400 



THE NEW INDIVIDUAL 



Part IV 



may differ genotypically. It is used commonly in analyzing F] dominants 

 by crossing them with their pure recessive parents, hence the name backcross. 

 In the preceding cross of guinea pigs, this would be a cross between a black 

 guinea pig of Fi and its pure white parent (Fig. 20.10). If the black is a 

 hybrid, the offspring are black and white guinea pigs in equal numbers. In 

 the hybrid black guinea pig of Fi, half of the sex cells carry a gene for black 

 (W) and half of them a gene for white (w). In the white parent P every sex 

 cell carries a gene for white (h'). The cross results in the half white and half 

 black of black and white animals (Fig. 20.10). This figure also shows a 

 similar result for another pair of contrasting characters, short-long hair, where 

 short is dominant to long. 



Dihybrid Cross. A dihybrid cross is one between organisms that differ 



Parents 



[•• 



Black male 

 carrying 2 

 black gems 



Gametes 



carrying 

 \(me black 



White female 

 carrying 2 

 white ^enis 



carrying 

 /"^^m white ^ 

 (O) 9^*^ (O 



First 

 generation 



Gametes 



Cross these two 



All black 

 carryin^one 

 black and 

 one white 

 gene 



(•) ^39s (O) 



Second 

 generation 



Ratio: 

 3 black 



to 

 1 white 



Fig. 20.9. Monohybrids, offspring of guinea pigs which differ in one color. The 

 original parents, P,, are pure-line blacks and pure-line whites, black W being domi- 

 nant over white h'. The resulting progeny show the behavior of a dominant gene. 

 They also show the effects of the all-important separateness of genes, that the 

 character white which was lost from sight in the first or F^ generation reappeared 

 unaffected in the Fo generation. Each animal of F^ is in appearance (phenotypi- 

 cally) black but in gene content (genotypically) black and white. When the hy- 

 brids of Fi are crossed, their offspring F2 show the typical Mendelian ratio of 

 three dominants to one recessive. When animals of the F2 generation are inter- 

 crossed, one-fourth are pure black, one-fourth pure white, and two-fourths black 

 (black and white). When crossed these animals that contain genes for black and 

 white produce blacks and whites in the 3:1 ratio. (Courtesy, Winchester: Genetics. 

 Boston, Houghton Mifflin Co., 1951.) 



