SEGREGATION AND DOMINANCE 



139 



When a homozygous individual with dark curly 

 hair crosses with a homozygous individual with light 

 straight hair, all the offspring have dark curly hair. 



The dark curly-haired individuals of this second 

 generation, however, are heterozygous with respect 

 to each of these two hair characters. When any two 

 individuals having this particular genotypic compo- 

 sition mate, therefore, 

 they may produce any 

 one of four possible 

 phenotypes dark 

 curly, dark straight, 

 light curly or light 

 straight haired individ- 

 uals. These four phe- 

 notypes in turn will 

 present nine different 

 genotypic combina- 

 tions out of sixteen pos- 

 sible cases, as shown in 

 Figure 46. 



Figure 45 further- 

 more serves to make 

 clear, first, the distinc- 

 tion between somato- 

 plasm and germplasm; 

 second, the maturation 

 of germ-cells ; third, the 

 segregation of gametes ; and fourth, the formation of 

 zygotes in sexual reproduction. 



The cells of the somatoplasm are represented as 



FIQ. 46. Diagrams showing the pos- 

 sible genotypic and phenotypic com- 

 binations resulting when two hetero- 

 zygous individuals, with dark curly 

 hair, mate. Symbols are the same 

 as in Figure 45. 



