sex chromosomes

Question 1

why do sexually antagonistic genes spread

Answer 1

Y linked genes only spread when it increases male fitness more than it decreases female fitness

inversion suppresses recombination and creates linkage of sexually antagonistic genes to sex determination allele in males, inversions favoured by selection since it reduces XY recombination,

Question 2

how can you tell if genes are old, how does this relate to sex genes

Answer 2

using Ks= number of synonymous substitutions per site (time ceased to recombine), Y chromosome shows 4 strata with different ages

Question 3

how does lack of recombination lead to degeneration on the Y

Answer 3

due to lack of recombination any deleterious mutations are linked with favoured alleles and cannot be removed: muller’s ratchet

this leads to evolution of Y chromosome being slow/weak

Question 4

what is the no recomination experiment

Answer 4

in male drosophila there is no recombination

use of markers to keep chromosomes 2 and 3 (80% genome) in males, after 35 generations adult survived 2 days post eclosion

a natural experiment: a piece of autosomal DNA recombined in drosophila onto Y chromosome creating neo Y chromosome, since no recombination in males has remained the same, 34% of genes are non functional, subject to accumulation of mutations

Question 5

how do sex chromes evolve in plants with males and females

Answer 5

in silene latifolia, has male and female sex associated with X and Y chromes

lower rate of degeneration due to gene expression in pollen, compared to sperm which show no gene expression

Question 6

how is Y specialised for male benefit

Answer 6

Y survivors are genes which have never been removed from the chrome, there are 12 ubiquitously expressed genes, 2 testis specific genes and 13 pseudogenes, there are palindromes which help prevent degredation