Chapter 5 - Sex Determination and Sex Chromosomes

Flashcards covering different sex determination systems, X and Y chromosomes, sex chromosome aberrations, sex ratios, Barr bodies, X inactivation, and Drosophila sex determination.

What are some different systems of sex determination found in nature?

Grasshoppers (XX F, XO M), Bees (haplodiploidy), Nematode worms (XX hermaphrodite, XO M), Snakes (ZZ M, ZW F), and Lizards (XX/XY or ZZ/ZW).

What is temperature-dependent sex determination (TSD)?

Sex is determined by the incubation temperature during a critical period of embryo development, found in crocodiles, most turtles, and some lizards.

What is a hypothesized mechanism for temperature-dependent sex determination (TSD)?

Temperature-dependent transcription of the aromatase gene, which converts androgens to estrogens.

How do some fish species demonstrate that sex is not always 'determined'?

Some fish change sex as they get older or larger, such as clownfish (largest is female) or bluehead wrass (largest becomes male).

What unique reproductive strategy is used by Cnemidophorus uniparens lizards?

Parthenogenesis, where eggs develop directly from oocytes by chromosome duplication, resulting in all female offspring genetically identical to their mothers.

What is a key difference between human X and Y chromosomes?

The Y chromosome typically contains approximately 75 genes, while the X chromosome contains 900–1400 genes.

What genetic condition is characterized by a 47,XXY karyotype, resulting in a tall, sterile male with a low IQ?

Klinefelter syndrome.

What genetic condition is characterized by a 45,X karyotype, resulting in a short, sterile female with a normal IQ?

Turner syndrome.

What gene is responsible for determining maleness in mammals?

The SRY gene, usually found on the Y chromosome.

What happens if the SRY gene is missing or has a null mutation in an XY individual?

The XY individual can develop as a normal female.

What is the primary sex ratio?

The proportion of males to females at conception.

What phenomenon prevents excessive expression of X-linked genes in females and involves the inactivation of one X chromosome?

Dosage compensation.

What is a Barr body?

A dark-staining body visible in the nuclei of interphase cells, representing an inactivated X chromosome, found only in female mammals.

What is the key idea behind the Lyon hypothesis?

One X chromosome in female mammals becomes randomly inactivated early in development, and all descendant cells maintain the same X inactivation pattern.

Why are calico and tortoiseshell cats almost always female?

They are genetic mosaics resulting from the random inactivation of one of the two X chromosomes in females, with each X carrying a different allele for coat color.

What gene is critical for X inactivation in mammals?

The Xist gene, which produces an RNA that spreads and inactivates the chromosome containing it.

How does X inactivation differ in marsupials compared to other mammals?

Marsupials exhibit absolute paternal X chromosome inactivation.

How is sex determined in Drosophila (fruit flies)?

Sex is determined by the ratio of the number of X chromosomes to the number of sets of autosomes.