84 



CHAPTER 12 



Assume first that females are XY and males 

 XX. The first cross is then dull red 99 (fe- 

 males) X"'"Y«'* by white cf cT (males) X"'X'^ 

 (Figure 12-4, A-1), and the Fi expected are 

 X"'*X"' sons and X"'Y'"'^ daughters, all dull 

 red-eyed, as observed. The reciprocal cross 

 (Figure 12-4, B-1) is, then, white 99 X'-'Y"- 

 by dull red d" & X'" X"'\ The Fi daughters 

 (X'^'Y'") are expected to be dull red-eyed, as 

 observed. However, the Fi sons (X"'"X"') are 

 expected to be dull red-eyed, whereas they 

 are actually white-eyed. Therefore, we must 

 reject this particular hypothesis for correlat- 

 ing sex chromosomes and eye color genes. 



Let us assume next the reverse situation — 

 that females are XX and males XY. The 

 same crosses are represented now as dull red 

 99 X'-'^X"" by white d^d X^-Y"- producing 

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

 red) sons (Figure 12-4, A-2); reciprocally, 

 white 99 X-X- by dull red dd x-^Y-* 

 gives X'""X«' (dull red) daughters and X"'Y«'^ 

 (dull red) sons (Figure 12-4, B-2). The 

 expected phenotype given last is contrary to 

 fact, the phenotype of the Fi sons being white, 

 not dull red. 



Since we cannot explain the observations 

 merely by identifying maleness with XX or 

 XY, we shall have to increase the number of 

 assumptions used in an attempt to accom- 

 plish this. Let us test two hypotheses simul- 

 taneously, namely that Drosophila males are 

 XY and that the Y chromosome can carry only 

 w, and cannot carry w+. The genotypes and 

 results of the first cross given in the last para- 

 graph remain the same (Figure 12-4, A-3). 

 The reciprocal cross (Figure 12-4, B-3) be- 

 comes white 99 X'^X"' by dull red d& 

 X'"'Y"' to produce X«"X«' (dull red) daughters 

 and X"'Y"' (white) sons, as observed. Since 

 these hypotheses fit the observations we may 

 accept them. 



There are a large number of other traits 

 which, hke white eyes, can be studied one at 

 a time in Drosophila. Their transmission 

 genetics also proves to be based upon a pair 



of genes on the sex chromosomes, each case 

 giving different results in Fi when pure lines 

 of the two alternatives are crossed recipro- 

 cally. Moreover, each case can be explained 

 by assuming that females are XX, males XY, 

 with the Y carrying the most recessive and 

 least effective allele known for the gene pair 

 under test, as is the case for white. The 

 finding, in dozens of different cases, that the 

 Y chromosome always behaves as though it 

 contains the least influential allele of the gene 

 pair, tempts the hypothesis that for the gene 

 pair under test the Y in fact contains no allele 

 at all! The very fact that a partially or com- 

 pletely dominant allele of such a gene is 

 normally never found on the Y of Drosophila 

 must mean that such alleles cannot be formed 

 there by mutation of the most recessive allele, 

 most simply because this recessive allele does 

 not exist on the Y. Accordingly, the Y can 

 routinely be considered to lack an allele of a 

 gene located on the X, and Figure 12-4, A-3 

 and B-3 should have Y substituted for each 

 Y-. 



In all the cases where the Y carries no allele 

 of a gene on the X, because sons receive their 

 single X from their mother, they will show 

 phenotypically whatever is contributed in the 

 X they receive from their mother. With re- 

 gard to these genes, therefore, a female is being 

 test crossed whenever (or to whomever) she 

 mates, since her genotype can be determined 

 directly from the phenotypes of her sons. 

 Genes present on the X chromosome and 

 absent on the Y are said to be hemizygous 

 in the Drosophila male, because half of the 

 zygotes he produces will receive these alleles 

 in the X he contributes, while the other half 

 will not because they receive the Y. Note 

 that the X of a Drosophila male is obtained 

 from his mother and is transmitted to each 

 of his daughters. 



In the case of chickens, nonbarred feather 

 99 X barred feather cf cf produces offspring 

 which are all barred — barred (B) being domi- 

 nant to nonbarred (b) (Figure 12-5 A). In the 



