biology Now, 2e

A Deadly Inheritance ■ 141

The SRY gene does not act alone; in both

males and females, other genes on the auto-

somes and sex chromosomes directly influence

the development of the sexual characteristics

that distinguish men and women. Still, the SRY

gene plays a crucial role because when present,

it causes other genes to produce male sexual

characteristics, but when absent, those genes

produce female sexual characteristics. Individ-

uals that have XY chromosomal makeup but a

nonfunctional SRY gene are considered inter-

sex; they are not reproductively functional males

or females, although they appear phenotypically

female (Figure 8.8).

Interestingly, there are no well-documented

cases of disease-causing Y-linked genes. X chro-

mosomes, however, contain genes known to

be involved in many human genetic traits and

located on the X chromosome, while only about

60 are located on the much smaller Y chromo-

some. These 1,240 genes are said to be sex-linked.

Sex-linked genes on the X chromosome are

X-l i n ke d. Sex-linked genes on the Y chromosome

are Y- l i n k e d. One of these Y-linked genes is the

SRY gene (short for “sex-determining region of

Y”). SRY functions as the “master sex switch,”

committing the sex of the developing embryo to

male. In the absence of this gene, a human embryo

develops as a female.

Figure 8.8

Advocates for intersex youth: interACT

Founded by attorney Anne Tamar-Mattis in 2006 under the name “Advocates

for Informed Choice,” interACT was created with a focused mission of ending

harmful medical interventions on intersex children. Its mission statement reads

“interACT uses innovative legal and other strategies, to advocate for the human

rights of children born with intersex traits.”

Paternal

homologue

AA =

r

P

A

b

c

r

P

A

b

C

Homozygous

dominant

Three gene pairs at

three different loci

bb =Homozygous

recessive

Cc = Heterozygous

Maternal

homologue

In a pair of homologous chromosomes,

one is inherited from the male parent,

and the other from the female parent.

A genetic locus is

the location of a

particular gene on

a chromosome.

At each genetic

locus, an

individual has two

alleles, one on

each homologous

chromosome.

Figure 8.7

Genetic loci on homologous

chromosomes

The genes shown here take up a larger portion of

the chromosome than they would if they were drawn

to scale. The average human chromosome has more

than a thousand different genes interspersed with

large stretches of noncoding DNA.

Q1: How do we know whether two

chromosomes are homologous?

Q2: In one sentence, explain how the terms

“gene,” “locus,” and “chromosome” are related.

Q3: If hair color were determined by a

single gene, what would be an example of

the gene’s alleles?