1. Matters of Sex 5 th Week: All embryos develop 2 unspecialized gonads 6 th Week: gene SRY (sex- determining region of Y) activated male develops X (1500 genes)Y (90 protein-encoding genes) XX (Homogametic sex)  12 th Week (females): Meiosis begins  25 th Week: Eggs at PI (primary oocytes); dormant for 10-15 years XY (Heterogametic sex)  Y (95% male-specific genes) Human Male (Hemizygous): only 1 set of X-linked genes  (X from his mother; Y from his father)

2. Heteromorphic Chromosomes –X,Y; denotes 1 sex or the other Unisexual (dioecious or gonochoric): only male or female organs Bisexual (monoecious or hermaphroditic); with male and female organs Intersex: intermediate sexual condition (usually sterile) X Linked Recessive Inheritance: Ichthyosis: brown, scaly appearance (Failure in removing cholesterol from skin cells) Color blindness in humans Hemophilia: blood clotting disorder Eye color (red/white) discovered in Drosophila melangastor by Thomas Hunt Morgan

3. Lesch-Nyhan Syndrome: abnormal nucleic acid metabolism; uric acid in blood; self mutilation of lips and fingers; incurable; mostly in males; mentally challenged  Onset: 6 to 8 months after birth Duchenne Muscular Dystrophy: progressive muscular wasting; fatal in 20’s  Onset: begins 3 to 5 years of age

4. X Linked Dominant Inheritance: Rare; expressed in females in 1 X; more severe in males; increased rates of miscarriage CGH (Congenital Generalized Hypertrichosis): extra hair follicles, denser hair  Mutant gene is atavistic

5. Barr Bodies: inactivated X (15 genes); cells from female cheek; interphase cells stained and Barr bodies seen The Lyon Hypothesis: In female mammals, which X chromosome is inactivated? Inactivation occurs randomly in somatic cells early in embryonic development

6. Sex Limited Traits: Affects a structure or function of the body that is present in only men or women Men (beard growth); Women (breast size) Sex Influenced Traits: An allele is dominant in one sex but recessive in the other Difference in expression due to hormonal differences between the sexes Pattern baldness in males