Lecture 13 - Sex Genetics: Simon Whitehall

Question 1

Eukaryotic asexual lineages have only emerged recently. What does this suggest?

Answer 1

This suggests that there is a high extinction rate.

Question 2

What is the main advantage of asexual reproduction?

Answer 2

All individuals can produce offspring, so the population can reproduce twice as fast.

Question 3

In which organism have successful sex selection experiments taken place?

Answer 3

Yeast

Question 4

What is the advantage of sexual reproduction?

Answer 4

Faster growth and better survival rate in harsh conditions.

Recombination during meiosis allows harmful mutations to be purged.

Makes the population more adaptable to external changes.

Question 5

What is Protenor Sex Determination?

Answer 5

Protenor Insect sex determination: aka
XX/XO mode of sex determination.

♀ somatic cell = 14 chromosomes with 2 X chromosomes

(Gametes have 7 chromosomes with one being an X)

♂ somatic cell = 13 chromosomes with 1 X chromosome

(Gametes have either 6 chromosomes with no X, or 7 chromosomes with one being an X)

Question 6

What is Lygaeus sex determination?

Answer 6

Lygaeus Insect sex determination aka
XX/XY mode of sex determination.

♀ somatic cell = 12 autosomes with 2 of these being X chromosomes.

Gamete = 6 autosomes (1 being an X)

♂ somatic cell = 12 autosomic cells with 1 X and 1 smaller heterochrome called Y.

Gamete = 6 Autosomes (1 being X or Y)

Question 7

What is Heterogametic Sex?

Answer 7

The Gender that produces unlike gametes (usually (♂, but not always)

Question 8

What is Homogametic Sex?

Answer 8

The gender that produces identical gametes (usually ♀)

Question 9

What does ZW and ZZ describe?

Answer 9

ZW = Heterogametic ♀

ZZ = Homogametic ♂

Question 10

What are some examples of organisms that defy convention when it comes to Hetero/Homogametic Sex?

Answer 10

Butterflies, Some fish and most birds.

Question 11

What are the 2 syndromes that provided insight into sex determination in humans?

Answer 11

Klinefelter & Turner Syndrome.

Question 12

How do the 2 syndromes arise?

Answer 12

Both Klinefelter & Turner Syndrome arise due to an abnormal number of X chromosomes (Aneuploidy) due to nonsdisjunction (failure of segregation of chromosomes during mitosis).

Question 13

What are the phenotypes/karyotypes associated with Klinefelter’s Syndrome?

Answer 13

Klinefelter: Multiple X’s,(potentially) multiple Y’s

(e.g) XXY, XXXY,XXYY

These would be designated:

47, XXY
48,XXXY
48,XXYY

Question 14

What are the phenotypes/karyotypes associated with Turner Syndrome?

Answer 14

Turner: Single X, No Y

Designated: 45,X0

0 indicates lack of Y chromosome.

Question 15

What are the 3 regions of the Y Chromosome?

Answer 15

PARS
MSY
SRY

Question 16

Where is Euchromatin and Heterochromatin found in the Y chromosome?

Answer 16

Heterochromatin is close to the telmoric end and the PAR, whereas Euchromatin is closer to the Centromere.

Question 17

In which organisms is sex not genetically determined, and by what means?

Answer 17

Some reptile species rely on temperature dependent sex determination, whereby temperature affects activity of enzymes and inhibitors which control production of steroid hormones (such as oestrogen).

Question 18

Explain dosage compensation.

Answer 18

Human males have only 1 X, whereas females have 2, so females produce twice as much of each gene product for X.

This is regulated by Barr/Chromatin Bodies in Females, whereby 1 X chromosome is kept inactive, (shows as darkened body under microscope).

The number of Barr Bodies is always Xn-1

Question 19

What is Lyonization?

Answer 19

Where 1 X chromosome in a female is kept inactive.

Question 20

What are Female cells described as?

Answer 20

They are described as being a X chromosome mosaic, whereby 1/2 the cells have an inactive paternal X, and 1/2 have an inactive maternal X.

Question 21

What is an example of a Lyonization disease?

Answer 21

Mosaic Disease - Anhidrotic ectodermal dysplasia, X linked mutation - in females this means 1/2 body has no sweat glands.

Question 22

How is Lyonisation achieved?

Answer 22

Inactivation initiated from XIC (X-inactivation centre)

-> produces 2 non-coding RNA transcripts

Xist
Tsix

Xist progressively coats 1 random X chromosome spreading outwards. Leads to packaging of 1 into dense compact form of chromatin called heterochromatin - which then silences it.

Question 23

What % of genes escape silencing?

Answer 23

15% escape inactivation.

Question 24

What is an autosome?

Answer 24

A non sex chromosome.

Question 25

What is PARS?

Answer 25

Pseudoautosomal regions that share homology with the X chromosome. These regions synapse and recombine with X chromosome during meiosis.

Question 26

What is MSY?

Answer 26

Male specific region of the Y - which does not synapse with the X chromosome.

Question 27

What is SRY?

Answer 27

Sex determining region Y - produces a product called testis determining factor (TDF) which triggers undifferentiated gonadal tissue of the embryo to form testes.