1. 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) 1

2. 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) 2

3. 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 3

4. 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 4

5. 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 5

6. 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 6

7. Figure 6.3
Figure 6.4

8. 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 8

9. Table 6.1

10. Sex Determination in Humans
Figure 6.4
Figure 6.6

11. 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 11

12. 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 12

13. X-linked Recessive Inheritance13

14. 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 14

15. Figure 6.5
Figure 6.7

16. Figure 6.6
Figure 6.8

17. X-linked Dominant Inheritance17

18. X-linked Dominant Traits
Incontinentia pigmenti
Figure 6.7 18

19. X-linked Dominant Traits
Congenital generalized hypertrichosis
Figure 6.8 19

20. 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 20

21. 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 21

22. 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 22

23. 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 23

24. 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 24

25. Figure 6.9
Figure 6.12

26. 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 26

27. 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 27

28. Imprints are erased during meiosis
Figure 6.11
Imprints are erased during meiosis
- Then reinstituted according to the sex of the individual 28

29. 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 29

30. 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 30