GENETICS

A Conceptual ApproachFOURTH EDITION

GENETICS

A Conceptual ApproachFOURTH EDITION

Benjamin A. Pierce

© 2012 W. H. Freeman and Company

CHAPTER 4Sex Determination and Sex-Linked

Characteristics

Chapter 4 Outline

• 4.1 Sex Is Determined by a Number of Different Mechanisms, 74

• 4.2 Sex-Linked Characteristics Are Determined by Genes on the Sex Chromosomes, 81

Sex Determination Mechanisms

• Hermaphroditism: both sexes in the same organism

•Monoecious: both male and female reproductive structures in the same organism

•Dioecious: either male or female reproductive structures in one organism

Chromosomal Sex-Determination Systems:Sex chromosomes and non-sex chromosomes

(autosomes)

• XX-XO system: • XX – female• XO – male• grasshoppers

• XX-XY system:• XX – female• XY – male•mammals

Concept Check 1

How does the heterogametic sex differ from the homogametic sex?

a. The heterogametic sex is male; the homogametic sex is female.

b. Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome.

c. Gametes of the heterogametic sex all contain a Y chromosome.

d. Gametes of the homogametic sex all contain an X chromosome.

Concept Check 1

How does the heterogametic sex differ from the homogametic sex?

a. The heterogametic sex is male; the homogametic sex is female.

b. Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome.

c. Gametes of the heterogametic sex all contain a Y chromosome.

d. Gametes of the homogametic sex all contain an X chromosome.

Chromosomal Sex-Determination Systems

• ZZ-ZW system:• ZZ – male• ZW – female• Birds, snakes, butterflies, some amphibians,

and fishes• Haplodiploidy system:

•Haploid set – male•Diploid set – female• Bees, wasps, and ants

Genic Sex-Determining System

• No sex chromosomes, only the sex-determining genes

Environmental Sex Determination

• Environmental factors

• Limpet’s position in the stack

• Temperature in turtles

Sex Determination in Drosophila melanogaster

• genic balance system

• X : A ratio (X, number of X chromosomes;

A, number of haploid sets of autosomes)

Concept Check 2

What is the sexual phenotype of a fruit fly that has XXYYY sex chromosomes and two sets of autosomes?

a. Male

b. Female

c. Intersex

d. Metamale

Concept Check 2

What is the sexual phenotype of a fruit fly that has XXYYY sex chromosomes and two sets of autosomes?

a. Male

b. Female

c. Intersex

d. Metamale

Sex Determination in Humans XX-XY

• SRY gene on the Y chromosome determines maleness

• Turner syndrome: XO; 1/3000 female births

• Klinefelter syndrome: XXY, or XXXY, or XXXXY, or XXYY; 1/1000 male births

• Poly-X females: 1/1000 female births

The Role of Sex Chromosomes

• The X chromosome contains genetic information essential for both sexes; at least one copy of an X is required.

• The male-determining gene is located on the Y chromosome. A single Y, even in the presence of several X, still produces a male phenotype.

• The absence of Y results in a female phenotype.

The male-determining gene in humans

• Sex-determining region Y (SRY) gene

• Androgen-insensitivity syndrome

•Caused by the defective androgen receptor

4.2 Sex-Linked Characteristics Are Determined by Genes on the Sex Chromosomes

• X –Linked characteristics– X-linked white eye in Drosophila

• Fig. 4.12

– X-linked color blindness in humans• Fig. 4.14

Concept Check 3

What was the genotype of the few live F1 red-eyed males obtained by Bridges when he crossed a white-eyed female with a red-eyed male?a. X+

b. XwX+Y

c. X+Y

d. X+X+Y

Concept Check 3

What was the genotype of the few live F1 red-eyed males obtained by Bridges when he crossed a white-eyed female with a red-eyed male?a. X+

b. XwX+Y

c. X+Y

d. X+X+Y

Concept Check 4

Hemophilia (reduced blood clotting) is an X-linked recessive disease in humans. A woman with hemophilia mates with a man who exhibits normal blood clotting. What is the probability that their child will have hemophilia?

Concept Check 4

1/2

Hemophilia (reduced blood clotting) is an X-linked recessive disease in humans. A woman with hemophilia mates with a man who exhibits normal blood clotting. What is the probability that their child will have hemophilia?

X-Linked Characteristics

• Dosage compensation: the amount of protein produced by X-linked genes and randomly inactivated in two sexes

• Fig. 4.17

4.2 Sex-Linked Characteristics Are Determined by Genes on the Sex Chromosomes

• Z-linked characteristics

• Y-linked characteristics

•Characteristics of the Y and Y-linked genetic markers