GENETIC SYSTEM: RELATION TO CHARACTERISTICS II3 



Thus the homozygotes aa and AA breed true, while the 

 heterozygotes, Aa, do not. 



Furthermore, as Mendel discovered, if the heterozygotes 

 Aa are mated with the recessive homozygotes aa, they give 

 two kinds of offspring, Aa and aa; half (Aa) being domi- 

 nant, the other half (aa) being recessive. If the hetero- 

 zygotes Aa are mated with the dominant homozygotes AA, 

 the offspring AA and Aa are all dominant, though half 

 (AA) are homozygotes, the other half (Aa) heterozy- 

 gotes. 



All these results are typical for the descendants of two 

 parents different in a single pair of chromosomes, one pair 

 being dominant, the other recessive^ For reference it will 

 be well to make a table of these results, characteristic for 

 Mendelian inheritance: 



Table i, for the Results of Mendelian Inheritance 



Parents: AA and aa; both homozygotic, 

 Fi : Aa; all alike, dominant heterozygotes. 



F2: AA -\- 2Aa -(- aa. Three-fourths of the individuals 



dominant, one-fourth recessive. Half the individuals homozygotes 

 (AA and aa), the other half heterozygotes (2Aa). 



Each type of homozygote bred by itself produces offspring like 

 itself. 



The heterozygotes Aa, when bred by themselves, produce again 

 offspring 



AA -f- 2Aa + aa. 

 Aa mated with aa yields again Aa -\- aa. 

 Aa mated with AA yields again Aa + AA. 



All of these results are found to hold true for thousands 

 of different characteristics. We may therefore summarize 

 the situation in the following general propositions: 



I. When two different individuals are crossed, in many 

 cases the differences between them follow the same method 

 of distribution as do the two members of a pair of auto- 

 somes — one being dominant, the other recessive. 



