Chapter 4, Problem 22P

Remarkably, the platypus has 10 sex chromosomes, the largest number found in any mammal. The female platypus has five pairs of different X chromosomes (X1–X5), while the male has X1–X5, and also five different Ys (Y1–Y5). During meiosis in the male, the five Xs always end up together in one gamete, and the five Ys always end up together in another gamete. To achieve this segregation, during prophase of meiosis I the sex chromosomes form a long chain, always in the order X1 Y1 X2 Y2 X3 Y3 X4 Y4 X5 Y5, in which the chromosomes are held together through pseudoautosomal regions (PARs).

a.	How many different PARs must exist to allow the formation of these chains? (Hint: To answer this question, try drawing the chain of chromosomes.)
b.	In terms of pairing ability of the PARs, explain the structural differences between the human and platypus sex chromosomes.