90 Heredity and Environment 



In subsequent generations the progeny of the pure round (RR) 

 breed true and produce only round-seeded peas, whereas the pro- 

 geny of the hybrid round-wrinkled (RW) split up into pure 

 round, hybrid round-wrinkled, and pure wrinkled in' the regular 

 Mendelian ratio of I RR : 2 R(W) : I WW (Fig. 30). 



When a pure recessive is crossed with a mixed dominant- 

 recessive (Fig. 29, d) another typical ratio results. Thus if a 

 wrinkled-seeded variety of pea is crossed with a hybrid between 

 a round-seeded and wrinkled-seeded one, round-seeded and 

 wrinkled-seeded peas are produced in the proportion of i : I. This 

 is due to the fact that the hybrid produces two kinds of germ 

 cells, the pure-bred but one, and the possible combinations of 

 these are as follows: 



9 germ cells W^ W 



^ 



X1 



germ cells K W 



Possible combinations 2,R(W}\ 



This ratio of 2 : 2 or I : I is approximately the. ratio of the two 

 sexes in many animals and plants, and there is good reason to be- 

 lieve that sex is a Mendelian character of this sort, in which one 

 parent is heterozygous for sex and the other homozygous (See 

 p. 163). 



2. Results of Crossings where there is more than one Pair of 

 Contrasting Characters. It rarely happens that two individuals 

 differ in a single character only; more frequently they differ in 

 many characters, and this leads to a great increase in the number 

 of types of offspring in the F 2 generation. But however many 

 pairs of contrasting characters the parents may show each pair 

 may be considered by itself as if it were the only contrasting pair, 

 and when this is done all the offspring may be classified accord- 

 ing to the regular Mendelian formula given above. 



When the parents differ, in one character only, the offspring 

 formed by their crossing are called monohybrids, when there are 



