230 PRINCIPLES OF ANIMAL BIOLOGY 



Then each sperm formula is written four times in one of the rows of 

 spaces across the chart. In each space are the genes found in one of the 

 sixteen kinds of F2 animals. They are written in the chart with the genes 

 from the egg separated by a dot from the genes from the spermatozoon; 

 but in other situations it is preferable to write the two genes of one pair 

 together, followed by the genes of the other pair. Some of the sixteen 

 formulas are identical Avith others, but they have been arrived at in 

 sixteen different ways. 



It remains only to indicate the appearance of the guinea pigs having 

 these genes. The little figures accompanying the gene formulas are 

 intended to do this. Nine of the sixteen are rough black, three rough 

 white, three smooth black, and one smooth white. It should be remem- 

 bered that these numbers are a ratio, 9:3:3:1, not absolute numbers. 

 They are so many sixteenths of the total number in F2. In a single 

 litter the least frecjuent kind (smooth white, the double recessive) could 

 easily be missing. 



Fig. 199. — Gray and ebony body and long and vestigial wings in Drosophila, combined in 



the four possible ways. 



A Two -pair Backcross. — ^As a basis of judgment of certain phenomena 

 to be described later, a backcross involving two independent pairs of 

 characters will be useful. The characters chosen for illustration are the 

 color of the body and the shape and size of the wngs of the fly Drosophila. 

 The body is normally of a brownish gray, but there is a very dark variety 

 known as ebony. The wings are ordinarily long and lie flat over the 

 back of the fly when at rest; but in one variation of them, called vestigial, 

 the wings are small and crumpled and project obliciuely outward from 

 the body. The vestigial wing is useless for flight; flies with such wings 

 merely crawl or jump. 



The four combinations into which these characters may enter are 

 shown in Fig. 199. Suppose that the cross be made between a gray long- 

 winged fly and an ebony vestigial- winged one. The Fi generation is 

 gray and long-winged, for these are the dominant characters of the two 

 pairs. If these Fi flies, which are heterozygous for both pairs of genes, 

 are mated with ebony vestigial flies, which are necessarily homozygous 

 for the two recessive genes, all four of the kinds of flies illustrated in 

 Fig. 199 are produced. Moreover, they are about equaUy numerous; 



