FACTS OF INHERITANCE 149 



both breeding true — and a number of forms, 

 usually like pure dominants, whicli, when inbred, 

 again produce " pure dominants," " pure 

 recessives," and " impure dominants " like them- 

 selves. In typical cases, where attention is paid 

 to one pair of contrasted characters, the proportions 

 of the " hybrids " always approximate to the 

 formula — 1 pure dominant : 2 impure dominants : 

 1 pure recessive. 

 This may be expressed in a simple schema : 



D X R Parents 



I ^1 



D(R) Hybrids. F» 



D(R) 



IDD + 2D(R) + IRR 2nd Generation F» 

 Extracted Impure Extracted 



Pure Dominants Pure 



Dominants X x Recessives 



I I I 



DD 1DD + 2D(R)+RR RR 3rd Generation F' 



Mendel explained his results by the ingeniously 

 simple hypothesis of segregation. He supposed 

 that the germ-cells of the hybrids segregated into 

 two contingents, one half bearing the dominant 

 character and one half bearing the recessive 

 character. If fertilisations follow the laws of 

 chance the second generation should theoretically 

 show the proportions which actually occur. When 

 there are two pairs of contrasted characters — 

 for instance, when a tall yellow-seeded (Dd) pea 

 is crossed with a dwarf green-seeded one (Rr) — 

 the offspring are tall yellows (Dd), combining the 



