Chapter 5 – Sex Determination and Sex Chromosomes
• Heteromorphic Chromosomes
– Eg. X and Y
• Unisexual, dioecious and gonochoric
– Contain only male OR female reproductive organs
• Bisexual, monoecious and hermaphroditic
– Contain both male AND female reproductive organs
– Individuals of intermediate sexual differentiation…usually sterile
Modes of Sex Determination
• Protenor mode (XX & XO)
• Lygaeus mode (XX & XY) or (ZZ & ZW)
• In both cases one sex produces two types of gametes (heterogametic sex) while the other
produces only one type of gamete (homogametic sex).
Klinefelter Syndrome 47,XXY
• Testes underdeveloped and fail to produce sperm
• Tall, long arms & legs, large hands & feet
• Slight decrease in intelligence (below normal range)
• Result of nondisjunction event in parent
Turner Syndrome 45,XO
• Female external genitalia and internal ducts
• 1 in 10,000 female births
• Ovaries not fully developed
• Short stature, skin flaps on back of neck and underdeveloped breasts. Shield-like chest.
• Intelligence is normal
• Result of nondisjunction event in parent.
• Occurs in about 1 of every 1,200 female births.
• Observe variable expression:
– Some XXX individuals are phenotypically normal, while others show underdeveloped
secondary sex characteristics and mental retardation can occur.
• Also see 48,XXXX and 49,XXXXX individuals…more pronounced symptoms
• Phenotypically male, usually over 6 feet tall, sometimes associated with lower intelligence and
behavioral problems. The Y Chromosome
• Originally thought not to have genes or homology to other chromosomes.
– Pseudoautosomal regions (PARs) share homology with X chromosome
• Allow pairing during meiosis
– The rest of the chromosome is called the male-specific region of the Y (MSY)
• Some genes in common with X, and some novel genes
• There is at least one very important gene that determin