Incomplete Sex Linkage in Man 



197 



noncrossover types will be a heterozygous female who will be 

 phenotypically and a homozygous recessive man. These two 

 types follow the crisscross pattern expected of ordinary sex- 

 linked genes, but, in addition, two crossover types will be pres- 

 ent which do not appear in sex linkage. They are a homozygous 

 recessive woman and a heterozygous, phenotypically man (Fig. 

 60a). These new types show that we are not dealing with X 



o- ■ ■ -0 



X 



O ■ +0 



o- - -o 



X 



o - 4-0 



o- Oo - 



noncrossovers 



o- --00-- \0 



recombinations 



0-- --00-- +0 



noncrossovers 



o- Oo - 



recombinations 



a 



Fig. 60. Scheme of inheritance of an incompletely sex-linked gene in 

 human beings, (a) A cross between a female homozygous for the recessive 

 gene o and a male with on the X chromosome and o on the homologous 

 part of the Y chromosome, (b) A cross between the oo female and a male 

 with o on the X and on the Y chromosome. 



chromosome inheritance alone. If sex is disregarded, the ratio 

 of 1 Oo : 1 00 makes the cross appear to be an ordinary test- 

 cross. When, however, the sex of the individuals is also taken 

 into account, the greater abundance of Oo females and oo males 

 shows that the gene is in the sex chromosome. The percentage 

 of recessive women and dominant men will vary with the gene 

 under investigation and will depend upon the distance the par- 

 ticular gene happens to be from the nonhomologous segments. 

 By the use of these different percentages, Haldane has con- 

 structed a linkage map for this homologous segment. 



If the male used in a test had the gene on the Y chromo- 

 some and the o gene on the X chromosome, the noncrossover 

 types would be recessive females and dominant males as in Y 

 chromosome inheritance. The appearance of dominant females 



