Our Sexual Selves

2. Our Sexual Selves • Maleness or femaleness is determined at conception • Another level of sexual identity comes from the control that hormones exert on development • Finally, both psychological and sociological components influence sexual feelings

3. Sexual Development During the fifth week of prenatal development, all embryos develop two sets of: - Unspecialized (indifferent) gonads - Reproductive ducts – Müllerian (female-specific) and Wolffian (male-specific) An embryo develops as a male or female based on the absence or presence of the Y chromosome - Specifically the SRY gene (sex-determining region of the Y chromosome)

4. Sex Chromosomes Determine Gender Human males are the heterogametic sex with different sex chromosomes, (XY) Human females are the homogametic sex (XX) In other species sex can be determined in many ways - For example, in birds and snakes, males are homogametic (ZZ), while females are heterogametic (ZW)

5. X and Y Chromosomes X chromosome - Contains > 1,500 genes - Larger than the Y chromosome - Acts as a homolog to Y in males Y chromosome - Contains 231 genes - Many DNA segments are palindromes and may destabilize DNA Figure 6.1

6. Anatomy of the Y Chromosome Pseudoautosomal regions (PAR1 and PAR2) - 5% of the chromosome - Contains genes shared with X chromosome Male specific region (MSY) - 95% of the chromosome - Contains majority of genes including SRY and AZF (needed for sperm production) Figure 6.2

7. SRY Gene Encodes a transcription factor protein Controls the expression of other genes Stimulates male development Developing testes secrete anti-Mullerian hormone and destroy female structures Testosterone and dihydrotesterone (DHT) are secreted and stimulate male structures

8. Abnormalities in Sexual Development Pseudohermaphroditism = Presence of male and female structures but at different stages of life - Androgen insensitivity syndrome = Lack of androgen receptors - 5-alpha reductase deficiency = Absence of DHT - Congenital adrenal hyperplasia = High levels of androgens

9. Figure 6.3 Figure 6.4

10. Homosexuality Homosexuality has been seen in all cultures for thousands of years Documented in 500 animal species Evidence suggests a complex input from both genes and the environment Research in this area is controversial Studies of identical and fraternal twins Identifying possible markers

11. Table 6.1

12. Sex Ratios The proportion of males to females in a human population Calculated by # of males / # of females multiplied by 1,000 Primary sex ratio – At conception Secondary sex ratio – At birth Tertiary sex ratio – At maturity Sex ratios can change markedly with age

13. Sex Determination in Humans Figure 6.4 Figure 6.6

14. Y-linked Traits Genes on the Y chromosome are said to be Y-linked Y-linked traits are very rare Transmitted from male to male No affected females Currently, identified Y-linked traits involve infertility and are not transmitted

15. X-linked Traits Possible genotypes X+X+ Homozyogus wild-type female X+Xm Heterozygous female carrier XmXm Homozygous mutant female X+Y  Hemizygous wild-type male XmY Hemizygous mutant male

16. X-linked Recessive Inheritance

17. X-linked Recessive Traits Examples: - Ichthyosis = Deficiency of an enzyme that removes cholesterol from skin - Color-blindness = Inability to see red and green colors - Hemophilia = Disorder of blood-clotting

18. Figure 6.5 Figure 6.7

19. Figure 6.6 Figure 6.8

20. X-linked Dominant Inheritance

21. X-linked Dominant Traits Incontinentia pigmenti Figure 6.7

22. X-linked Dominant Traits Congenital generalized hypertrichosis Figure 6.8

23. Solving Genetic Problems Steps to follow: 1) Look at the inheritance pattern 2) Draw a pedigree 3) List genotypes and phenotypes and their probabilities 4) Assign genotypes and phenotypes 5) Determine how alleles separate into gametes 6) Use Punnett square to determine ratios 7) Repeat for next generation

24. Sex-Limited Traits Traits that affect a structure or function occurring only in one sex The gene may be autosomal or X-linked Examples: - Beard growth - Milk production - Preeclampsia in pregnancy

25. Sex-Influenced Traits Traits in which the phenotype expressed by a heterozygote is influenced by sex Allele is dominant in one sex but recessive in the other Example: - Pattern baldness in humans - A heterozygous male is bald, but a heterozygous female is not

26. X Inactivation Females have two alleles for X chromosome genes but males have only one In mammals, X inactivation balances this inequality and one X chromosome is randomly inactivated in each cell The inactivated X chromosome is called a Barr body

27. X Inactivation X inactivation occurs early in prenatal development It is an example of an epigenetic change - An inherited change that does not alter the DNA base sequence The XIST gene encodes an RNA that binds to and inactivates the X chromosome

28. Figure 6.9 Figure 6.12

29. X Inactivation Animation Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Figure 2.3

30. X Inactivation A female that expresses the phenotype corresponding to an X-linked gene is a manifesting heterozygote X inactivation is obvious in calico cats Figure 6.10

31. Genomic Imprinting The phenotype of an individual differs depending on the gene’s parental origin Genes are imprinted by an epigenetic event: DNA methylation - Methyl (CH3) groups bind to DNA and suppress gene expression in a pattern determined by the individual’s sex

32. Imprints are erased during meiosis - Then reinstituted according to the sex of the individual Figure 6.11

33. Importance of Genomic Imprinting Function of imprinting isn’t well understood, but it may play a role in development Research suggests that it takes two opposite sex parents to produce a healthy embryo - Male genome controls placenta development - Female genome controls embryo development Genomic imprinting may also explain incomplete penetrance

34. Imprinting and Human Disease Two distinct syndromes result from a small deletion in chromosome 15 - Prader-Willi syndrome - Deletion inherited from father - Angelman syndrome - Deletion inherited from mother The two syndromes may also result from uniparental disomy

35. Imprinting and Human Disease Deletion on chromosome 15 reveals imprinting Figure 6.14