Sex Chromosomes and Sex-Linked Genes 



93 



9 X w+ X w+ by white 6 X"Y' r producing 

 X" + X"' (dull-red) daughters and X" + Y'" 

 (dull-red) sons (Figure 7-3, A-2); recip- 

 rocally, white 9 X"'X' r by dull-red $ 

 X w+ Y w+ produces X w+ X w (dull-red) daugh- 

 ters and X W Y W * (dull-red) sons (Figure 

 7-3, B-2). Again an expected phenotype 

 is contrary to fact — the phenotype of the Fi 

 sons being white, not dull-red. 



Since we cannot explain these observa- 

 tions by identifying maleness with XX or 

 XY alone, we must increase the number of 

 assumptions. Let us then test two hypoth- 

 eses simultaneously: (1) Assuming that 

 Drosophila males are XY and (2) the Y 

 chromosome carries w but no other allele, 

 then the genotypes and results of the first 

 cross described in the preceding paragraph 

 remain unchanged (Figure 7-3, A-3); the 

 reciprocal cross (Figure 7-3, B-3) becomes 

 white 9 X"X"" by dull-red <$ X M ' + Y\ 

 producing X"X" (dull-red) daughters and 

 X I( Y"" (white) sons. Since these hypoth- 

 eses fit the observations, we may accept 

 them. 



The recombination genetics of several 

 traits in Drosophila other than sex and eye 

 color also proves to be based upon a pair of 

 genes in the sex chromosomes; that is. each 

 case gives different results in Fi when lines 

 pure for different alternatives are crossed 

 reciprocally. Moreover, each case can be 

 explained by assuming that females are XX, 

 males XY, and that the Y carries the most 

 recessive and least effective allele known for 

 the gene pair under test, as is w in the case 

 of eye color. In such cases, the absence 

 from the Y of a partially or completely domi- 

 nant allele must mean that such alleles can- 

 not be produced by mutation of the most 

 recessive allele simply because this recessive 

 allele does not exist on the Y. Accordingly, 

 the Y ordinarily lacks an allele of a gene 

 located on the X; therefore, in Figure 7-3, 

 A-3 and B-3 a Y should be substituted 

 for each Y" . 



Whenever the Y carries no allele of a gene 

 on the X, sons will express phenotypically 

 whatever allele is contained in the single X 

 each son receives from his mother. With 

 regard to these genes, therefore, a Drosoph- 

 ila female is being test crossed whenever 

 and to whomever she mates, since her X 

 chromosome genotype can be determined di- 

 rectly from the phenotypes of her sons. An 

 otherwise diploid individual carrying one or 

 more unpaired genes is said to be hemi- 

 zygous in this respect. For example, a gene 

 in the X chromosome with no allele in the 

 Y is hemizygous in the Drosophila male; 

 half of the zygotes he produces will receive 

 this allele in the X he contributes, whereas 

 the half receiving the Y will not get one. 

 The X of a Drosophila male is obtained 

 from his mother and transmitted to each of 

 his daughters; the Y is transmitted from 

 father to son. 



In poultry a mating of a female with non- 

 barred feathers to a male with barred feathers 

 produces offspring which are all barred — 

 barred (B) being dominant to nonbarred 

 (b) ( Figure 7-4 A ) . In the reciprocal cross 

 (Figure 7-4), barred 9 by nonbarred $, 

 all sons are barred and all daughters non- 

 barred. Here also the results of reciprocal 

 matings differ, so that we are dealing again 

 with sex-linkage. In the reciprocal cross, 

 note that the exceptional F x are nonbarred, 

 showing the recessive trait as in the case of 

 Drosophila. But, in poultry the sex is op- 

 posite, since the exceptional Fi are females. 

 (The exceptional F! Drosophila were white- 

 eyed males.) To explain these results we 

 must assume that in poultry, as in Drosoph- 

 ila, sex is determined by XX vs. XY, and 

 that the X chromosome does and the Y 

 chromosome does not contain a gene for 

 barred or nonbarred feathers. But, contrary 

 to Drosophila, poultry males are XX and 

 females, XY. 



The genotypes of the bird crosses are, on 

 these hypotheses, X 6 Y (nonbarred 9 ) by 



