Several investigators have discovered that the ratio 

 9:3:3:1 does not always occur in di-hybrids. This devia- 

 tion is now explained by Morgan as due to gametic coupling 

 or crossing-over of the chromosomes in the synapsis stage of 

 nuclear formation. (See page 123). 



Problem. — Determine the Fi and F2 generations when 

 black hornless cattle are crossed with red horned cattle, 

 when black is dominant to red, and hornless to horned. 



Problem 2. — Determiile the Fi and Fo generations 

 when smooth white guinea pigs are crossed with rough 

 black guinea pigs, when rough and black are dominant. 



COT) # g 



000 



000 



9 



Fig. 16. — Cross between yellow-round and 

 green-wrinkled peas, giving the 9: 3- 3- 1 

 ratio in F2. (Morgan.). 



Three Pairs of Characters in Hybridism— Trihybrids 



Mendel showed that when three pairs of characters are 

 inyolvedthe law still holds goods. In the case of peas, when 

 yellow-smooih-tall peas were crossed with green-wrinkled- 

 dwarf peas the hybrids were all yellow-sniooth-tall 



On the assumption that a gamete can contain onlv one 

 of eachpair of characters the probabilities are that there will 

 pe^an equal number of eight kinds of male gametes in the 



^^w^'- ^^^-1 X^H' YWT, YWD, GST, GSD. GWT a 

 GWD. Similarly for the female gametes. Accordingly 

 foHow"^ ^' generation will be constituted as 



8S 



