558 GENETICS AND EVOLUTION 



are heterozygous for both hair length and hair color genes and have the 

 genotype BlaSs. Each ol the h\ individuals will produce four kinds of 

 gametes, BS, Bs, bS and bs, and there will be equal numbers of each 

 type. When two of these Fi individuals are mated, there will be sixteen 

 possible combinations in the Fo (Fig. 32.2), with a phenotypic ratio of 

 y black, short : 3 black, long : 3 brown, short : 1 brown, long. This 

 9:3:3:1 ratio is characteristic of the second generation of a cross of 

 individuals differing in two traits whose genes are located in non- 

 homologous chromosomes. This is, of course, a probability ratio, which 

 means that there are nine chances out of sixteen that any particular 

 offspring will have black, short hair, three chances out of sixteen that it 

 will have black, long hair, three chances out of sixteen that it will have 

 brown, short hair, and one chance in sixteen that it will have brown, 

 long hair. The genetic mechanism underlying Mendel's Second Law, 

 the Law of Independent Assortment, should now be clear. 



The results of crosses with three or more different pairs of genes 

 may be predicted by similar reasoning. The Fi individuals of a trihybrid 

 cross will produce eight different kinds of gametes in equal numbers, 

 and the random union of eight types of sperm and eight types of eggs 

 gives 64 different combinations of genes in the Fo generation. In the pea 

 plant studied by Mendel, the crossing of a plant with round, yellow 

 seeds and long stems (YYRRLL) and a plant with wrinkled, green seeds 

 and short stems (yyrrll) yields Fi individuals with the genotype YyRrLI, 

 all with round, yellow seeds and long stems. When these plants are self- 

 fertilized, offspring are produced in the ratio of 27 yellow, round, 

 long : 9 yellow, round, short : 9 yellow, wrinkled, long : 9 green, 

 round, long : 3 yellow, wrinkled, short : 3 green, round, short : 3 green, 

 wrinkled, long : and 1 green, wrinkled short. 



Set up a Punnett square with the eight types of eggs across the top 

 and the eight types of sperm down the sides. Fill in the 64 squares with 

 the appropriate Fo genotypes and add up the phenotypes. Compare the 

 phenotypic ratio you obtain with the one given here. 



275. Problem Solving 



The science of genetics resembles mathematics in that when one 

 has a firm grasp of the few basic principles involved he can solve a wide 

 variety of problems. These basic principles include: (1) Inheritance is 

 biparental; both parents contribute to the genetic constitution of the 

 offspring. (2) Genes are not altered by existing together in a hetero- 

 zygote. (3) Each individvial has two of each kind of gene, but each gamete 

 has only one of each kind. (4) Two pairs of genes located in different 

 chromosomes are inherited independently. (5) Gametes unite at random; 

 there is neither attraction nor repulsion between an egg and a sperm 

 containing identical genes. 



In working genetics problems, it is helpful to use the following pro- 

 cedure: 



1. Write down the symbols used for each gene. 



2. Determine the genotypes of the parents, deducing them from 



