HEREDITY 



579 



Expressed 



trait 



(Phenotype) 



Genetic 



constitution 



(Genotype) 



aabb 



AABB 



aai 



AABB 



Figure 406. Sketch showing the difference between phenotype and genotype. The potential 

 difference in skin pigmentation of a white person and a Negro is controlled by genes (genotype), 

 but the actual color of the skin (phenotype) will depend greatly on the extent of the individ- 

 ual's exposure to sunlight. 



individual with black curly hair, the off- 

 spring will all have black curly hair. Similar 

 results are obtained with respect to any 

 trait when the parents are homozygous, 

 since the genes that control the develop- 

 ment of the trait are all of one type. Since 

 some organisms are known to have thou- 

 sands of different kinds of genes, any cross 

 between individuals may serve as a test of 

 the inheritance of any or of all the traits 

 in which the two individuals differ. The in- 

 vestigator may concentrate his attention on 

 as many, or as few traits, as he wishes. Usu- 

 ally the smaller the number of traits studied 

 simultaneously, the easier it makes the inter- 

 pretations. No more than two pairs of traits 

 will be used in this book. We will start 

 with the simplest possible situation in which 

 the inheritance of one pair of alleles is stud- 

 ied. 



Monohybrids 



A monohybrid is the result of a cross be- 

 tween parents differing in respect to a single 

 gene. One of Mendel's (Fig. 463) experi- 

 ments with peas will serve to illustrate this 



situation. He crossed tall pea plants with 

 short pea plants. Figure 408 presents a dia- 

 gram of the results. Here each parent con- 

 tains two genes for height; one parent has 

 the dominant genes (SS) for tallness; the 

 other, recessive genes (ss) for shortness. 

 The offspring in the first filial (L. filia, 

 daughter; filius, son) generation (Fi) are 

 all alike, each receiving a dominant gene 

 (S) from the tall parent, and a recessive 

 gene (s) from the short parent. These off- 

 spring are all tall since the gene for tallness 

 (S) is dominant; but they carry within 

 them the recessive gene (s) for shortness. 



During maturation, as indicated in Fig. 

 404, 2 types of male gametes and 2 types of 

 female gametes are produced by the mem- 

 bers of the Fi generation; one half of each 

 are provided with a gene for shortness (s) 

 and the other half with the gene for tall- 

 ness (S). Fertilization brings about the re- 

 sults shown, that is, 25 per cent of the 

 zygotes will receive 2 genes for tallness 

 (SS); 25 per cent will receive 2 genes foi 

 shortness (ss), and 50 per cent will receive 

 1 gene for tallness and 1 for shortness (Ss). 

 The F2 generation will consist of 75 per 



