,S>.\ Chromosomes and Sex-Linked denes 



\)\ 



demonstrating that the two pairs of genes 

 are segregating independently. The same 

 result and conclusions hold for the cross of 

 ci ci e? by ci ci e * e. Consider, next, 

 crosses in which the sex and wing venation 

 traits are studied simultaneously in recip- 

 rocal matings — ci+ci XX by ci ci XY and 

 ci ci XY by ci ci XX. In both cases the 

 result is a 1:1:1:1 ratio of cubitus male, 

 cubitus female, normal male, normal female. 

 Here, then, the sex genes segregate inde- 

 pendently of the genes for cubitus. There- 

 fore, according to our hypothesis, the ci 

 alleles are located autosomally. 



Similarly, each of the reciprocal crosses — 

 e fXXbyee XY and e • e X Y by e e XX 

 — also gives a 1:1:1:1 ratio, indicating that 

 the gene for ebony is located autosomally. 

 Since the genes for ebony and cubitus segre- 

 gate independently of each other, they must 

 be located on different pairs of autosomes. 



Even though we cannot yet specify which 

 sex is XX or XY, the last two types of 

 crosses can be described as reciprocally made 

 backcrosses of a monohybrid; that is, one 

 time the hybrid parent was the male, the 

 other time the hybrid parent was the female. 

 In both cases the two traits appear in a 1:1 

 ratio among the sons, and in a 1:1 ratio 

 among the daughters. 



At this point, an earlier statement (p. 

 32) — that all crosses give the same results 

 when made reciprocally — can be understood 

 to mean that the observed phenotypes 

 and their proportions are the same for 

 sons and daughters even though the crosses 

 were made reciprocally. So, for example, 

 in a cross of the dihybrids ci+ci e + e by 

 ci+ci e + e there would be a 9:3:3:1 ratio 

 among the sons and a 9 : 3 : 3 : 1 ratio among 

 the daughters because the parents' sex genes 

 were located in the sex chromosomes, where- 

 as the other gene pairs were in nonhomolo- 

 gous pairs of autosomes. Previously, all the 

 crosses we dealt with involved autosomal 

 recombination. Because autosomal genes 



A 



B 



P -'9* whi^ P "hi,e CJ*^ 



lei'? 9 



/hite 



dull 

 red 



0*0* 



?? 



figure 7-2. Phenotypic results of reciprocal 

 matings involving eye color. & 6 nudes. 

 9 9 = females. 



always segregate independently of the sex 

 genes, sex did not influence the results; that 

 is, the phenotypic results of autosomal re- 

 combination crosses are the same for sons 

 and daughters even though reciprocal mat- 

 ings are made. 



But consider the results of crosses involv- 

 ing the dull-red (w+) and white (w) eye 

 color alleles. Using pure lines, dull-red 9 

 by white i (Figure 7-2 A) produces all 

 dull-red sons and daughters in F,, as ex- 

 pected, since w+ is dominant. However, 

 the reciprocal cross (Figure 7-2B), white 

 9 by dull-red 6 , produces only white sons 

 and dull-red daughters. Although the first 

 cross gives the same result for sons as for 

 daughters, the second (reciprocal) cross 

 gives different results: sons resemble their 

 mothers; daughters resemble their fathers. 

 Because such different results are never ob- 

 tained from reciprocal matings involving 

 autosomal genes, we can conclude that w+ 

 and its alleles are not located autosomally. 



Let us assume that the gene for white eye 

 is located in the sex chromosomes and, 

 therefore, is sex-linked and investigate the 

 consequences of this on the gene's transmis- 

 sion relative to the sex phenotype. 1 



If we assume that females are XY and 

 males XX, the first cross then is dull-red 

 female X""Y"" by white male X"X"' (Fig- 

 ure 7-3, A-l ), and the F, expected are 



1 See T. H. Morgan ( 1910). 



