Chapter 5: Sex Determination and Sex Chromosomes

1. Chapter 5:Sex Determination and Sex Chromosomes Susan Chabot Honors Genetics 2013-2014

2. Chapter Concepts Sexual reproduction, which greatly enhances genetic variation, requires a mechanism for sexual differentiation. Usually, specific genes on a single chromosome determine male-ness or female-ness during development. A variety of mutations can lead to sexual dimorphism (differences). In humans, the presence of extra X or Y chromosomes above the normal diploid number can be tolerated but will lead to distinctive phenotypes.

3. Chapter Concepts There is NOT a 1-to-1 ration of males to females at conception; the ratio tends to favor males at conception. Female mammals inherit 2 X-chromosomes at conception; the additional information in inactivated in early development. Temperature of incubation for reptilian eggs will determine the sex of offspring.

4. Introduction • Heteromorphic Chromosomes: • It is the GENES on the chromosomes that determine sex of the organism. • MOST of these genes are on the sex chromosomes but some can be found on the autosomes.

5. Introduction • Heteromorphic Chromosomes: characterize one sex or the other; XY pair in mammals. • It is the GENES on the chromosomes that determine sex of the organism. • MOST of these genes are on the sex chromosomes but some can be found on the autosomes.

6. Vocabulary • Primary Sexual Differentiation: • Secondary Sexual Differentiation:

7. Vocabulary • Primary Sexual Differentiation: involves production and maturation of the gonads and gametes. • Secondary Sexual Differentiation: involves overall appearance of the organism; non-reproductive organs

8. Vocabulary • Unisexual/Dioecious/Gonohoric: • Bisexual/Monoecious/Hermaphroditic: • Intersex:

9. Vocabulary • Unisexual/Dioecious/Gonochoric: organism that contains only male or only female reproductive organs. • Bisexual/Monoecious/Hermaphroditic: organism that contains both male and female reproductive organs. These organisms can produce both eggs and sperm. • Intersex: organisms of intermediate sexual condition; often sterile.

10. The Y Chromosome Determines Maleness in Humans • Human DIPLOID number is 46; HAPLOID number is 23. • Remember, chromosomes are only visible during MITOSIS and MEIOSIS. Visualization of chromosomes during cell division will allow for the construction of a karyotype, seen below.

11. Klinefelter Syndrome • 47, XXY • 1 in 600 male births! • Result of nondisjunction during Meiosis. • Tend to be tall, long arms, large hands and feet • Outward appearance of male • Undeveloped testes that fail to produce sperm. • Female secondary sex characteristics are not suppressed; rounded hips, slight breast enlargement. • Below average intelligence.

13. Turner Syndrome • 45, X • 1 in 2000 female births! • Result of nondisjunction during Meiosis • Short stature, skin flaps on back of neck, broad shield-like chest • Outward appearance of female • Undeveloped ovaries, may not produce eggs. • Normal intelligence

15. Nondisjunction

16. Other Conditions of 23rd pair Nondisjunction • 47, XXX • 1 in 1000 female births • Would not realize the condition was present without a karyotype performed for confirmation. • Normal primary and secondary sexual development. • Rare cases of under-developed secondary sex characteristics, mental retardation. • 47, XYY • Above average height, below normal intelligence. • Study done in 1960’s to show predisposition to criminal behavior due to this karyotype