THE PRINCIPLES OF HEREDITY 521 



somes. The X-carrying sperms produce females, while those sperms 

 without an X produce males. 



Sex-Linked Inheritance 



Some of the genes on the X-chromosome (or Z-chromosome) are 

 directly concerned with sex determination, but the majority of them 

 are similar to the genes on the other chromosomes. They affect many 

 characteristics which have nothing to do with sex. In man, for instance, 

 there is a gene on the X-chromosome which is concerned with color 

 vision. There is one recessive form of this gene which produces de- 

 fective color vision known as color blindness. Persons who express this 

 gene have difficulty distinguishing between red and green. There is 

 no counterpart of this gene on the Y-chromosome. Most of the genes 

 on the X-chromosome in a man are present in the single (haploid) 

 condition, because the smaller Y-chromosome does not possess the 

 homologous genes. This means that a man will express these genes 

 regardless of whether they are dominant or recessive when they are 

 present in pairs in the female. Therefore, if a man receives a gene for 

 color blindness from his mother he will be color-blind for he does not 

 receive a partner for this gene from his father. A woman, on the other 

 hand, must receive a gene for color blindness from both parents, for 

 this gene is recessive to the gene for normal color vision. Because of 

 this there are always more color-blind men than women with this af- 

 fliction — there are about sixteen color-blind men to one color-blind 

 woman. Fig. 32.10 shows how inheritance of sex-linked characteristics 

 is diagramed. 



Hemophilia is another human characteristic which is sex-linked. 

 This is something called "bleeders" disease because the blood clots very 

 slowly, and there is excessive bleeding whenever an injury occurs. 

 This gene occurs in some of the royal families of Europe and causes 

 many an untimely death among the males of this line. 



REVIEW QUESTIONS 



1. What is meant by the expression "dominant" and "recessive" as applied to 

 inherited characters ? 



2. By means of a diagram show how a white guinea pig can be produced by 

 a cross between two black guinea pigs. 



3. Suppose you have some cats that are gray and some cats that are yellow. 

 Tell how you would breed them to determine which of these two characters is 

 dominant and which is recessive. 



4. Short hair is dominant to long hair in guinea pigs. Show the results of a 

 cross between a pure (homozygous) short-haired male and a pure long-haired female. 



5. Cross the offspring from problem 4 among themselves and show the expected 

 results. 



