Gene Segregation 



11 



all offspring 25% will be CC and 25% Co. 

 Those offspring receiving c from the female 

 (50% of offspring) will have an equal chance 

 of receiving C or c from the male, so that the 

 contribution to all the offspring genotypes 

 will be 25% Co and 25% cc from this source. 

 On this basis the F2 would be predicted to 

 contain 25% of individuals that are CC, 50% 

 that are Cc, and 25% cc. This expectation 

 can be expressed as relative frequencies 

 in several ways: K CC : ji Cc : ji cc, or 

 \ CC : 2 Cc : I cc, or .25 CC : .50 Cc : .25 cc. 

 As already reasoned CC and Cc are pheno- 

 typically indistinguishable, having colored 

 flowers, so that phenotypically 75% of the 

 F2 would be colored and 25% would be color- 

 less. What is their relative frequency in the 

 Fo actually observed? 



Although a penny has in theory a 50% 

 chance of falling head up and a 50% chance 

 of falling tail up, you realize that a sufficiently 

 large number of tosses is required to actually 

 obtain approximately 50% heads, 50% tails. 

 So, in the present case, an accurate test of the 

 theoretical expectation of 75% colored and 

 25% colorless will be obtained only if a 

 sufficiently large sample of offspring is scored. 

 Accordingly, instead of scoring just the off- 

 spring of one P2 we shall total the results for 

 the offspring of all P2. And when this is 

 done, it turns out that the actual results 

 (among 929 plants, 75.9% were colored and 

 24.1% colorless) are very close to expectation. 



It should be emphasized that obtaining or 

 not obtaining the phenotypic ratio % colored 

 to K colorless is a critical test neither for genes 

 being paired, nor of their untaintability, nor of 

 their segregation — these properties of genes 

 having been previously established on other 

 grounds. The ratio merely tests the ideas 

 that segregation of paired uncontaminable 

 genes results in an equal chance for offspring 

 to receive either haploid product of segrega- 

 tion from a parent and that the haploid prod- 

 ucts from different parents come together 

 at random to restore the diploid condition. 



If all the assumptions so far made are 

 correct, the 75% of F2 which are colored 

 should be of two genotypes, % CC, breeding 

 like pure line CC individuals, and % breed- 

 ing like the Fi Cc individuals. Accordingly, 

 each F2 colored plant is permitted to self- 

 fertilize and, in fact, very nearly % produce 

 only colored F3 whereas % produce F3 of 

 both colored and colorless types. The theo- 

 retical genotypic ratio expected in the F2, 

 % CC : H Cc : % cc, is, in this way, fully 

 confirmed in experience. The gene model 

 we have proposed to explain these pheno- 

 typic results is summarized in Figure 2-1. 

 It is convenient to introduce two additional 

 terms at this time. A homozygote is an indi- 

 vidual that is pure with respect to the genes 

 in question, like CC or cc, while a hetero- 

 zygote, or hybrid, is impure in this respect, 

 like Cc. 



An independent test of all the hypotheses 

 presented in this Chapter can be made in 

 the following way. Fi colored plants are 

 crossed to colorless plants, this cross being 

 symbolized genetically: Fi Cc X cc. As 

 the result of segregation half of the off- 

 spring should receive C and half c from the 

 Cc parent, and all should receive a c from 

 the cc parent. So, the genotypes of the off- 

 spring from this cross should be, theoret- 

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

 time, and the expected phenotypic ratio 

 should be, then, )i colored : ){ colorless. 

 This expectation is actually observed (85 

 colored : 81 colorless). 



The next question we may ask is, are the 

 principles we have established generally 

 applicable? Thus far they apply strictly only 

 to the genie determination of flower color 

 in garden peas. Now it is possible to test all 

 these ideas six additional times, using six 

 other traits each of which occurs in two 

 clear-cut alternatives and fulfills the pre- 

 requisites for suitability already described. 

 In each case, when two appropriate pure 

 lines were crossed the Fi hybrids produced 



