Segregation of Alleles 



35 



paired genes in question, like CC or cc, 

 whereas a heterozygote, or hybrid, is im- 

 pure in this respect, like Cc. 



An independent test of all the genetic 

 hypotheses presented so far can be made in 

 the following way. F, colored plants are 

 crossed with colorless plants, this cross being 

 symbolized genetically: F, Cc X cc. As the 

 result of segregation half of the offspring 

 should receive C and half c from the Cc 

 parent, and all should receive c from the cc 

 parent. The genotypes of the offspring from 

 this cross should be, theoretically, Cc 50% 

 of the time and cc 50% of the time, and the 

 expected phenotypic ratio should be, there- 

 fore, 1 {» colored: l / 2 colorless. This expecta- 

 tion is fulfilled experimentally (85 colored: 

 81 colorless). 



Are the principles just established gen- 

 erally applicable? Thus far they apply 

 strictly only to the genetic determination of 

 flower color in garden peas. All these ideas 

 can be tested six additional times, using six 

 other traits in garden peas, each of which 

 occurs in two clearcut alternatives and ful- 

 fills the prerequisites for suitability already 

 described. In each case, when two appro- 

 priate pure lines are crossed, the Fi hybrids 

 produced are phenotypically uniform, as be- 

 fore. Moreover, self-fertilization of the F, 

 produces Fj in approximately the expected 

 1:2:1 genotypic ratio. 



Recall that the Cc phenotype is indistin- 

 guishable from CC. In Cc individuals the 

 phenotypic expression of c is masked by the 

 expression of C. The ability of a gene to 

 express itself phenotypically in the presence 

 of a different allele is described in terms of 

 dominance. In the case of flower color, C 

 produces a dominant effect when present 

 with c, whose effect is, accordingly, recessive. 

 It should be emphasized that our concept of 

 the gene does not depend upon the occur- 

 rence or nonoccurrence of dominance. In- 

 deed, testing our genetic postulates has been 

 made more complicated by the fact that the 



effect of C is, for all intents and purposes, 

 completely dominant to that of c. The Fi Cc 

 expressed only C, and the presence of c was 

 detected only by breeding F, individuals, and 

 observing cc progeny. Only by breeding the 

 colored F L > were we able to determine that ' :{ 

 were CC and % Cc. Dominance, then, re- 

 fers to the phenotypic expression of genes 

 in heterozygous condition and has no rela- 

 tion to their integrity, replication, or mech- 

 anism of transmission. For convenience, 

 dominant and recessive will be used here- 

 after to refer to genes, but the precise mean- 

 ing of these terms should always be kept 

 in mind. 



As mentioned, six other traits have been 

 used to test the general applicability of the 

 gene concept. In each case it happened that 

 one allele was dominant to the alternative 

 one in the hybrid. It is tempting to con- 

 clude that dominance is a universal phe- 

 nomenon since it was found to hold for 

 each of seven different traits based on genes 

 in the garden pea. Before making this de- 

 cision, however, examine the results with re- 

 gard to feather color of breeding certain 

 chickens. Here black X white produces 

 blue-gray F]. Mating two blue-gray Fx pro- 

 duces in F_. ] 4 black, 1 /> blue-gray, and % 

 white. In this case complete dominance 

 does not occur, so that complete dominance 

 is not a rule for the phenotypic expression 

 of alleles in heterozygotes. Whenever domi- 

 nance is incomplete or absent, genotypes can 

 be stated with certainty from a knowledge 

 of phenotypes. 



Cross-fertilization made it possible to 

 show that genes occur as pairs, which be- 

 come unpaired after segregation, then re- 

 combine to form pairs in the offspring. In 

 other words, the view that the genetic ma- 

 terial contains separable paired units is based 

 upon the recombination which these units 

 undergo in cross-fertilizing species. The 

 meaning of the term genetic recombination 

 ought to be considered at this point. The 



