Chapter 5 - Sex Determination and Sex Chromosomes

Flashcards covering sex determination mechanisms, sex chromosome aberrations, sex ratios, and dosage compensation, including X inactivation and Barr bodies, from Chapter 5 of General Genetics.

How do grasshoppers, crickets, and roaches determine sex?

Females are XX and males are XO.

What is haplodiploidy and in which organisms is it found?

Haplodiploidy is a sex-determination system where females develop from fertilized eggs (diploid) and males develop from unfertilized eggs (haploid), found in bees, ants, and wasps.

How is sex determined in many snakes?

Males are ZZ and females are 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 hypothesis regarding the mechanism of temperature-dependent sex determination?

Aromatase converts androgens to estrogens, and there might be temperature-dependent transcription of the aromatase gene.

How do clownfish determine sex within their group?

The largest group member is female, the second largest is male, and all others are non-breeding males.

How do coral gobies (Gobiodon histrio) adapt their sex for breeding?

If a mate dies or leaves, the resident will court the next fish, and one will change sex to accommodate.

What is parthenogenesis, as seen in Cnemidophorus uniparens?

Cnemidophorus uniparens are all female, and eggs develop directly from oocytes by chromosome duplication, producing offspring that are genetically identical to their mothers.

How do X and Y chromosomes differ in gene content?

The Y chromosome has ~75 genes, while the X chromosome has 900–1400 genes.

What region provides enough homology for X and Y chromosomes to synapse during meiosis?

The pseudoautosomal regions (PARs).

What is Klinefelter syndrome?

A sex chromosome aberration (47,XXY) resulting in male individuals who are tall, have undeveloped testes (sterile), and often a low IQ.

What is Turner syndrome?

A sex chromosome aberration (45,X) resulting in female individuals who are short, have rudimentary ovaries (sterile), and a normal IQ.

What is the 'male gene' in mammals?

The SRY gene, which is usually found on the Y chromosome and is responsible for initiating male development.

What happens if the SRY gene becomes translocated to an X chromosome in an XX individual?

The XX individual can be a phenotypically normal (but sterile) male.

What is the primary sex ratio?

The proportion of males to females at conception, which is typically greater than 1 in human populations.

What is the secondary sex ratio?

The proportion of males to females that is born, which is typically greater than 1 in human populations.

What is dosage compensation?

A mechanism that prevents excessive expression of X-linked genes in humans and other mammals, ensuring that males and females have equivalent amounts of X-linked gene products.

What are Barr bodies?

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

What is the Lyon hypothesis?

Proposed by Mary Lyon and Lianne Russell, it states that one X chromosome becomes inactivated early in development, and the choice of which X is inactivated is random for each cell, leading to female mammals being genetic mosaics.

Why are calico and tortoiseshell cats almost always female?

These coat patterns result from random X inactivation (Lyon hypothesis) because the alleles for black and orange coat color are carried on different X chromosomes.

What is the role of the Xist gene in X inactivation?

When active, the Xist gene transcribes an RNA (but is not translated into protein) that spreads and inactivates the chromosome containing it by forming a 'molecular cage'.

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

Marsupials exhibit absolute paternal X chromosome inactivation, meaning the paternal X chromosome is always inactivated.

How is sex determined in fruit flies (Drosophila)?

Sex is determined by the ratio of the number of X chromosomes to the number of sets of autosomes (X:Autosome Balance).