312 



EVOLUTION AND GENETICS 



a 



20 



3J-54C 



A 



B 

 C 



elude that C is five times as far from A as from B. Whether A and 

 B are on the same side of C or on opposite sides is not disclosed 

 by these data but if an additional test gives 20 per cent of 

 crossing over between A and B we may conclude that they are on 

 the same side (Fig. 179). 



Some genes of Drosophila are shown to be widely separated, 

 yet cross over much less than is to be expected in such cases 



when handled together. Double or multi- 

 ple crossing over accounts for these cases 

 without invalidating the data derived from 

 the study of more intimately associated 

 characters. 



Sex-Linkage. This interesting type of 

 linkage results from the retention by sex 

 chromosomes of genes related with wholly 

 different characters. The inheritance of 

 sex-linked characters is, as the term sug- 

 gests, intimately associated with the sex 

 of the individuals in a given generation. 

 Our now familiar example, Drosophila, 

 affords numerous illustrations. 



In this species one of the chromo- 

 somes, commonly called the x chromosome, 

 is associated in the male with a dissimilar y 

 chromosome (Fig. 168). The y chromo- 

 some apparently lacks the functional 



powers of the x chromosome, although the 

 age of crossing over and , ,• , i • ,• 



the location of genes in ^^o are synaptic mates, and is sometimes 



the chromosomes. looked upon as a degenerate x chromosome. 



In the female there are two x chromosomes. 



The gene for the mutant character white eyes in Drosophila is 

 located in the x chromosome. This character is recessive to the 

 normal red eyes of the wild fly, consequently a white-eyed female 

 is a homozygous recessive. When mated with a red-eyed male 

 all females of the Fi generation are red-eyed, but all males are 

 white-eyed. The results in this and the F2 generation are graphi- 

 cally expressed in Figure 180. Fi males produce two types of 

 gametes, one containing the x chromosome bearing the gene for 

 recessive white eyes, the other bearing the y chromosome without 

 genes. The female produces two types of eggs, each with the x 



Fig. 179. — Diagram to 

 illustrate the relation- 

 ship between percent- 



