GENETICS 



225 



trary, have a double set, two genes for each character. When egg and 

 spermatozoon unite in fertihzation, the zygote receives a double set of 

 genes, and these are handed on as a double set to all the cells of the 

 body of the individual produced from that zygote. With this under- 

 standing of the mechanism of heredity we may now turn to some con- 

 crete examples of its operation. 



Yell 01^ Body 

 Nhife £ye 



I Oroi/ Body 

 Red Eye 



Norma/Win^ 



^NonmalU/inq 



Verm f a on 

 E.ye 



MiniaturQ 

 Nino 



RedEye 

 Miniature 



Rudimen-' 

 fan^Ninq 



Forked 

 Brisf/es 



Co/n/JsfeEi^e 



Rud I men- 

 fa r^l^in^ 



Forked 

 Bnisiles 



Ban £y6 



Fig. 192. — Diagrams of two homologous chromosomes of the vinegar fly Drosophila. 

 Some of the genes are represented, and are in their proper order through the length of the 

 chromosome. Homologous genes are located at the same level in the two chromosomes. 



Simple Inheritance. — Among guinea pigs there are different color 

 varieties which breed true so long as animals of the same color are mated 

 Avdth one another. One of these true-breeding strains is black (Fig. 193), 

 another one is albino or wdiite, from the absence of all of the ordinary 

 pigments in skin and hair and the iris of the eyes. If a black animal is 

 mated with a white one, the offspring are all black. This result is 

 described by saying that black is dominant, white recessive. The hybrid 



