326 



Ee SEARCH IN Animal Breeding. 



[July, 



RESEARCH IN ANIMAL BREEDING. 



IV. 



E. C. Ptjnnett, F.E.S., 

 ' Professor of Genetics, University of Camhridge. 



In the previous articles of this series, puhlished in the 

 April and May issues of the Journal, Professor Punnett 

 dealt iDith the coat colours in cattle and the crossing of 

 polled with horned cattle as illustrations of simple Mendelian 

 inheritance. In the June issue of the Journal a description 

 icas given of the experiments undertaken with poultry and 

 rahhits which icere designed to investigate the inheritance of 

 weight and coat patterns. 



One of the most striking points of difference between the 

 higher animals and plants is that in the former the sexes are 

 separate, while the latter are most often hermaphrodite. A.sso- 

 ciated wdth the bisexual mode of reproduction are peculiar 

 features of heredity which have formed the subject of active 

 investigation in recent years. As the result of much w^ork in 

 different parts of the world, the tangle of sex and its dependent 

 characters is gradually being unravelled. In the first place we 

 recognise sex itself as being inherited on Mendelian lines. 

 Speaking generally, one of the features of sex-heredity is that 

 the two sexes are produced in equal numbers. Male and female 

 give males and females in like proportion, and it will be remem- 

 bered (p. 15) that recessive and impure dominant give reces- 

 ^sives and impure dominants in like proportion. Hence the con- 

 ception that one sex is recessive and the other an iuipure 

 dominant. The pure dominant can never arise, for male cannot 

 be fertilised by male, nor female by female. Then comes the ques- 

 tion, w hich sex is to be regarded as recessive, and which the impure 

 dominant — w^hich is the sex that produces germ-cells all of the 

 same sex, and which the one that produces equal numbers of 

 two kinds of germ-cells differing in their sex-determining pro- 

 perties? Experience has shown that there is no general rule 

 for all animals. In man the male produces two kinds of sperms, 

 but in the case of poultry it is the hen that produces two kinds of 

 eggs; on the other hand women and cocks agree in that each 

 produces only one kind of germ-cell in respect of sex-determina- 

 tion. In man the two kinds of sperm decide the sex of the child; 

 in the fowl the two kinds of egg determine whether there shall 

 hatch out a cockerel chick or a pullet. 



