8. Sex chromosomes Flashcards
Explain sex chromosomes in evolutionary terms
Sex chromosomes - prior autosomes which gained sex-specific gene segments - still have pseudoautosomal regions where they pair up and can recombine
Explain the differences in sex chromosome systems across species
Do all organisms have sex chromosomes?
No:
- asexual organisms - no sexual reproduction - form clones - ex: bacteria
- hermaphrodites - both sexes at once - ex: worms
- sex environmentally determined - not genetically - ex: fish / crocodiles
Did sex chromosomes evolve once in animals or multiple times?
Sex chromosomes evolved multiple times - different sex determination systems
How divergent are human X and Y sex chromosomes?
Very divergent compared to other homologs - diverged over time - tended to do so in gene groups - few X-Y gene pairs remain across chromosomes
What is the structure of human sex chromosomes?
- pseudoautosomal regions - use for pairing up + minimal recombination
- centromeres
- genes - X gene rich - Y gene poor
Explain how human sex chromosomes evolved
Evolved from a pari of autosomes - default X chromosome - random male-specific mutation of one homolog - MSY (in humans SRY) - recombination between sex-specific regions supressed by sequence inversion -> gene duplication + further mutation accummulation -> loss of genes from MSY - Y chromosome shrinkage
Why sex chromosomes have minimal recombination?
Sex chromosomes only recombine at pseudoautosomal regions - no recombination in sex-specific genes - not needed for another sex to gain the genes - recombination supressed by sequence inversion - not alignment between homologous sequences
How divergent are sequences along the sex chromosomes?
The further away from pseudoautosomal regions - the more divergent are sequences between X and Y chromosomes - lower levels of recombination
How divergent are same sex chromosomes between closely related species?
Y chromosomes more divergent between closely related species than X chromosomes - more Y rearrangements over short time period
Where do the most mutations in sex chromosomes come from and why?
More mutations from fathers - more cell divisions in spermatogenesis than in oogenesis - more mutations in sex chromosomes
How are rates of evolution determined in sex chromosomes?
What are the trends of rates of evolution?
Lecture summary
