92 " Gifnandroiiiorpltism " and Kindred Problems 



known is always one of a pair of uniovular twins, the other being 

 a normal bull, whereas the other twin in the case of the female free- 

 martin is a heifer. 



The male fi'ee-martin usually resembles a spayed heifer in external 

 appearance, the sex glands are testes usually devoid of spermatozoa, and 

 the other internal sex organs arc of predominantly female type. This 

 form is the " heifer free-martin." The " steer free-martin," externally 

 like an ox, but internally like the other form, is much rarer. The female 

 free-martin has ovaries, but the genital organs are of predominantly 

 male type. 



Berry Hart {Proc. Roy. Soc. Edinburgh, XXX, p. 238) says that 

 " the free-martin with a potent bull twin is the result of a division of a 

 male zygote, so that the somatic determinants are equally divided and 

 the genital determinants unequally divided, the potent going to one 

 twin, the potent bull, the non-potent genital determinants to the free- 

 martin." He offers a similar explanation for the female free-martin. 

 The two forms of male free-martin may be exf)lained by supposing that 

 there is more than one factor for secondaiy sexual characters, and that 

 the unequal division may affect all of them in one case and only some 

 of them in the other. 



In man I only know of one case, in which segregation of Mendelian 

 unit characters, comparable to that which occurs in heterochroism of 

 Lepidoptera, has happened. It is a case described by Nettleship of 

 female uniovulai' twins, one of whom was colour-blind and the other 

 normal. He gives incontrovertible proof that they really were twins 

 of this kind. 



According to Doncaster colour-blindness is recessive to normal vision 

 and an explanation of its rare occurrence in females is given by Jenkins 

 {Journal of Genetics, in, p. 121). 



The best explanation of this unique case is that an unequal division 

 of chromosomes took place at the first cleavage, so that the factor for 

 colour-blindness was segregated into one cell, that for normal vision 

 into the other. Instead of the result being an individual with one side 

 colour-blind, and the other normal, the two cleavage cells became 

 separated. Each half grew into a complete individual, the one colour- 

 blind, the Other normal, but in all other respects alike. 



That it was not due to the loss of a character by a sudden mutation, 

 as Lang thinks, is proved by the fact that the fether was colour-blind, 

 and came of a family many of whose members were similarly affected, 

 and the mother's grandmother was probably colour-blind also. 



