510 



THE PRINCIPLES OF HEREDITY 



Parents 



■ Pure red hull 

 carrying 2 red 

 genes in each cell 



Gametes 



'Sperm carry ■inip 



fa one red gene fa 



W each W 



Pure white cow 

 carrying 2 white 

 genes in each cell 



Eggs carrying 



©one white 

 gene each i q 



First 

 Generation 



All roan 

 carrying one 

 red ana one 

 white gene 



Gametes 



faSperm carrying G\Sperm carrying/" N 

 \J red gene \y white gene | 



Egg carrying 

 red gene 



Second 

 Generation 



Egg carrying 

 white gene 



I Red 



2 Roan 



I White 



Fig. 32.4. Diagram showing intermediate inheritance of hair color in short-horned 

 cattle. Note that the combination of red and white genes gives roan. 



will be type A. Thus, we have both intermediate and dominant-recessive 

 inheritance in the case of the genes for blood types in man. 



Independent Segregation of Genes 



When two or more genes are traced at the same time it is usually 

 found that they follow the pattern of the monohybrid crosses inde- 

 pendently of one another. We can make another cross with guinea pigs 

 to illustrate this important fact. There is a pair of genes which in- 

 fluence the length of the hair on the body. The dominant form of this 

 gene produces short hair and the recessive form produces long hair. 

 Now, suppose we cross a guinea pig that has black short hair (homo- 

 zygous for both pairs of genes) with a guinea pig that has white long 

 hair. We know that the latter is homozygous for both genes because 



