1 1 2 GENETICS 



them gave both yellow and green offspring, in the propor- 

 tion of 3 yellows to i green. 



The reason for this Is clear when we consider the chromo- 

 somes on which the characters depend. If A represents the 

 chromosome (autosome) that gives the dominant yellow 

 color, while a represents the chromosome that gives the 

 recessive green, the original parents were AA and aa, the 

 Fi generation were all Aa, while the F2 generation showed 

 the proportions 



AA + 2Aa + aa 



The recessive individuals aa are homozygotes; their two 

 chromosomes are alike, being aa. When germ cells are pro- 

 duced by these homozygotic individuals, all these germ cells 

 must contain the chromosome a, and when two of these 

 unite in self-fertilization, of course they produce in all 

 cases again individuals aa. These are of course recessives 

 (green), like their parents; that is, the green parents aa 

 "breed true." 



Similarly the dominant yellows, AA (constituting one- 

 third of all the dominants) are homozygotes. Their germ 

 cells all contain the chromosome A, and the offspring pro- 

 duced by union of such germ cells, in self-fertilization, are 

 all AA, and so dominant yellow like the parents. One-third 

 of all the dominants thus breed true. 



But the individuals Aa are heterozygotic; that Is they 

 have two kinds of chromosomes, A and a, in the pair. Thus 

 they produce two kinds of germ cells, A and a, in equal 

 numbers. And when these germ cells unite, they necessarily 

 give, as we saw on an earlier page, offspring of three kinds, 

 in the proportions 



AA + 2Aa + aa 



Of these, AA and 2Aa will be dominants (yellow), while 

 aa will be recessive (green). 



