2: Sex chromosomes

Question 1

Give an organism with male heterogamety

Answer 1

(X & Y) humans

Question 2

Give an organism with female heterogamety

Answer 2

(Z & W) Birds

Question 3

Give an organism with polygeneic determination

Answer 3

(XYW) Rats, Cichlids

Question 4

Describe the haplodiploidy system

Answer 4

e.g ants, bees etc.
- Females come from diploid eggs that have been fertilised
- Males come from haploid eggs that have not been fertilised

Question 5

Give 3 external conditions that can alter sex determination

Answer 5

1. Intracellular parasitism (in some arthropods)
   - Bacteria in mature eggs, and so kills developing males or turns them female
2. Temperature (in alligators)
   - Males = ~33 C
   - Females = ~30 C
3. Social factors
   - e.g sequential hermaphroditism in Blue banded goby

Question 6

Describe sex chromosome evolution

Answer 6

1. Start off as normal autosomes, then one aquires a sex determining gene
2. Suppression of recombination (sex chromosomes don’t recombine with each other) → bit of a mystery
3. Accumulation of mutations (due to no recomb) and sex specific traits
4. 3 stratas form
5. Divergence and accumulation of new mutations
6. Sex limited chromosome degrades (e.g Y in humans)

Question 7

In sex chromosome evolution, what is a strata?

Answer 7

The region around the sex determining gene

Question 8

What causes the degredation of the Y and W chromosomes?

Answer 8

Reduced efficacy of selection (due to no recombination)

Question 9

What prevents the Y chromosome from disappearing completley?

Answer 9

Despite random loss, the important genes are maintained by selection
- A copy of the Y chromosome that can’t produce sperm for instance will not be passed onto the next gen

Question 10

What are the effects of random loss on the Y chromosome?

Answer 10

• Fewer genes = smaller mutational target (less mutations)
• Slows the rate of degradation over time
• Increased power of sex-specific selection on Y (and W)
• Strongly selected on for role in reproduction

Question 11

Describe gene conversion on the Y chromosome

Answer 11

→ unidirectional transfer of genetic material from a ‘donor’ sequence to the homologous ‘acceptor’
- sequences replace each other (non-reciprocal intra-chromosomal recomb)
- Y recombines with itself

Question 12

What has gene conversion on the Y resulted in?

Answer 12

• It has 8 major palindromic regions
• Also has some other near-perfect mirrored repeats (almost palindromic)
• Contain ampliconic regions (multiple copies of the same genes over and over) → back-ups!

Question 13

Gene movement onto the Y

Answer 13

→ duplication of autosomal genes onto the Y chromosome
- Gene gains on the Y were more frequent than gene losses
- Sex specific genes move over