Monohybrids 83 



It can be seen, therefore, that all the offspring of a homozygous 

 dominant will be phenotypically dominant irrespective of the 

 genetic constitution of the other parent, since all the gametes of 

 the homozygous dominant would contain the dominant gene. 



If a homozygous wild type is crossed with a curved, all the 

 offspring will be heterozygous and will look like the dominant 

 parent. The generation of the parents is called the parental, or 

 Pi, generation; the generation of the offspring is known as the 

 first filial, or Fi, generation (pronounced "eff one"). Since all 

 the Fi flies are alike genotypically, all will produce the same 

 kinds of gametes. Since all are heterozygous, one half the 

 gametes of each fly will be C and the other half will be c. If two 

 Fi flies are mated together, the grandchildren of the original two 

 parents will be produced. This generation is known as the second 

 filial, or F2, generation. One half the eggs of the Fi fly which is 

 used as a female will contain C and one half will contain c. Simi- 

 larly, the male Fi fly wdll produce C and c sperm in equal 

 numbers. 



Whether a C-bearing sperm unites with a C or with a c egg 

 is a matter of pure chance as the probabilities are theoretically 

 even. The same is true for the sperm which bear c. The chance 

 that a C egg will be fertilized by a C or by a c sperm is exactly 

 even, and the same is true for a c egg. Therefore, four combina- 

 tions are possible in the F2 and will exist in equal numbers: 

 CC, Cc, cC, and cc. Since Cc and cC flies are alike genotypically, 

 the nature of the F2 can be written as a ratio of ICC : 2Cc : Ice. 

 In terms of fractions, the F2 population will be y4:CC : Y2CC : 

 y4cc. These ratios, however, are genotypic. As both homozy- 

 gous C and heterozygous flies are phenotypically alike, the pheno- 

 typic F2 ratio is 3 normal-winged : 1 curved or % normal and % 

 curved. This is shown diagrammatically in Fig. 28, in which the 

 method of arriving at the F2 is determined by the conventional 

 "checkerboard." It must be understood that if only four flies 

 are produced, three of them will not necessarily be normal and 

 one curved. This ratio is theoretical, based on chance, and 

 means that out of a large number of cases, approximately three- 

 quarters will be normal and approximately one-quarter will be 

 curved. The larger the number of F2 flies, the more nearly the 

 numbers obtained may be expected to approximate the theoretical 

 ratio. 



