52 MENDELISM AND 



sequent to fertilisation, but now there is good 

 reason to believe that generally the sex of the in- 

 dividual is determined at fertilisation, though as 

 we shall see there is evidence that it may in certain 

 cases be changed at a later stage. 



In Mendelian experiments, a heterozygote in- 

 dividual is one arising from gametes containing 

 opposite members of a pair of characters, in other 

 words, from the union of a gamete carrying a 

 dominant with another carrying a recessive. A 

 pure recessive individual is one arising from the 

 union of two gametes both carrying recessives. If 

 a heterozygote is bred with a pure recessive the 

 offspring are half heterozygote and half recessive. 

 The heterozygote individual in typical cases shows 

 the dominant character. In the formation of its 

 gametes when the reduction division of the chromo- 

 somes takes place, half of them receive the dominant 

 character, half the recessive. When the division 

 in the gametes of the recessive individual takes 

 place its gametes all contain the recessive character. 

 Thus, if we indicate the dominant character by D 

 and the recessive by d, the constitution of the two 

 individuals is 



Dd and dd. 

 The gametes they produce are 



D-^-d and d+d, 

 and the fertilisations are therefore 



Dd, Dd, dd, dd, 

 or heterozygote dominants and pure recessives in 

 equal numbers. 



It is evident that the reproduction of the sexes 

 is very similar to this. One of the remarkable facts 

 about sex is that, although the uniting gametes 



