RULES AND RATIOS OF INHERITANCE 



209 



genes that differ In the two parents. The two parents may 

 thus be called AA and aa, the Fi generation Aa, the F2 gen- 

 eration AA -1- 2Aa -\- aa. It is only when the conditions are 



ff 



Purple 



Fl 



Red 



Figure 44. Diagrams to illustrate the situation as to chromosomes 

 and genes in a cross between parents differing in a single pair of 

 autosomal genes, AA and aa. The usual algebraic expressions for 

 such crosses, AA x aa = Aa, are essentially abbreviated diagrams 

 of this type, with the omission of all parts of the chromosomes in 

 which the two parents are alike. 



complex that it is necessary to represent the chromosomes, 

 as in figure 44.) 



2. Parents differing in a single pair of genes located in 

 the X-chromosome. — The same character red eye-color, just 

 considered, gives in other cases a different type of single- 



