FERTILITY OF PARENT SPECIES AND HYBRIDS. 93 



per cent aa, or fertility in one-half of the F2 males. In Mendelian nota- 

 tion it would be: 



A+A wild gametes. 



a -j- a tame gametes. 



^- + ^- * -Id ^yg°t-{Sf?maL. 



A + a J wild eggs. 



a -j- a tame sperm. 



50 p. ct. fertile males. 



Aa + aa J wild zygotes 



60 p. ct. sterile females. 

 100 p. ct. fertile females. 



Furthermore, although all j wild females would be fertile, half of them 

 would transmit sterility in the next back-cross to guinea-pigs. If the 

 two classes of females occurred in about equal frequency (as one would 

 expect) then 75 per cent of the | wild males would be fertile. Express- 

 ing this mating in the usual terms, it would read as follows: 



Aa + aa J wild female zy 



A+a+a + a J wild eggs. 



a-j-a tame sperm 



{75 p. ct. fertile males. 

 25 p. ct. sterile males. 

 100 p. ct. fertile females. 



Now, if the numbers were large, and the different zygotic classes of 



I wild females were represented in the expected proportions, then 



seven-eighths or 87.5 per cent of the ^V ^iW males should be fertile. 



2""' — 1 

 In any generation i males should be fertile (n being the number 



of the hybrid generation) . 



Table 77 gives the probable percentages of fertile males expected in 

 each generation from the Fi to the F9 inclusive, it being supposed that 

 very large numbers are involved and that the females of any generation 

 are distributed approximately in the expected proportions of the differ- 

 ent zygotic classes. Our actual experimental data show that the case 

 is far from being as simple as this, for the percentage of fertile males 

 in each generation does not agree with the series expected on the basis 

 of one factor as given in table 77. Furthermore, on the basis of one 

 factor, the males would also be divided into two distinct classes: sterile 

 (Aa) and fertile (aa). It was shown that this was not the case. The 

 hypothesis, at least in this simple form, does not agree with the facts. 



Now, if the sterility of the males had been due to disturbances 

 between either one or both of two Mendelian pairs of factors or pairs 

 of homologous chromosomes, then we should have expected one-fourth 

 of the F2 \ wild males to be fertile. If we represent the two factors 

 from the wild as A and B, and the two from the tame as a and b,then 

 the mating of the wild, AABB, with the tame, aabb, would give hetero- 



