i8 INTRODUCTION TO SEXUAL PHYSIOLOGY 



Sperms < 



Black 



White 



White 



Black 



> Ova 



Individuals whicli transmit characters of tlie same kind, and 

 in respect of tliis character produce only one kind of gamete, are 

 said to be homozygous, whereas those which transmit two (or 

 more) alternative characters, and produce two (or more) gametes 

 corresponding to them, are called heterozygous. Thus the blue 

 Andalusian fowl is heterozygous for colour. 



In the illustrations just given consideration has been restricted 

 to a single pair of genes, but actually the parental individuals 

 generally difier in respect of a number of character-pairs, and 

 the genes which determine these are inherited independently. 

 Thus, if we cross Aberdeen-Angus and Hereford cattle — two 

 breeds in which the most striking differences are that, whereas 

 the Aberdeen is polled and black and has a face which is not 

 specially marked, the Hereford is horned and red and has a 

 white face — aU the individuals in the F^ generation are polled 

 and black and white faced (these characters being dominant) ; in 

 the Fg generation, however, there are no less than twenty-seven 

 possibilities in the way of combinations, though actually only 

 eight appear, as certain of the animals which have different 

 constitutions are superficially alike. It may be stated in general 

 that when two parents which differ in regard to more than one 

 character-pair are crossed, the F^ generation will exhibit dominant 

 characters irrespective of which parent contributed them. 



The fact that so many character-pairs assert themselves 

 independently of one another is supposed to be due to the respec- 

 tive genes being carried in different chromosomes. It follows 

 from this that the number of pairs of characters which can at the 

 same time assort independently in any organism is equal to the 

 number of chromosomes in its cells (Burlingame). Moreover, 

 characters the genes of which are located in the same chromosome 

 are usually linked in heredity. This is the case with the so-called 

 sex-linked characters which are ordinarily transmitted by one 



