1. Sex Determination and Sex Chromosomes
Chapter 7 Lecture
Concepts of Genetics
Tenth Edition
Sex Determination and Sex Chromosomes

2. Caster Semenya

3. 7.1 Life Cycles Depend on Sexual Differentiation
7.1.1 Chlamydomonas

4. Figure 7-1 The life cycle of Chlamydomonas
Figure 7-1 The life cycle of Chlamydomonas. Unfavorable conditions stimulate the formation of isogametes of opposite mating type that may fuse in fertilization. The resulting zygote undergoes meiosis, producing two haploid cells of each mating type. The photograph shows vegetative cells of this green alga.
Figure 7.1

5. llustration of mating types during fertilization in Chlamydomonas
llustration of mating types during fertilization in Chlamydomonas. Mating will occur only when plus (+) and minus (–) cells are together.

6. Figure 7-2 The life cycle of maize (Zea mays)
Figure 7-2 The life cycle of maize (Zea mays). The diploid sporophyte bears stamens and pistils that give rise to haploid microspores and megaspores, which develop into the pollen grain and the embryo sac that ultimately house the sperm and oocyte, respectively. Following fertilization, the embryo develops within the kernel and is nourished by the endosperm. Germination of the kernel gives rise to a new sporophyte (the mature corn plant), and the cycle repeats itself.
Figure 7.2 6

7. 7.1 Life Cycles Depend on Sexual Differentiation
7.1.3 Caenorhabditis elegans

8. Figure 7-3 (a) Photomicrograph of a hermaphroditic nematode, C
Figure 7-3 (a) Photomicrograph of a hermaphroditic nematode, C. elegans; (b) The outcomes of self-fertilization in a hermaphrodite and a mating of a hermaphrodite and a male worm.
Figure 7.3

9. 7.2 X and Y Chromosomes Were First Linked to Sex Determination Early in the Twentieth Century

10. Figure 7.4

11. Figure 7-4a (a) The Protenor mode of sex determination where the heterogametic sex (the male in this example) is XO and produces gametes with or without the X chromosome; (b) The Lygaeus mode of sex determination, where the heterogametic sex (again, the male in this example) is XY and produces gametes with either an X or a Y chromosome. In both cases, the chromosome composition of the offspring determines its sex.

12. Figure 7-4b (a) The Protenor mode of sex determination where the heterogametic sex (the male in this example) is XO and produces gametes with or without the X chromosome; (b) The Lygaeus mode of sex determination, where the heterogametic sex (again, the male in this example) is XY and produces gametes with either an X or a Y chromosome. In both cases, the chromosome composition of the offspring determines its sex.

13. 7.3 The Y Chromosome Determines Maleness in Humans

14. Figure 7-5 The traditional human karyotypes derived from a normal female and a normal male. Each contains 22 pairs of autosomes and two sex chromosomes. The female (a) contains two X chromosomes, while the male (b) contains one X and one Y chromosome (see arrows).
Figure 7.5

15. Figure 7-5a The traditional human karyotypes derived from a normal female and a normal male. Each contains 22 pairs of autosomes and two sex chromosomes. The female (a) contains two X chromosomes, while the male (b) contains one X and one Y chromosome (see arrows).
Figure 7.5a

16. Figure 7-5b The traditional human karyotypes derived from a normal female and a normal male. Each contains 22 pairs of autosomes and two sex chromosomes. The female (a) contains two X chromosomes, while the male (b) contains one X and one Y chromosome (see arrows).
Figure 7.5b

17. 7.3 The Y Chromosome Determines Maleness in Humans
7.3.1 Klinefelter and Turner Syndromes

18. Figure 7-6a The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X).
Figure 7.6a

19. Figure 7-6b The karyotypes and phenotypic depictions of individuals with (a) Klinefelter syndrome (47,XXY) and (b) Turner syndrome (45,X).
Figure 7.6b

20. 7.3 The Y Chromosome Determines Maleness in Humans
,XXX Syndrome
,XYY Condition
7.3.4 Sexual Differentiation in Humans
7.3.5 The Y Chromosome and Male Development

21. Table 7.1 Frequency of XYY Individuals in Various Settings

22. The presence or absence of the Y chromosome and SRY determines sexual differentiation in humans. Early human embryos are sexually
indifferent. Bipotential gonads can form either ovaries or testes depending on the presence or absence of the SRY genotype. With its presence in XY embryos,
bipotential gonads form testes and, subsequently, male reproductive organs and ducts. In the absence of SRY, in XX embryos, the female reproductive
organs and ducts are formed.

23. Figure 7-7 The various regions of the human Y chromosome.

25. 7.4 The Ratio of Males to Females in Humans Is Not 1.0

26. 7.5 Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammals
7.5.1 Barr Bodies

27. Figure 7-8 Photomicrographs comparing cheek epithelial cell nuclei from a male that fails to reveal Barr bodies (bottom) with a female that demonstrates Barr bodies (indicated by an arrow in the top image). This structure, also called a sex chromatin body, represents an inactivated X chromosome.
Figure 7.8

28. Figure 7-9 Barr body occurrence in various human karyotypes, where all X chromosomes except one (N –1) are inactivated.
Figure 7.9

29. 7.5 Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammals
7.5.2 The Lyon Hypothesis

30. Figure 7-10 (a) A calico cat, where the random distribution of orange and black patches illustrates the Lyon hypothesis. The white patches are due to another gene; (b) A tortoiseshell cat, which lacks the white patches characterizing calicos.
Figure 7.10

31. Depiction of the absence of sweat glands (shaded regions) in a female heterozygous for the X-linked condition anhidrotic ectodermal dysplasia. The locations vary from female to female, based on the random pattern of X chromosome inactivation during early development, resulting in unique mosaic distributions of sweat glands in heterozygotes.

32. 7.5 Dosage Compensation Prevents Excessive Expression of X-Linked Genes in Humans and Other Mammals
7.5.3 The Mechanism of Inactivation

33. Figure 7-11 (a) Transient pairing of X chromosomes may be required for initiating X-inactivation. (b) Deleting the Tsix gene of the Xic locus prevents X–X pairing and leads to chaotic X-inactivation. (c) Blocking X–X pairing by addition of Xic-containing transgenes blocks X–X pairing and prevents X-inactivation.
Figure 7.11

34. 7.6 The Ratio of X Chromosomes to Sets of Autosomes Determines Sex in Drosophila

35. Figure 7-12 Chromosome compositions, the ratios of X chromosomes to sets of autosomes, and the resultant sexual morphology in Drosophila melanogaster. The normal diploid male chromosome composition is shown as a reference on the left (XY/2A).
Figure 7.12

36. 7.6 The Ratio of X Chromosomes to Sets of Autosomes Determines Sex in Drosophila
7.6.1 Dosage Compensation in Drosophila
7.6.2 Drosophila Mosaics

37. Figure 7.13

38. Figure 7-14 A bilateral gynandromorph of Drosophila melanogaster formed following the loss of one X chromosome in one of the two cells during the first mitotic division. The left side of the fly, composed of male cells containing a single X, expresses the mutant white-eye and miniature-wing alleles. The right side is composed of female cells containing two X chromosomes heterozygous for the two recessive alleles.
Figure 7.14

39. 7.7 Temperature Variation Controls Sex Determination in Reptiles

40. Figure 7-15 Three different patterns of temperature-dependent sex determination (TSD) in reptiles, as described in the text. The relative pivotal temperature Tp is crucial to sex determination during a critical point during embryonic development FT = female-determining temperature; MT = male-determining temperature).
Figure 7.15