(2) a heterozygote yellow w'lthahomozygote green. Of the 

 male gametes half will contain the factor for yellow, the 

 other half for green; all the female gametes will contain the 

 factor for green. The possible matings may be represented 

 as follows: 



Female 

 Gametes 



Y 



= Male Gametes. 



= 2 YG + 2 GG 



Phenotypically half are 

 yellow and half are green, but 

 genotypically half are impure 

 yellow and half are pure green. 



Two Pairs of Characters in Dihybrids 



"When two pairs of characters hybridize, such as 

 wrinkled-green peas with smooth-yellow, the Mendelian 

 law still holds good. On the assumption that a gamete 

 can contain only one of each pair of factors, one-fourth of 

 the male gametes will contain the factors for smoothness 

 and greenness (SG), one-fourth the factors for smoothness 

 and yellowness (SY), one-fourth the factors for wrinkled- 

 ness and greennes (WG) and one-fourth for wrinkledness 

 and yellowness (WY).i Similarly for the female gametes. 

 When these gametes unite, the possible combinations may 

 be represented as follows: — 



or SSGG -f 2SSYG + 2SWGG + 4SWYG + SSYY + 

 2SWYY+ 2WWYG + WWGG + WWYY— nine differ- 

 ent combinations or genotypes. However, as S is domm- 

 ant to W, and Y to G, there will be 9 smooth-yellow, 3 

 smooth-green, 3 wrinkled-yellow and 1 wrinkled-green 

 phenotypes, four in all. The ratio is, therefore, 9:3:3:1. 

 (Fig. 16). 



(])— In the calculation of Mendelian results. Castle gives this import- 

 ant bit of advice- "It is all es.sential to determine first the kmds 

 of gametes each 'parent is expected to produce. The subsequent 

 calculation is easy and certain." 



87 . 



