GENERATION AND DEVELOPMENT 73s 



viz., for each of the distinguishing characteristics the dominant was 

 to the recessive as three to one in the second filial generation, F2. 



Dominance has been illustrated by Mendel's experiments on peas, 

 in which it has been seen that the resulting zygote is a pure 

 dominant when both gametes are pure for the particular character, 

 and impure when only one gamete imparts the dominant variety. 

 There is no difference in appearance between the pure and impure 

 dominant above mentioned, but the test of breeding from them at 

 once settles the question. In some cases a zygote differs from either 

 parent by being intermediate in size or physiological properties, but 

 their offspring follow the ordinary Mendelian law — viz., they repro- 

 duce the parental and the intermediate characteristics in the second 

 generation. The parental characteristics in the next generation 

 breed true ; the intermediates behave as in the first generation — 

 in fact, in accordance with the classical illustration of the peas. 

 In such cases dominance is absent, and it is usual to state the fact 

 by saying that dominance is not a necessary feature of Mendelian 

 inheritance. 



The following are examples of dominance in animals : 



Bay dominant to chestnut in horses. 



Grey dominant to black in rabbits and mice. 



Black dominant to white plumage in fowls. 



Polled dominant to horns in cattle. 



Short coat dominant to long coat in rabbits, sheep, and dogs. 



Feathered legs dominant to clean legs in poultry. 



Extra toe dominant to normal condition in poultry. 



Six lumbar vertebrae dominant to five lumbar in the horse. 



Convex nasal bones dominant to concave in the horse. 



Colour dominant to albinism in all animals. 



No explanation can at present be offered why dominance is 

 present in some cases, absent in others, and irregular in a third. 

 In the following example of irregular dominance in sheep it is 

 supposed that sex is the determining cause. Dorset horned and 

 Suffolk hornless sheep were crossed, and the first generation resulted 

 in the rams being horned and the ewes hornless. Bred among 

 themselves, the several generations produced rams of which only 

 three-fourths were horned, and ewes of which one-quarter bore 

 horns. In this case horns or their absence depended upon sex, 

 horns being dominant in the ram and recessive in the ewe. This 

 was proved by mating a hornless ram with the hornless ewes of the 

 first generation, the resulting progeny showing that half the rams 

 were hornless and all the ewes. 



Only an outline of Mendelism has been sketched above, and 

 nothing but its simpler side explained. When applied to many 

 cases of inheritance, Mendelism is not found to behave with the 

 simplicity above indicated. For instance, it is no longer considered 

 necessary to assume the existence of two factors to represent con- 

 trasting characters. Tallness or shortness in the pea is not due 

 to a factor for tallness and another for shortness, but to one factor 

 in two possible conditions — e.g., present or absent. If present, the 

 pea is tall ; if absent, it is short. Two or more distinct factors in 

 the gamete may also act on each other, and so influence the resulting 

 zygote. This is known as the interaction of factors, and is a question 

 of supreme importance to the Mendelist. Factors may also repel 

 or attract one another ; this is the theory designated the repulsion 

 and coupling of factors. These are mentioned in order to illustrate 



