1 14 GENETICS 



2. This distribution is what is known as typical Men- 

 delian heredity. 



3. The two sexes do not differ in these respects. Each 

 sex has the same proportion of dominants and recesslves. 



4. The rules of Mendelian Inheritance are essentially 

 the rules of distribution of the members of pairs of auto- 

 somes. 



Characteristics that depend on two or more pairs of 

 autosomes: We have thus far followed the distribution of 

 characteristics that depend on differences In a single pair 

 of autosomes. But most organisms have several or many 

 pairs of autosomes: the fruit-fly has three pairs, man has 

 twenty-three. Do some characteristics depend on differ- 

 ences In one pair of autosomes, others on differences in 

 other pairs? Have the different pairs of autosomes thus 

 different functions in development and inheritance? 



We find that the different pairs of autosomes have in- 

 deed different functions, and that some characteristics de- 

 pend on one pair, some on another. This Is shown In many 

 cases In which two parents differ in two diverse character- 

 istics. One characteristic may then act as if dependent on 

 one pair of autosomes, the other on another pair. As an 

 example of this, we may take a case studied by Mendel in 

 peas. 



One of the original pea plants had seeds that were (i) 

 yellow, and (2) round, or smooth. These, as it turns out, 

 are both dominant characters. The other plants, crossed 

 with those just mentioned, had recessive characters; the 

 seeds were (i) green, and (2) wrinkled. 



When these two kinds of pea plants were mated to- 

 gether, it turned out, as we shall see, that yellow and green 

 behave In Inheritances If due to a difference in the two 

 parents In the effects of a certain pair of autosomes; that is, 

 they are alleles. Round and wrinkled behave as If due to a 



