292 ANIMAL BIOLOGY 



we get the result directly; that is an individual's progeny are either 

 all dominants or dominants and recessives in 3 : 1 ratio, and thus 

 the gametic constitution of the parent is immediately known. 

 However, in the case of animals, where self-fertilization is impos- 

 sible, the determination can be made by mating the dominants 

 with recessives ; for a homozygous (pure) dominant then will give 

 all dominants, while a heterozygous (hybrid) dominant will give 

 half dominants and half recessives. Thus: 



Gametes » 



Gametes « d^ x d d^ \. 



Possible zygotes = 100% Dd 50% Dd+50% dd 



So far we have considered inheritance in monohybrids, that is 

 cases involving one pair of alternative characters that can be 

 interpreted as the resultant of one pair of genes termed al- 

 lelomorphs, but now we proceed to cases where two, three, 

 or more pairs of genes are concerned, known as dihybrids, trihy- 

 brids, etc. 



2. Dihybrids 



Mendel investigated inheritance in dihybrids by crossing, for 

 example, a Pea producing yellow round seeds with one producing 

 green wrinkled seeds. The plants in the Fi generations bear only 

 yellow round seeds, and therefore yellow and round are each 

 dominant characters when paired with green and wrinkled. After 

 self-fertilization such hybrid plants produce offspring (F 2 ) with 

 seeds showing all the possible combinations of the four characters, 

 and in the ratio of 9 yellow round to 3 yellow wrinkled to 3 green 

 round to 1 green wrinkled. (Fig. 183A.) 



This logically can only be interpreted as indicating that one of 

 the original parent plants bore gametes all containing the genes 

 for yellow and for round peas (YR), while the other parent plant 

 bore gametes all containing the genes for green and for wrinkled 

 (yr). Such being the case, the resulting zygote is YRyr, and the 

 hybrid which it forms develops gametes with all the possible com- 

 binations of these genes (except, of course, Rr and Yy) which are 

 YR, Yr, yR, and yr - - there is an independent assortment of 

 the genes as evidenced by the new combinations Yr and yR. Now, 



