Lecture 13 - Sex Genetics: Simon Whitehall Flashcards
Eukaryotic asexual lineages have only emerged recently. What does this suggest?
This suggests that there is a high extinction rate.
What is the main advantage of asexual reproduction?
All individuals can produce offspring, so the population can reproduce twice as fast.
In which organism have successful sex selection experiments taken place?
Yeast
What is the advantage of sexual reproduction?
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.
What is Protenor Sex Determination?
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)
What is Lygaeus sex determination?
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)
What is Heterogametic Sex?
The Gender that produces unlike gametes (usually (♂, but not always)
What is Homogametic Sex?
The gender that produces identical gametes (usually ♀)
What does ZW and ZZ describe?
ZW = Heterogametic ♀
ZZ = Homogametic ♂
What are some examples of organisms that defy convention when it comes to Hetero/Homogametic Sex?
Butterflies, Some fish and most birds.
What are the 2 syndromes that provided insight into sex determination in humans?
Klinefelter & Turner Syndrome.
How do the 2 syndromes arise?
Both Klinefelter & Turner Syndrome arise due to an abnormal number of X chromosomes (Aneuploidy) due to nonsdisjunction (failure of segregation of chromosomes during mitosis).
What are the phenotypes/karyotypes associated with Klinefelter’s Syndrome?
Klinefelter: Multiple X’s,(potentially) multiple Y’s
(e.g) XXY, XXXY,XXYY
These would be designated:
47, XXY
48,XXXY
48,XXYY
What are the phenotypes/karyotypes associated with Turner Syndrome?
Turner: Single X, No Y
Designated: 45,X0
0 indicates lack of Y chromosome.
What are the 3 regions of the Y Chromosome?
PARS
MSY
SRY
Where is Euchromatin and Heterochromatin found in the Y chromosome?
Heterochromatin is close to the telmoric end and the PAR, whereas Euchromatin is closer to the Centromere.
In which organisms is sex not genetically determined, and by what means?
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).
Explain dosage compensation.
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
What is Lyonization?
Where 1 X chromosome in a female is kept inactive.
What are Female cells described as?
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.
What is an example of a Lyonization disease?
Mosaic Disease - Anhidrotic ectodermal dysplasia, X linked mutation - in females this means 1/2 body has no sweat glands.
How is Lyonisation achieved?
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.
What % of genes escape silencing?
15% escape inactivation.
What is an autosome?
A non sex chromosome.
What is PARS?
Pseudoautosomal regions that share homology with the X chromosome. These regions synapse and recombine with X chromosome during meiosis.
What is MSY?
Male specific region of the Y - which does not synapse with the X chromosome.
What is SRY?
Sex determining region Y - produces a product called testis determining factor (TDF) which triggers undifferentiated gonadal tissue of the embryo to form testes.
