35 o EXPERIMENTAL STUDY OF INHERITANCE 



But the exactness in the proportions of the three groups — i D 

 (pure dominants) + 2 D(R) (dominants with recessive character 

 latent) + i R (pure recessives) — lends very strong support to 

 Mendel's simple theory. 



§ 3. Elaborations 



Impure Dominants bred with Pure Types. — In the typical 

 cases discussed above, a hybrid form D(R) — an impure dominant 

 — is supposed to be self- fertilised or inbred. The results are accord- 

 ing to the formula i DD (pure or extracted dominants) + 2 D(R) 

 (dominant-recessives ) + i RR (pure or extracted recessives). 



But let us suppose the impure dominant or dominant- recessive 

 D(R) to be bred with a pure type — e.g. RR (extracted recessive) 

 (in technical phrase, a heterozygote unites with a homozygote). 

 The impure dominant has, by hypothesis, equal numbers of 

 two kinds of germ-cell — let us say, of egg-cell. The pure type 

 has only one kind of germ-cell — let us say, of sperm-cell. The 

 chances of fertilisation should be as follows : 



w ® + « egg-cells of impure dominant ; 

 w © + w sperm-cells of pure recessive : 

 The result will be 



n ova (D fertilised by n sperms Q = n offspring 

 n ova fertilised by n sperms = n offspring 



That is to say, equal numbers of impure dominants and pure recessives. 



" This is what actually happens on crossing a fowl having a 

 single comb (RR) with one having a heterozygous ' rose comb.' " 



Or let us suppose the impure dominant D(R) to be bred 

 with a pure dominant DD : 



^ © + w Q egg-cells of impure dominant ; 



n ® + w Q) sperm-cells of pure dominant : 



The result will be w -J) -t- « (J) equal numbers of impure dominants and 

 pure dominants. 



" Here again experiment has borne out theory." Therefore, 



as Mr. Punnett says, " the generalisation known as the principle 



of gametic segregation may be regarded as firmly established 



on the phenomena exhibited by plants and animals when strains 



are crossed which possess pairs of differentiating characters." 



