PHENOMENA OF INHERITANCE 33 l 



Other Mendelian Ratios 



When a pure dominant is crossed with a mixed dominant-recessive 

 all the offspring show the dominant character, though one half are pure 

 dominant and the other half dominant-recessives. Thus if a pure 

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

 and a wrinkled seeded one, all the progeny are round-seeded, though one 

 half of them carry the factor for wrinkled seed ; this may be graphically 

 represented as follows : 



5 germ cells Rk /R 



V I = Possible 



J 1 germ cells R W combinations 



2RR:2R(W). 



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

 true and produce only round-seeded peas, whereas the progeny of the 

 hybrid round and wrinkled (RW) split up into pure round, hybrid 

 round and wrinkled, and pure wrinkled in the regular Mendelian ratio 

 of 1RR:2R(W) : 11717 (Fig. 52, C). 



When a pure recessive is crossed with a mixed dominant-recessive 

 another typical ratio results. Thus if a wrinkled-seeded variety of pea 

 is crossed with a hybrid between a round and wrinkled seeded one, 

 round-seeded and wrinkled-seeded peas are produced in the proportion, 

 of 1:1. 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 : 



5 germ cells W\ yW 



V I = Possible 



<§ germ cells R 17 combinations 



2RW : 21717 



This ratio of 1 : 1 is approximately the ratio of the two sexes in many 

 animals and plants, and there is good reason to believe that sex is a 

 Mendelian character of this sort, in which one plant is heterozygous for 

 sex and the other homozygous. 



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

 Character. — 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 according to the regular Mendelian formula given above. 



