16) Sex

Question 1

What are the different ways of sex determination?

Answer 1

1) Environmental:
Sex of crocodile babies depends on temperature at which the eggs are incubated over the middle third of their development, 31 degrees- 34 degrees celcius= Male
Risky: Species do not have long term control

2) Genotypic
Genes responsible for sex determination
Sex chromosomes= Evolved from autosomes independently
X chromosome is relatively larger than Y

Question 2

Why are sex chromosomes unusual?

Answer 2

Main point in sexual reproduction= Increase variation
Sex is different as there is no advantage to allow recombination between sex chromosomes

Recombination is suppressed in Y chromosome= Y chromosome does not carry as many genes

Sex is binary trait= No advantage to mixing male and female traits

Sex is still evolving= a lot of species are still evolving towards having complete separation of male and female

Question 3

Why does having no recombination of Y a bad thing?

Answer 3

No repair of Y
IF Y chromosome has accumulated mutations that have knocked out most of its genes and turned them into pseudogenes= Degenerate

Question 4

What happens in humans since females essentially have twice as much gene product as males (due to having XX)?

Answer 4

Shut down one of the X chromosomes in females
One chromosomes is turned off= Becomes Barr Body
X inactivation happens to one chromosome per cell at random in early development= 12 days after conception

Question 5

What is an example of sex linked disease?

Answer 5

Haemophilia which is blood clotting factor= More prevalent in males as they only need 1 recessive allele to be expressed as they only have one X

Question 6

Why are all calico cats female?

Answer 6

Cats which are spotted or particoloured coat
Around 12 days after conception= X inactivation happens by which time= 100 cells in embryo
In each cell, one X chromosome at random is inactivated= maintained epigenetically in all daughter cells that arise from those embryonic cells

Gene for coat colour is on X chromosome:
O allele= Reddish colour
o allele= Brownish colour
So if cat is heterozygous at colour gene= Mosaic nature of X inactivation will mean that 100 cells will develop patches
Each patch= Will have one or the other X chromosome active and so when you get them paired up= Will have both

White spotting gene= sometimes expressed to produce white and sometimes it isnt= 3 colours of calico cat