vi MENDEL'S LAW 169 



The F x and F 2 represent what is actually found in the breeding- 

 pen. To bring the notation into conformity with cytological observation 

 we must suppose that the colour of the feather is dependent on a 

 " factor " or gene residing in a chromosome, and we may call this chromo- 

 some A when the factor is present in the black-producing form, and a 

 when it is present in the white-producing condition. 



In the case of the original pure (homozygous) black parent, both of 

 the homologous chromosomes are present in the A form, and in the white 

 parent in the a form. Hence the gametes of the two birds contain one 

 A or one a chromosome respectively, and the zygote contains both A 

 and a, i.e. it is a hybrid or heterozygote. When this hybrid comes to 

 form its gametes, A and a being homologous chromosomes, pair together 

 in syndesis, A going to one spermatocyte II. (or oocyte II.) and a going 

 to the other (or ist polar body). Thus the segregation of the hereditary 

 factors, and hence of the characteristics which they represent, is brought 

 about. In the second division of the meiotic phase each of the chromo- 

 somes of course divides longitudinally, and therefore gametogenesis 

 has proceeded (having regard to this pair of homologous chromosomes 

 only) according to the following scheme : 



A + a somatic cell or germ track. 



I 



Aa syndesis. 



/ \ 



A a spermatocyte II., oocyte II., and polar body I. 



^ spermatozoon, or mature egg and polar bodies 



I. and II. 



In the male, therefore, we have obviously an equal number of 

 spermatozoa carrying the A and the a chromosomes respectively, and 

 as in the female it is a matter of chance which chromosome stays in the 

 egg and which goes into the polar body, we have, on an average, 

 approximate equality of the two kinds of female gametes also. If now 

 two of these hybrids are mated together, we get the following 

 possible combinations of gametes, all of which will occur, on the 

 average, in equal numbers : 



A $ may be fertilized by A $ = zygote A A black. 



A ^ " " a ^ = " aA \ 2revs 



? A 3 = Aa} 



a$ ,, ,, a $ = ,, aa white. 



If two or more independently inheritable characters are under 

 consideration we can again describe, by an interchangeable notation, 



