34 



( II \l'l p.r 3 



P CC x cc (Cross - fertilization) 



I J 



O. all C all c (Gametes) 



all Cc 



Cc x Cc (Self-fertilization of F, 



A A 



Vi C, Vt c Vi C, Vic 



Male gametes 

 Vi C 7* c 



Female ' 7 C 



gametes ,/ 

 3 Vi c 



or ' 4 CC »/j Cc V, cc 



when * t 



P 3 self- breeds breeds breeds 



fertilized like like like 



P, CC P Cc P cc 



figure 3—1. Genotypic model proposed to 

 explain the phenotypic results of certain crosses 

 involving colored and colorless flowers in pea 

 plants. 



also have an equal chance of receiving C or 

 c from the female, so that the contribution 

 to all the offspring genotypes will be 25% 

 Cc and 25% cc from this source. On this 

 basis the F L . would be predicted to contain 

 25% of individuals that arc CC. 50% that 

 are Cc, and 25% cc. This expectation can 

 be expressed as relative frequencies in 

 several ways: % CC: '{> Cc % cc, or 1 CC: 

 2 Cc : 1 cc, or .25 CC : .50 Cc : .25 cc. As al- 

 ready reasoned CC and Cc are phenotyp- 

 ically indistinguishable, having colored flow- 

 ers, so that phenotypically 75% of the F 2 

 would be colored and 25% would be color- 

 less. What is their relative frequency in the 

 F 2 actually observed? 



Although a penny has in theory a 5() r i 

 chance of falling head up and a 50% chance 

 oi falling tail up, obviously a sufficiently 

 large Dumber of tosses is required to obtain 

 approximately 5095 heads, 509? tails. Sim- 

 ilarly, an accurate test of the theoretical ex- 

 pectation of 75% colored and 25% color- 

 less will be obtained only if a sufficiently 

 large sample of offspring is scored. Instead 

 of scoring the offspring of just one P 2 , then, 

 the results for the offspring of all P 2 should 

 be totalled. When this is done, the actual 

 F 2 results (among 929 plants. 75.9% are 

 colored and 24. 1 % colorless ) are very close 

 to the expectation. 



It should be emphasized that the concept 

 of paired, untaintable, segregating genes has 

 not depended upon obtaining any particular 

 phenotypic ratio for the F 2 . Granting these 

 characteristics of the genetic material, ob- 

 taining or not obtaining the phenotypic ratio 

 :; , colored to % colorless merely tests the 

 suppositions ( 1 ) that there is an equal 

 chance for offspring to receive either haploid 

 product of gene segregation from a parent, 

 and (2) that the haploid products from dif- 

 ferent parents come together at random to 

 restore the diploid condition. 



If all the assumptions so far made about 

 genetic material are correct, the 75% of F 2 

 that are colored should have one of two 

 genotypes: Vs of them should be CC, breed- 

 ing like pure-line CC individuals, and % 

 should be Cc. breeding like the F, Cc indi- 

 viduals. Accordingly, each F 2 colored plant 

 is permitted to self-fertilize and, in fact, very 

 nearly K produce only colored F :i . whereas 

 -;. produce F ;! of both colored and colorless 

 types. The theoretical genotypic ratio ex- 

 pected in the F L ., % CC:' 1 ^ ^ c l A cc > is, in 

 this way, fully confirmed in experience. The 

 gene model proposed to explain these pheno- 

 typic results is summarized in Figure 3—1. 

 It is convenient to introduce two additional 

 terms at this time. A homozygote is an 

 individual that is pure with respect to the 



