304 



ANIMAL BIOLOGY 



tion in the gametes and the diploid in the zygote. This close paral- 

 lelism of character and chromosome behavior affords further 

 proof that the chromosomes through their constituent genes de- 

 termine the physical basis of inheritance, and that segregation 

 and related phenomena are facts. For all practical purposes, 

 A, B, C, D, and a, b, c, d, in Figures 167 and 189 may be inter- 

 preted either as chromosomes or as characters (genes). 



Turning now to the inheritance of characters whose genes are 

 borne by the same chromosome : these would seem to be indissolv- 



Zygote, (5 

 Polocyte 



{ °%) jMaturation 



Q 



Maturation /^ 



Spermatogonium. (°a 



Mature Egg 



Zygote , (j> 



Oogonium 



Fig. 190. — Diagram to show the relation of the two classes of sperm in 

 fertilization. The formation of gametes in the male is shown at the left, in the 

 female at the right; fertilization, producing the male or female zygote, in the 

 center. Somatic chromosome number assumed to be six. X chromosome 

 (large) and Y chromosome (small) in black. 



ably linked together. And since the chromosome number is usually 

 not large — there are twenty -four chromosomes in the gametes of 

 Man — compared with that of heritable characters, we would 

 expect sometimes to find characters linked together. That is, not 

 separately inherited, or independently assorted, as are yellow and 

 round in our example. In reality many cases are known in which 

 characters are usually inherited together. The inheritance of sex 

 and sex-linked characters will make the main point clear, and at 

 the same time serve to bring before us the essential facts in regard 

 to the determination of sex. 



1. Sex Determination 



Intensive studies of the chromosomes in the somatic cells and 

 in the germ cells before the maturation divisions have shown that 



