CHAPTER 2 



THE SEX-CHROMOSOMES AND 

 SEX-DETERMINATION 



In a very large number and in a wide variety of species it has 

 been shown that the male is to be distinguished from the 

 female by constant differences in the chromosome content 

 of the nuclei of their component cells. This difference takes 

 several forms. In certain species one sex possesses one 

 chromosome less than does the other, that is to say in one 

 sex the chromosomes are all paired whilst in the other one 

 member of one pair is lacking. In other species both sexes 

 possess the same number of chromosomes, existing in pairs, 

 but in one sex one particular pair consists of chromosomes 

 unequal in size and shape. 



The single unpaired chromosome found in one sex and 

 the pair in the other to which it corresponds, and the pair of 

 chromosomes in respect of which the sexes differ, are known 

 as the sex-chromosomes in order to distinguish them from 

 the rest of the chromosomes in respect of which both male 

 and female are alike. These are known as the autosomes. 

 The single chromosome found in one sex and the pair in the 

 other to which it corresponds and, in the case of the species 

 in which the chromosome number is the same in both sexes, 

 that chromosome which is found in both male and female, 

 are known as X- chromosomes. The unequal mate of the X 

 in one of the sexes is known as the Y-chromosome. Thus m 

 respect of sex-chromosome constitution the sexes can be 

 described as: 



XO:XX 

 or XY:XX 



The third form which this difference between the sexes 



takes is that in which the sex-chromosomes are represented 



not by single elements but by groups which during gameto- 



genesis behave as one compound chromosome. Whatever 



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