1. Sex Determination and Sex-Linked Characteristics
Benjamin A. Pierce
GENETICS
A Conceptual Approach
FIFTH EDITION
CHAPTER 4
Sex Determination and Sex-Linked Characteristics
© 2014 W. H. Freeman and Company

2. Sex in the platypus is determined by sex chromosomes
Sex in the platypus is determined by sex chromosomes. Females have 10 X chromosomes, whereas males have 5 X and 5 Y chromosomes.

3. Female platypuses have 10 X chromosomes; males have 5 X’s and 5 y’s

4. Sex Determination and Sex-linked Characteristics
There are several different mechanisms of sex determination
The X and Y chromosomes pair during meiosis, even though they are not homologous (the genes located on each are different)

5. Sex Determination
Sexual reproduction: alternates between haploid and diploid states (Fig. 4.3)
Most organisms have two sexual phenotypes, male and female (Fig. 4.4)

8. 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

9. Chromosomal Sex-Determination Systems: Sex Chromosomes and Non-Sex Chromosomes (Autosomes)
XX-XO system:
XX – female
XO – male
grasshoppers
XX-XY system:
XY – male
mammals

13. Concept Check 1
How does the heterogametic sex differ from the homogametic sex?
The heterogametic sex is male; the homogametic sex is female.
Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome.
Gametes of the heterogametic sex all contain a Y chromosome.
Gametes of the homogametic sex all contain an X chromosome.

14. Concept Check 1
How does the heterogametic sex differ from the homogametic sex?
The heterogametic sex is male; the homogametic sex is female.
Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome.
Gametes of the heterogametic sex all contain a Y chromosome.
Gametes of the homogametic sex all contain an X chromosome.

15. 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

16. Genic Sex-Determining System
No sex chromosomes, only the sex-determining genes
Found in some plants, fungi, protozoans, and fish

17. Environmental Sex Determination
Environmental factors
Limpet’s position in the stack
Temperature in turtles

18. Figure 4.7 In Crepidula fornicata, the common slipper limpet, sex is determined by an environmental factor—the limpet’s position in a stack of limpets.

20. Sex Determination in Drosophila melanogaster
Genic balance system
X : A ratio (X, number of X chromosomes;
A, number of haploid sets of autosomes)

22. Concept Check 2
What is the sexual phenotype of a fruit fly that has XXYYY sex chromosomes and two sets of autosomes?
Male
Female
Intersex
Metamale

23. Concept Check 2
What is the sexual phenotype of a fruit fly that has XXYYY sex chromosomes and two sets of autosomes?
Male
Female
Intersex
Metamale

24. 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

27. 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’s, still produces a male phenotype.
The absence of Y results in a female phenotype.

28. The Male-Determining Gene in Humans
Sex-determining region Y (SRY) gene
Androgen-insensitivity syndrome
Caused by the defective androgen receptor

29. Figure 4.10 The SRY gene is on the Y chromosome and causes the development of male characteristics.

30. X-Linked characteristics
Sex-Linked Characteristics Are Determined by Genes on the Sex Chromosomes
X-Linked characteristics
X-linked white eye in Drosophila
Figure 4.12
X-linked color blindness in humans
Figure 4.14

31. Figure 4. 12 Morgan’s X-linked crosses for white eyes in fruit flies
Figure 4.12 Morgan’s X-linked crosses for white eyes in fruit flies. (a) Original and F1 crosses. (b) Reciprocal crosses.

32. Figure 4. 12 Morgan’s X-linked crosses for white eyes in fruit flies
Figure 4.12 Morgan’s X-linked crosses for white eyes in fruit flies. (a) Original and F1 crosses. (b) Reciprocal crosses.

33. Figure 4. 12 Morgan’s X-linked crosses for white eyes in fruit flies
Figure 4.12 Morgan’s X-linked crosses for white eyes in fruit flies. (a) Original and F1 crosses. (b) Reciprocal crosses.

34. 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? X+
XwX+Y
X+Y
X+X+Y

35. 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? X+
XwX+Y
X+Y
X+X+Y

36. Figure 4.14 Red-green color blindness is inherited as an X-linked recessive trait in humans.

37. Figure 4.14 Red-green color blindness is inherited as an X-linked recessive trait in humans.

38. 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?

39. 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?
1/2

40. Y-Linked Characteristics
Only present in males
All male offspring will exhibit the trait
Y chromosome lost DNA over time
Important for sex determination in SRY

42. X-Linked Characteristics
Dosage compensation: the amount of protein produced by X-linked genes and randomly inactivated in two sexes
Figure 4.17
Lyon hypothesis

43. Figure 4. 17 A Barr body is an inactivated X chromosome
Figure 4.17 A Barr body is an inactivated X chromosome. (a) Female cell with a Barr body (indicated by arrow). (b) Male cell without a Barr body.

44. Random X Inactivation