HEREDITY 



581 



Parents 





Parents 



Gametes 

 of parents 



F, All tall 



Gametes 

 of F, 



Gametes 

 of parents 



F, All tall 



Gametes 

 off, 



1 Homozygous tail: 2 Heterozygous tall: 1 Homozygous short 

 3 Tall plants: 1 Short plant 



Figure 408. Inheritance in height in a cross between a tall and a short variety of the edible 

 garden pea. Tall stem is dominant to short stem. S represents the gene for tallness; s represents 

 the gene for shortness. 



takes place is shown by use of a checker- 

 board (Fig, 409), which is a common 

 method used by geneticists in illustrating 

 the combinations of genes during hybridiza- 

 tion. As shown in Fig. 409, the Fo will con- 

 sist of 9 genetically different combinations, 

 and their phenotypes will be as follows: 9 

 yellow-round; 3 yellow-wrinkled; 3 green- 

 round; and 1 green-wrinkled. 



Menders laws 



These are simple illustrations of what 

 are commonly called Mendel's laws. Men- 

 del's First Law is the law of segregation 

 (separation); it states that there is a separa- 

 tion of the members of a pair of genes dur- 

 ing maturation so that each gamete of an 



individual contains one gene from each pair. 

 Thus when two such gametes unite at fer- 

 tilization, we shall have two genes for each 

 trait brought back together in the individ- 

 ual. This is true because of reduction divi- 

 sion. Mendel's Second Law is the law of 

 independent assortment; it states that the 

 distribution of each pair of genes to the 

 gamete is entirely independent of the dis- 

 tribution of any other pair. This second law 

 holds only if the pairs of genes are on sepa- 

 rate (nonhomologous) chromosomes. As in- 

 dicated in a later section on linkage, this 

 condition is not always met. According to 

 Mendel's laws, genes in the gametes behave 

 in heredity as units and are distributed 

 among the offspring according to mathe- 

 matical rules. Thus with monohybrid par- 



