402 

 Parents 



THE NEW INDIVIDUAL 



Black short 

 haind male 

 (.homozygous) 



Part IV 



White long haired 



female 



(homozygous) 



Gametes 



First 

 generation 



Qametes 



Eggs 



(D(D® 



^o" 



Second 

 generation 



All black 

 short haired 

 ^heterozygous) 



Ratio: 

 9 Black sliort 

 3 Black long 

 3 White slwrt 

 I White lonj 



Fig. 20.11. Offspring of dihybrid guinea pigs, a male with short black hair and 

 a female with long white hair. The genes for short and black are dominant. Chart 

 of the combinations of genes that produce them with the combinations in the sex 

 cells. (Courtesy, Winchester: Genetics. Boston, Houghton Mifflin Co., 1951.) 



Multiple Hybrids. Three independent pairs of contrasted characters are 

 governed by the same principles as two. The only difference is the greater 

 variety of gametes and the larger number of possible combinations of genes. 

 Animals in general are hybrids for hundreds of different characters; human 

 beings are probably the greatest mixture of all. If an animal had only ten 

 pairs of contrasting characters, each pair on a different pair of homologous 

 chromosomes, it could produce 1,024 types of gametes which could in turn 

 form 1,048,576 combinations. But animals actually have thousands of char- 

 acters. This is the reason that no two children inherit the same combination 

 of traits unless they are identical twins (Fig. 19.20). 



Linkage of Genes 



Linked genes are those that are located in the same chromosome and in- 

 herited together. This is an exception to MendeFs Independent Assortment of 

 Genes which still holds for genes that are located on different chromosomes. 



