Sex and the Single Chromosome Flashcards
What are autosomes?
Have nothing to do with sex
What chromosomes do females have?
- 22 pairs of autosomes and XX
- All gametes 22A + X
- Homogametic
What chromosomes do males have?
- 22 pairs of autosomes and XY
- Half gametes 22A + X and half 22A + Y
- Heterogametic
Why do sex ratios balance out?
- Rare sex has an evolutionary advantage to get genes into the next generation
- Ratio balances back out
What are four human chromosome abnormalities?
- 44AOY: Dies
- 44AXO: Turner’s syndrome
- 44AXXY: Klinefelter’s syndrome
- 44AXXX: Triple X
- 44AXYY: Double Y
What is Turner’s syndrome?
- Female phenotype
- Sterile
- Late sexual maturity
- Webbing of skin on neck
- Upturned fingernails
What is Klinefelter’s syndrome?
- Variable male phenotype
- Sterile
What is triple X?
- Normal female phenotype
What is double Y?
- Normal male phenotype
How is sex determined?
- By the Sex Determining Region (SRY) gene
- Default development is female
What is required for males?
- Testis determining factor: SRY locus on Y chromosome
- Receptor for male steroid hormones: androgen receptor on X chromosome
What genes are involved in sex linkage?
Genes on the part of X and Y chromosomes that do not pair
Why is sex disadvantageous?
Two-fold cost of sex
- Female that has only females will get more genes into the next generation
- Sexual female will have half children who may not pass on her genes directly
Require a mate
- Look for mates
- Compete for mates
- Finding mates at low population density
What is anisogamy?
Difference in the size of eggs and sperm
Why is anisogamy important?
- Large gametes: more food and more robust
- Small gametes: can produce more of them
- Intermediate size has no advantage
- Outcome is evolutionary arms race
What are the benefits of sex?
- DNA repair
- Muller’s ratchet
- The tangled bank
- The Red Queen hypothesis
What are the benefits of DNA repair
- Proof reading when homologous chromosomes pair
- BUT could happen without sexual reproduction
What is the consideration of Muller’s ratchet
- Prevents accumulation of mutations
- Undesirable mutations are more likely than favourable ones
- Sex allows elimination of undesirable mutations and accumulation of favourable ones
- BUT the good of the species argument doesn’t work because evolution works on an individual basis
What is the benefit of the tangled bank?
Variable gametes mean:
- Offspring won’t compete with parents or each other
- Able to fit into ecological niches
What is the Red Queen hypothesis?
- Pathogens and parasites are small compared to the organisms they infect
- Larger organisms take longer to replicate
- Need variability to defeat pathogens
- Have to ‘run faster to stay in the same place’
How is it thought the Y chromosome will evolve?
- Y chromosome has very few genes (mostly just SRY)
- Evolutionary pressure to lose functional genes to other chromosomes where they are protected by diploidy
- Hypothesised Y may eventually be lost
