170 CYTOLOGY CHAP. 



the results of breeding and the distribution of the factors in gameto- 

 genesis, on the assumption that these are located in different chromosomes. 

 Thus supposing we are dealing with two characters say colour (green 

 or yellow) and surface (smooth or wrinkled) of the pea cotyledon we 

 may call the chromosome in which resides the colour factor by the 

 first letter of the alphabet A or a in accordance with whether the factor 

 is present in ^ts yellow (^4) or green (a) producing form, and similarly 

 we may call the chromosome which contains the surface factor B 

 (smooth) or b (wrinkled). Regarding these two chromosomes only, the 

 pure parent plants, say the one green and smooth, the other yellow 

 and wrinkled, possess chromosomes aaBB and A Abb respectively, their 

 gametes being of course aB and Ab, and the hybrid offspring AaBb. 



Now in the meiosis of this hybrid, chromosomes A -a pair together in 

 syndesis and likewise B-b, and at the reduction division A and a go 

 one to each spermatocyte II. and B and b behave similarly. As now 

 it is a. matter of chance whether B goes into the same secondary 

 spermatocyte as A or as a, there are formed four different classes of 

 spermatocytes II. and therefore of gametes, namely, AB, Ab, aB and ab. 

 These gametes, uniting at random when the hybrids are bred together, 

 combine to give the same classes of zygotes, and in the same pro- 

 portion as the various classes of individuals found in F 2 in the actual 

 breeding experiment. 



Since this is a treatise on cytology and not on heredity, it would be 

 out of place to give any account of the immense number of character- 

 istics, both in animals and in plants, which have now been found to 

 be inherited in accordance with Mendel's Law. One or two interesting 

 side lines of evidence as to the correspondence between the distribution 

 of characteristics in heredity and of chromosomes in gametogenesis, 

 may, however, be mentioned. 



(i) The Genetics of a Tetraploid Plant 



Gregory (1914) obtained two tetraploid individuals of Primula 

 sinensis (of independent origin) , and as they proved to be heterozygous 

 for certain characteristics, he was able to carry out breeding experiments 

 with them. As we have just seen, if we fix our attention on a single 

 characteristic, represented in the idioplasm by a factor which we may 

 call A, or a, according to the form in which it is present, the formula 

 for a pure homozygous individual is A A or aa, and for the heterozygote 

 A a. In a tetraploid individual, however, if its quadruple set of chromo- 

 somes means a quadruple set of factors, the formulae for the pure forms 

 will be AAAA and aaaa, while there are three kinds of heterozygotes 

 possible, namely, AAAa, AAaa and Aaaa. 



