GENETICS 



231 



about one-fourth of the backcross family are of each of these kinds. 

 Figure 200 gives the explanation. The four kinds of eggs produced by 

 the Fi flies are about equally numerous because the two pairs of genes 

 are distributed at random in the reduction divisions of their germ cells. 

 Whatever ratio exists among these eggs must also prevail among the 

 backcross offspring produced from them — hence the equal numbers of 

 the four kinds of flies in that generation. 



F, 



Eggs 



Gray 

 Long 



EEW 



EV 



Gray 

 Lons 



EeVv 



EV 

 Ev 

 eV 



Ebony 

 Vestigial 



eevv 



ev 



Ebony 

 Vestigial 



eevv 



ev Soerm 



BC 



J^EeW i^Eevv J^eeVv J^eevv 



Gray 

 Long 



Gray 

 Vestigial 



Ebony 

 Long 



Ebony 

 Vestigia 



Fig. 200. — Inheritance of two pairs of characters in a mating between a double hetero- 

 zygote and a double recessive. The characters are gray and ebony body and long and 

 vestigial wing in Drosophila. 



Interactions of Genes. — The two pairs of genes studied in guinea 

 pigs, and the two in Drosophila, appear to be entirely independent of 

 each other in the production of their characters. An animal with gene 

 W is black, regardless of the slope of its hair; and one whose formula is 

 rr is smooth, no matter whether it is white or black. A long-winged 

 fly may be either ebony or gray, and an ebony fly either long- or vestigial- 

 ^vinged. Very often, however, the action of one gene is modified by 

 some other specific gene if they are both present in the same individual. 

 A striking example is found in the combs of fowls. When a pea-combed 

 fowl (Fig. 201, upper left) is crossed with a single-combed one (lower 

 right), their offspring are pea-combed, and the F2 generation is three- 

 fourths pea and one-fourth single. These results indicate that pea comb 



