genetics a conceptual approach fourth edition genetics a conceptual approach fourth edition benjamin...
TRANSCRIPT
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