Chapter *12 

 SEX-LINKAGE 



WE HAVE already found that 

 different pairs of genes seg- 

 regate independently, and 

 have hypothesized that this behavior is due to 

 different pairs of genes being located in differ- 

 ent pairs of chromosomes. You may now ask 

 what the genetic basis for sex is. In the case 

 of the garden pea we cannot obtain the answer 

 from a study of just the two alternatives, male- 

 ness and femaleness, since all the pea plants 

 dealt with were bisexual. So long as there are 

 only two alternatives for the sex trait and both 

 occur in every individual, there can be no 

 phenotypic differences produced by segrega- 

 tion and recombination, and a genie basis for 

 sex cannot be determined. We can, however, 

 attempt a study of the genetic basis for sex, 

 say in Drosophila, where the typical individual 

 is either male or female (Figure 12-1). 

 When normal males and females are mated, 

 their progeny are in the approximate pheno- 

 typic ratio of male : female as 1:1. This 

 permits the hypothesis that sex is determined 

 by a single gene pair, and that one of the 

 sexes of Drosophila is a homozygote and the 

 other is a heterozygote. At the moment, 

 however, we cannot say which sex carries 

 which genotype. In accordance with our 

 view that chromosomes contain the genes, 

 there should be one pair of chromosomes con- 

 cerned with sex. Let us call the homologous 

 pair of chromosomes, which the homozygote 

 for the sex genes carries, the XX pair, and the 

 pair carried by the heterozygote, the XY pair. 

 Segregation and random fertilization will pro- 

 duce equal numbers of XX and XY individ- 

 81 



FIGURE 12-1. Normal (wild-type) Drosophila me\- 

 anogaster male (A) and female (B). (Drawn by 

 E. M. Wallace.) 



uals. Since the X and the Y chromosomes 

 carry the genes for sex, these can be called 

 sex chromosomes, while the other chromo- 

 somes which an individual carries can be 

 called autosomes (A). Since Drosophila 

 melanogaster has a diploid chromosome num- 

 ber of four pairs, each individual can now 

 be represented as either XX -f 3AA or XY 

 -f 3AA. 



Consider first crosses involving the reces- 

 sives cubitus interruptus, ci, and ebony body 

 color, e, and their dominant alleles c/+ 

 (normal wing venation) and e+ (gray body 

 color). Let one cross be c/+c/ e+e X ci ci e e, 

 in which one parent is dihybrid and the other 

 parent is the double recessive (Figure 12-2). 



