72 TABOO AND GENETICS 



are the reverse of this ratio — one horned to 

 three hornless. This is an example of Mendel's 

 principle of segregation — factors may be mixed 

 in breeding, but they do not lose their identity, 

 and hence tend to be sorted out or segregated 

 again in succeeding generations. 



In the horned Dorsets, we must suppose that 

 both males and females carry a dual factor for 

 horns — technically, are homozygous for horns. 

 The hornless Suffolks, on the contrary, are 

 homozygous for absence of horns. Thus the 

 dual factor in the zygotes or fertilized eggs at 

 the basis of the first filial (hybrid) generation 

 consits of a single factor for horns and a single 

 factor for their absence. If we represent horns 

 by H and absence of horns by A, Dorsets have 

 a factor HH, Suffolks A A and the hybrids HA. 



All the males in this generation have horns, 

 which means that a single " dose " of the factor 

 H will produce horns in a male, or that they 

 are dominant in males. But a single dose will 

 not produce horns in a female — that is, horns 

 are recessive in females — the factor is present 

 but unexpressed. 



Mating two HA hybrids, the H and A of 

 course split apart in the formation of the 

 gametes, as the HH and A A did in the previous 

 generation ; so that we get an equal number of 

 single H and A factors. In reuniting in fer- 

 tihzed eggs, the chance is just half and half that 



