332 AN INTRODUCTION TO MODERN GENETICS 



females. The ratio of ^ affected males to cf' affected females, appropriate 

 for a recessive gene, can be verified in the case of colour blindness, but 

 in very few others. Many of the sex-Unked genes usually considered to 

 be recessive cannot strictly be proved to be so. For instance, hsemo- 

 philia (failure of the blood to clot) is found only in the male sex. It is 

 not handed on from father to son and therefore is probably due to a 

 sex-linked gene though its failure to show in females may indicate 

 that it is also sex-limited in expression (there is some evidence that the 

 presence of the female sex hormone prevents the expression of haemo- 

 philia).^ In this case the question of recessiveness and dominance does 

 not arise, and the gene may be classed as indeterminate. In other cases. 



Fig. 138. Provisional Map of the Human X Chromosome (and of 

 ^ the homologous part of the Y). — The genes indicated are: qc achroma- 

 topsia, xe xeroderma pigmentosa, og Oguchi's disease, ep epidermolysis 

 °3 bullosa, Re, re dominant and recessive retinitis pigmentosa. The map 

 «f> was derived from investigation of partial sex linkage. 



(From Haldane.) 



28 Rc.rt 



it is known that a gene has some degree of dominance, since some 

 effect is shown in the heterozygous female, but it is not known whether 

 the dominance is complete or whether the homozygous female would 

 show even more marked effects. Levit^ has proposed calling such genes 

 "conditionally dominant." 



The quantitative requirement that, from a mating involving a sex- 

 linked recessive Aa x AY, the affected males should form a quarter of 

 the offspring, can be tested in some cases (e.g. hsemophilia, some forms 

 of night blindness, etc.), correction being made as before for the possi- 

 bility of missing some families in which by chance no affected offspring 

 occur. 



Genes inherited in the Y chromosome have also been described in 

 man. If ihey are confined to the Y, they pass from father to son and do 

 not appear in females, but the linkage is usually incomplete and crossing- 

 over takes place between the X and Y (p. 8o). The principle of the 



* Pratt 1932. 2 Levit 1936. 



