Evolution of X and Y Chromosomes from Homologous Autosomes

Evolution of the Y chromosome. Today, the SRY gene determines male sex. Homologous regions of the chromosomes are shown in pink; mya, million years ago. Monotremes are egg-laying mammals; marsupials are pouched mammals.Evolution of the Y chromosome. Today, the SRY gene determines male sex. Homologous regions of the chromosomes are shown in pink; mya, million years ago. Monotremes are egg-laying mammals; marsupials are pouched mammals.

(autosome pair) Ancestral reptiles >350 mya A Before 350 mya, sex was determined by temperature, not by chromosome differences. SRY Ancestral reptiles 350 mya B The SRY gene begins to evolve 350 mya. The DNA sequences of the chromo— somes diverge as other mutations accumulate. —> Y X Y X Y X Y X area that _, cannot cross over Monotremes Marsupials Monkeys Humans 320—240 mya 170—130 mya 130—80 mya 50—30 mya C By 320—240 mya, the DNA sequences of the chromosomes are so different that the pair can no lon— ger cross over in one region. The Y chromosome begins to get shorter. D Three more times, the pair stops crossing over in yet another region. Each time, the DNA sequences of the chromosomes diverge, and the Y chromosome short— ens. Today, the pair crosses over only at a small region near the ends.