198 Chromosome Maps 



and recessive males (Fig. 606) could be accounted for only by 

 crossing over between alleles in the X and Y chromosomes. 



Nine genes have been plotted on the homologous segment of 

 the X and Y chromosomes (Table 6). The gene for complete 

 color blindness or day blindness prevents the afflicted person 

 from distinguishing any color and must not be confused with 

 Daltonism. Xeroderma pigmentosum is a condition in which 

 a person's skin and eyes are abnormally sensitive to light. It 

 leads generally to a fatal malignant disease of the skin. This 

 gene is almost completely recessive, but the heterozygotes are 

 often heavily freckled. Oguchi's disease is a form of night 

 blindness found mainly in Japan, but also reported in Europe. 

 This condition is characterized by a peculiar golden appearance 

 of the light-adapted fundus which disappears on dark adapta- 

 tion. Spastic paraplegia, resulting in a motor weakness of the 

 lower limbs, may be caused by both dominant and recessive 

 genes. The dominant gene is located in an autosome, but the 

 recessive type appears to be partially sex linked and to result 

 from three or more alleles determining different ages of onset. 

 A group of diseases are characterized by the formation of bullae 

 in the skin. In the dystrophic form of epidermolysis bullosa the 

 bullae are formed in deep layers and give rise to scars. The 

 dystrophic form results from both dominant and recessive non- 

 allelic genes, but the recessive form is probably incompletely sex 

 linked, is more severe than the dominant type, and is often 

 fatal. Retinitis pigmentosa is a type of night blindness involv- 

 ing a contraction of the visual field, thickening of the retinal 

 blood vessel, the production of ophthalmoscopically visible pig- 

 mentation of the retina, and often complete blindness. It may 

 result from several dominant and recessive genes. In some pedi- 

 grees the dominant form appears to be incompletely sex linked 

 with a crossover value of about 28 per cent. Two of the re- 

 cessive forms result from autosomal genes, and one of these also 

 produces deafness. A recessive type without deafness appears 

 to result from a recessive allele of the incompletely sex-linked 

 dominant gene for it shows the same percentage of crossing 

 over. Hereditary hemorrhagic diathesis is another character 

 that appears to be incompletely sex linked. The condition is 

 generally characterized by purpura and by prolonged spon- 



