482 



GENETICS 



of a character in an adult organism, it is carried by the chromosomes. 

 These hypothetical determiners of inherited characters are known 

 as Mendolian units, factors, or genes. It is believed that many 

 genes are found in a single chromosome and that they are arranged 

 in a row along the length of the chromosome. They are, there- 



Zygotes from 



which Pj 



developed 



Gametes of Pj 



Zygotes of F, 



Gametes of F, 



Zygotes of F^ 



Red-eyed females Red-eyed male White-eyed male 



Fig. 257. — Diagram illustrating independent segregation and possible recom- 

 binations of X-chromosomes carrying genes determining eye-color, in a cross 

 between a red-eyed female Drosophila and a white-eyed male. (C/. Fig. 250, 



p. 472.) This is the reciprocal of the cross shown in Fig. 258. 



X. X-chromosome carrying gene for red eye-color; J^, X-chroniosome carrying gene for 

 white eye-color; ][ , Y-chromosome which is confined to males and does not carry genes. 



fore, equally distributed to all cells that are derived from a zygote 

 by mitosis. The mechanism of independent assortment of the 

 members of chromosome pairs during maturation of the germ cells, 

 together with the possibilities of recombination, furnishes a morpho- 

 logical basis, within the cell, upon which Mendelian ratios may be 

 explained. 



