The proposed evolutionary development of the mammalian

The proposed evolutionary development of the mammalian Y chromosome through four major structural rearrangementsThe proposed evolutionary development of the mammalian Y chromosome through four major structural rearrangements

Time Identical chromosomes able to recombine (to swap segments) ———> SRY gene arises First structural recombination Matching areas still able to recombine . 3, 240—320 " 130—170 ‘ 350 million 3 million ‘ million years ago : years ago years ago _:”_, Second structural change: change: further recombination Present SRY moves the short arm of the Y ' 80—130 \ 30—50 >W million million j years ago years ago ;. Third structural change: additional recombination Fourth structural change: recom- bination failure Centromere fai|ure and SRY failure and SRY failure and Y SRY and severe SRY chromosome decay (second chromosome shrinkage shrinkage (first stratum) degradation (fourth stratum) stratum) (third stratum) Autosome Nascent Nascent Non-matching regions pair in Y X unable to recombine reptilemammal Y X Y X Y X Y X common ancestor Monotremes— Marsupial— Monkey— Human— mammal placental nonsimian monkey divergence mammal diverqence divergence divergence