Chapter 5 - Sex Determination and Sex Chromosomes

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Flashcards covering key concepts related to sex determination and sex chromosomes from Chapter 5 of General Genetics (PCB3063) lecture notes.

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26 Terms

1
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How is sex typically determined in grasshoppers, crickets, and roaches?

XX is female, and XO is male.

2
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What is haplodiploidy, and in what insects is it a method of sex determination?

Haplodiploidy is a sex determination system where males are haploid (n) and females are diploid (2n), found in bees, ants, and wasps.

3
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Describe sex determination in nematode worms.

XX individuals are hermaphroditic, and XO individuals are male.

4
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How is sex determined in many snake species?

ZZ is male, and ZW is female.

5
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What is Temperature-Dependent Sex Determination (TSD), and in which animals is it observed?

TSD is a system where sex is determined by the incubation temperature during a critical period of embryo development, seen in crocodiles, most turtles, and some lizards.

6
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What is the proposed hypothesis regarding aromatase's role in Temperature-Dependent Sex Determination (TSD)?

Aromatase converts androgens to estrogens, and its gene's transcription might be temperature-dependent.

7
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How does sex change occur in clownfish?

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

8
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Describe sex change in the coral goby (Gobiodon histrio).

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

9
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What is parthenogenesis, as observed in Cnemidophorus uniparens?

Parthenogenesis is a form of asexual reproduction where eggs develop directly from oocytes by chromosome duplication without cell division, resulting in all-female offspring genetically identical to their mothers.

10
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Are X and Y chromosomes completely homologous?

No, they are not completely homologous in the normal sense.

11
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How do X and Y chromosomes segregate normally at meiosis despite not being completely homologous?

They have enough homology for synapsis, particularly in the Pseudoautosomal Regions (PARs).

12
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Approximately how many genes are found on the Y chromosome compared to the X chromosome?

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

13
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Describe the characteristics of Klinefelter syndrome.

Individuals with Klinefelter syndrome have a 47,XXY karyotype, are male, tall, have immature testes leading to sterility, and often have a low IQ (occurs in 1/700 male births).

14
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What are the characteristics of an individual with Turner syndrome?

Individuals with Turner syndrome have a 45,X karyotype, are female, short, have rudimentary ovaries leading to sterility, and typically have a normal IQ (occurs in 1/3000 female births).

15
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What is the primary 'male gene' in mammals, and where is it typically located?

The SRY gene is the primary 'male gene' in mammals, and it is usually located on the Y chromosome.

16
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What can happen if an XX individual has the SRY gene translocated to another chromosome?

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

17
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Define primary sex ratio.

The proportion of males to females at conception.

18
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Define secondary sex ratio.

The proportion of males to females that is born.

19
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What is dosage compensation?

A mechanism that prevents excessive expression of X-linked genes in females compared to males.

20
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What is a Barr body, and in which sex is it observed in mammals?

A Barr body is a dark-staining body visible in the nuclei during interphase, and it is observed only in female mammals.

21
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What is the Lyon hypothesis?

The Lyon hypothesis proposes that in female mammals, one of the two X chromosomes is randomly inactivated early in development in each somatic cell, and this inactivation is maintained in all descendant cells.

22
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Why are female mammals considered genetic mosaics?

Due to random X-inactivation (Lyon hypothesis), different cells in a female mammal may express genes from either the paternal or maternal X chromosome, leading to a mosaic expression pattern.

23
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Why are calico and tortoiseshell cats almost always female?

These coat patterns result from X-inactivation in female cats, where one X chromosome carries the allele for black coat color and the other for orange. Random inactivation in different cells leads to patches of black and orange fur.

24
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What is the role of the Xist gene in X inactivation?

When the Xist gene is active, it is transcribed into an RNA that coats the X chromosome from which it was transcribed, leading to the inactivation of that chromosome.

25
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How does X inactivation differ in marsupials compared to other mammals?

In marsupials, there is absolute paternal X chromosome inactivation, meaning the X chromosome inherited from the father is always inactivated.

26
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How is sex determined in Drosophila (fruit flies)?

Sex in Drosophila is determined by the ratio of the number of X chromosomes to the number of sets of autosomes (the X:autosome balance).