EVOLUTION 



IIOI 



recessive character. But of the three showing the dominant character, 

 one will be "a pure extracted dominant", yielding only dominants 

 when paired with another like itself, while the other two, the "impure 

 dominants" will, if inbred, yield pure dominants, pure recessives, and 

 impure dominants again, in the average 1:2:1 ratio. 



D(R) X D(R) 



1 



I DD + 2 D(R) + RR 



i i 



DD DD 



The "extracted pure recessives" will yield only pure recessives 

 if inbred or bred with others of similar history. 



A giant pea crossed with a dwarf pea yields giant offspring only. 

 Tallness is dominant, shortness is recessive. The hybrid offspring 

 (F^), left to self-fertilise, will yield in the second filial generation 

 (F*), 25 per cent, pure tails, 25 per cent, pure dwarfs, and 50 per 

 cent, impure tails. 



The reason for the precise ratios (often closely verified in an aver- 

 age of largish numbers) is that the germ-cells of the hybrids (F^) 

 become segretated (in the maturation divisions) into two contingents, 

 one set with the gene for the dominant character, and the other set 

 with the gene for the recessive characters. If fertiUsation is fortuitous, 

 an egg- cell with a D gene may be fertilised by a sperm with the D 

 gene, or by a sperm with the R gene, and an egg-cell with an R gene 

 may be fertilised by a sperm with the R gene or by a sperm with a D 

 gene. Thus the results must be in the ratio 25 DD : 50 D(R) : 25 RR. 

 When the two parents differ in more than two contrasted charac- 

 ters the formula will, of course, be more complicated. 



The complete scheme for monohybridism — a pair of contrasted 

 but corresponding features (or allelomorphs) stands thus : 



Parents 



First FiUal Generation 



Second Filial 1 

 Generation J 



25% 

 DD 



X X 



+ 



D X R 



D(R) 



X X 



D(R) 



50% 

 D(R) 



X X 



+ 



25% 

 RR 



Third Filial \ 0/ t^t^ / t^t^ , t^Jox , ^Dr>^ -oo 0/ 



Generation J ^^^ % ^^ (^5 DD + 5o D(R) + 25 RR) RR 100 % 



