164 GENETICS 



are bred together they produce a certain percentage 

 of recessives lacking yellow which would not happen if 

 they were pure yellow. Hundreds of yellow individuals 

 have been tested but they always produce in addition 

 to yellow some non-yellow, that is, black, brown or gray 

 individuals. That the non-yellow individuals are re- 

 cessive is shown by the fact that when inbred, they 

 produce no yellow offspring, therefore, yellow is domi- 

 nant. 



In a Mendelian monohybrid cross, as has been pre- 

 viously pointed out, the expectation is that in the sec- 

 ond generation one fourth of the offspring will be re- 

 cessives (DR X DR = DD + 2 DR + RR), but when 

 yellow mice are bred together, the percentage of re- 

 cessives approximates one-third instead of one-fourth. 

 Little, in a total of over 1200 young produced by yel- 

 low parents, obtained almost exactly two-thirds yel- 

 low. This apparent exception to the Mendelian ratio 

 finds an explanation, however, when it is assumed that 

 D (yellow) is a lethal gene when present m duplex 

 (DD) form. The DDs drop out entirely which leaves 

 the proportion approximately two DRs and one RR. 

 This supposition is further supported by the fact that 

 the litters of young from yellow mice are, on an aver- 

 age, only three-fourths as large as normal litters of 

 mice, which is exactly what would be expected if one- 

 fourth of the possible gametic combinations (DD) fail 

 to produce offspring. Moreover, evidence of the death 

 m utero of the pure yellow mice has been produced by 

 Ibsen and Stiegleder, '17. 



