1 62 READINGS IN BIOLOGICAL SCIENCE 



is merely a matter of chance. In other words, the reduced number of 

 chromosomes in any germ cell is a random assortment of the original 

 chromosomes of maternal and paternal origin. The only necessity is that 

 each final germ cell {gamete) have one of each kind of chromosome, since 

 each kind carries special hereditary determiners and has its own particular 

 role to play in the development of the new individual. Thus, for instance, 

 if we arbitrarily represent the chromosomes of a given individual by ABC 

 abc, and regard A, B and C as of paternal and a,b, and c as of maternal 

 origin, then in synapsis only A and a can pair together, B and b, and C 

 and c; but each pair operates independently of the other so that in the 

 ensuing reduction division either member of a pair may get into a cell with 

 either member of the other pairs. That is, the line-up for division at a 

 given reduction might be any of the following: ABC, ABc, Abc, AbC. 



abc abC aBC aBc 

 This would yield the following eight kinds of gametes, ABC, abc, ABc, 

 abC, Abc, aBC, AbC, aBc, each kind of chromosome required to cover the 

 entire field of characters necessary to a complete organism. Since ova and 

 sperm would be equally likely to have these eight types of gametes, the 

 possible number of combinations for such germ cells would be 8 X 8, or 



64- 

 Computed on this basis, in man with his 24 pairs of chromosomes the 



number of different combinations producible would be 282,429,536,481. 

 The fear of standardization in mankind expressed by some of our literary 

 folk, does not, therefore, seem very alarming. Sex makes anything like com- 

 plete standardization virtually impossible. 



As noted, the X and Y chromosomes of man constitute a pair originally, 

 but after the reduction division half of the total number of sperm cells 

 will contain an X, the other half, a Y. Females of species such as man, how- 

 ever, are characterized by the possession of two X's and no Y. This means 

 that after the reduction division every ovum will carry an X. It is evident, 

 therefore, that there are equal chances of producing an XX type or an XY 

 type of fertilized ovum. The XX type develop into females, the XY t)^pe 

 into males. Thus sex is automatically launched on its course at the very 

 inception of development. Since there is no Y element in many species of 

 animal, X substance seems in some way, to be the determining factor. 



Sex chromosomes are not only agents of the sex-determining mechanism; 

 they also carry the determiners of certain hereditary characteristics. Such 

 traits display what is termed sex-lmked inheritance. The form of color 

 blindness characterized by inability to distinguish red from green is such 

 a sex-linked trait. In the color-blind man the single X carries the gene of 

 this defect. Mated to a woman with both X chromosomes normal (XX), 

 their daughters although getting the paternal defective X, also get a normal 

 X from the mother, and this normal X is sufficient to insure normality of 

 vision in all daughters (XX), or in other words, the character is what is 



