Chapter 7 



SEX CHROMOSOMES 

 AND SEX-LINKED GENES 



S 



|i\( i sex has phenotypic alter- 

 natives (maleness and female- 



ness ) . the genetie basis for sex 

 can be investigated. This basis cannot be 

 determined by studying garden pea plants 

 because they are bisexual; that is. both sex- 

 ual alternatives occur in one individual, and 

 no phenotypic differences will be produced 

 by genetic recombination. The typical 

 Drosophila individual, however, being either 

 male or female (Figure 2-6, p. 23), can be 

 used to study the genetic basis for sex. 



When normal males and females mate to- 

 gether, the male: female ratio of their prog- 

 eny is approximately 1:1. This suggests the 

 simplest hypothesis — that sex in Drosophila 

 is determined by a single gene pair, one of 

 the sexes being a homozygote and the other 

 a heterozygote. For the moment, however. 



which sex corresponds to which genotype 

 cannot be designated. 



In accordance with the view that chromo- 

 somes contain the genes, one pair of chro- 

 mosomes should be concerned with sex. Let 

 us call the presumed homologous pair of 

 chromosomes carried by the homozygote 

 lor the sex genes the XX pair and those 

 carried by the heterozygote, the XY pair. 

 Segregation and random cross fertilization 

 then would produce equal numbers of XX 

 and XY progeny. The X and the Y chro- 

 mosomes presumed to carry the genes for 

 sex can be called sex chromosomes; each of 

 the other chromosomes which an individual 

 carries can be called an autosome (A). 

 Since Drosophila melanogaster has a diploid 

 chromosome number of four pairs, each 

 individual can be represented as either 

 XX + 3AA or XY + 3AA. 



Sex-Linked Genes 



Consider the results of certain crosses in- 

 volving the recessives cubitus interruptus 

 (ci) and ebony body color (e) and their 

 dominant alleles ci + (normal wing venation) 

 and e + (gray body color). One cross, 

 ci+ci e + ehy ci ci e e — a dihybrid parent and 

 a double recessive parent (Figure 7-1 ) — 

 produces offspring in a 1:1:1:1 ratio thus 



+ + 

 ci ci e e 



ci ci e e 



+ + + + 



'Aci e, V4ci e, V-ici e, 'Aci e 



ci e 



+ + 



Aci ci e e 



Aci ci e e 



'Aci ci e e 



'Aci ci e e 



FIGURE 7-1 . Results of hack- 

 crossing a dihybrid. 



90 



