APPENDIX B 359 



Mendel's laws have been tested and found true as regards a 

 number of characters of cross-bred plants and animals. In 

 peas, yellow cotyledons, round seeds, and coloured seed-skins are 

 dominant over green cotyledons, wrinkled seeds, and white seed- 

 skins. In flowers colour is generally dominant over white. 

 Similarly pigmented coats in mice and rabbits are dominant over 

 albino coats. In rabbits the normal short hair is dominant over 

 the long or Angora coat. In poultry the " rose " comb is domi- 

 nant over the "single" comb. In mice the waltzing habit is 

 recessive to the normal. In stocks the hairy leaf is dominant 

 over the smooth leaf. In guinea pigs the rough coat is dominant 

 over the smooth, and the short over the long. " Among guinea 

 pigs there occurs a series of alternative pigment types which show 

 Mendelian relations to one another. If we write them in this 

 order, (1) agouti (i.e. black ticked with yellow, the ancestral or 

 wild type of coat), (2) black, (3) yellow, (4) albino, we may say 

 that each is dominant over all which follow it, and recessive in 

 relation to all that precede it." l Many other examples might 

 be given. 



The essential features of the Mendelian phenomena are, (1) that 

 certain of the characters of the crossed varieties do not blend, 

 but segregate, in offspring, and (2) that these characters may be 

 inherited independently of one another; they are what are 

 technically termed allelomorphs, separate inheritable units, each 

 of which in one set of allelomorphs is capable of displacing its 

 opposite number in another set. But, while dominance is com- 

 plete in some cases, it is incomplete in many others. Thus if 

 short-haired guinea pigs be crossed with the long-haired or 

 recessive type, some of the long-haired descendants have hair 

 shorter than their long-haired ancestors ; whereas some of the 

 short-haired descendants have longer hair than their ancestral 

 type. Rough-haired cross-breds are often less rough than their 

 types. Cross-breds between white and pigmented individuals 

 may have coats spotted with white. In Mr. C. C. Hurst's 

 experiments on poultry 2 it was found with regard to a number 

 of Mendelian characters that dominance was complete in 3 6 '3 per 

 cent, of the cross-breeds of the first generation and incomplete 

 in 61-4 per cent. According to Mendelian law, all cross-breeds 

 of the first generation should exhibit only dominant characters j 

 but " the appearance of a few impure recessives in the first cross 

 indicates that the normally recessive character (normal foot) may 

 sometimes dominate." 3 In many instances the first cross-breed 

 generation resembles the common ancestral type, not the 

 dominant parent, and Mendelian segregation occurs only subse- 



1 Castle, Popular Science Monthly, July 1905, p. 197. 



2 Report, II, to the Evolution Committee of the Royal Society, 1905, 

 p. 137." 



3 Ibid., p. 154. 



