Chap. 20 



Normal 



THE PHYSICAL BASIS OF HEREDITY 

 Color blind Color blind 



407 



Nocmal 





Normal 



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9 



XX 



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9 

 KX 



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XX 





XY 



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Fig. 20.15. The criss-cross inheritance of color-bUndness from a color-blind 

 man, via his daughter, a carrier, to his grandson. 



Color-blindness is a recessive c to normal vision C. The gene for color-blindness 

 is carried only on X-chromosomes. A man is color blind because he inherits one 

 X-chromosome carrying color-blindness and no gene for normal vision in the Y- 

 chromosome. A woman is a carrier because she inherits two X-chromosomes, C and 

 c with the C of normal vision the dominant one. If the mother is a carrier and 

 father is color blind, their daughter may be color blind, a rare occurrence. (After 

 Dunn. Courtesy, Sinnoit, Dunn, and Dobzhansky: Principles of Genetics, ed. 4. 

 New York, McGraw-Hill Book Co., 1950.) 



of Spain, two of whose sons had hemophilia, including the Crown Prince 

 Alfonso (IV. 16). There appears to be no record of hemophilia among the 

 ancestors of Queen Victoria, and the gene for the disease is believed to have 

 arisen as a mutation. If her consort, Prince Albert, had carried a gene he 

 would have had the disease. 



Sex-influenced Inheritance — Baldness. There are many types of baldness; 

 some of them are inherited. Its most striking character is its much greater fre- 

 quency in men than in women (Fig. 20.17). It seems probable that hereditary 

 baldness is due to a gene that behaves like a dominant in men and like a 

 limited recessive in women. The different expression of the genes in men and 

 women is evidently due to a difference in hormones that makes them more 

 or less sensitive to their inherited genes. Eunuchs (castrated men) seldom 

 become bald. In women, the sparsity of male hormones is said to keep the 

 hair, even though the genes for baldness may be present; in men the ex- 

 cess of male hormone makes the hair follicles sensitive to the genes of bald- 

 ness. 



