The chromosomes in both somatic and germ-line cells of the sand rat (Psammomys obesus) were studied. The diploid number of chromosomes was 48. The autosomes consisted of 4 pairs of large metacentric chromosomes, 10 pairs of medium to small metacentric and submetacentric chromosomes and 9 pairs of telocentric chromosomes. The X chromosomes in both sexes were the largest of the entire complement. The Y chromosome was a mediumsized metacentric chromosome. The two X chromosomes in the female cells were similar in size and morphology but differed in the DNA replication pattern. Studies of the meiotic chromosomes in the male germ-line cells revealed 23 autosome bivalents and the sex chromosomes either as an XY bivalent or X and Y univalents.
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