Q: Build a map of the chromosome(s) containingthe genes below andindicate the or

Question

Q: Build a map of the chromosome(s) containingthe genes
     below andindicate the order of the genes andthe
     distance betweenthem. All gene pair distancesnot
     shown were50. Genes    distance
A -C         10
B-E         15
B- F         45
C -G        40
D -G         20 Q: Build a map of the chromosome(s) containingthe genes
     below andindicate the order of the genes andthe
     distance betweenthem. All gene pair distancesnot
     shown were50. Genes    distance
A -C         10
B-E         15
B- F         45
C -G        40
D -G         20 Genes    distance
A -C         10
B-E         15
B- F         45
C -G        40
D -G         20 Genes    distance
A -C         10
B-E         15
B- F         45
C -G        40
D -G         20 Genes    distance
A -C         10
B-E         15
B- F         45
C -G        40
D -G         20

Explanation / Answer

To help understand the recombination between sex chromosomes,and its consequences for
of Y-linked genes, we have studied four X-Y gene pairs,including one gene not previously characterized in plants. We used the X-linkedcopies to build a genetic map of the X chromosomes, with a markerin the pseudoautosomal region (PAR) to orient the map. The mapcovers a large part of the X chromosomes—at least 50 centimorgans. Exceptfor a recent rearrangement in S. dioica, the gene order isthe same in the X chromosomes of all three species. Silent sitedivergence between the DNA sequences of the X and Y copies of the different genes increases with the genes'distances from the PAR, suggesting progressive restriction of recombination between the X and Y chromosomes. This wasconfirmed by phylogenetic analyses of the four genes, which also revealed that the least-diverged X-Y pair couldhave ceased recombining independently in the dioecious species after their split. Analysis of amino acid replacementsvs. synonymous changes showed that, with one possible exception, the Y-linked copies appear to be functional in allthree species, but there are nevertheless some signs of degenerative processes affecting the genes that have beenY-linked for the longest times. Although the X-Y system evolved quite recently in Silene (less than 10million years ago) compared to mammals (about 320 millionyears ago), our results suggest that similar processes have been atwork in the evolution of sex chromosomes in plants and mammals, and shed some light on the molecular mechanismssuppressing recombination between X and Y chromosomes.

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