310 Cell as Basis oj Organic Activity 



male ofifspring receives the factor for a character from the female 

 parent. This inheritance, often called "crisscross inheritance," may 

 appear in a male in one generation, be carried by the female of the next, 

 and then reappear in the males of the third generation. 



Hemophilia, a condition in which the blood does not clot properly, 

 is also sex-linked and is inherited in the same manner as color blindness. 



Sex-Limited Characters. — Genes carried on the X chromosome 

 that do not show dominance produce sex-limited characters. The reason 

 for this is simple: a different phenotypic character shows up in the 

 heterozygous condition. Since this takes two X chromosomes it can 

 appear only in females. An example of this is coat color in cats. In 

 this case, consider B as representing a gene for black while b is a gene 

 for yellow. A female cat may be X"X^ X^X^ or X'X\ that is, the 

 coat color may be black, tortoise shell, or yellow. The male cat can 

 be only X^'Y or X^Y, that is, black or yellow. Thus a cat that is 

 tortoise shell must contain two genes and hence is a female. 



Some characters while not sex-linked are sex influenced. One type 

 of baldness in human beings is of this kind. The gene affecting this 

 character is carried on one of the autosomes; however, its expression 

 is controlled by the sex of the individual. In males, it is dominant and 

 in females it is recessive. 



WZ Sex Determination. — The XY type of sex inheritance occurs 

 in most animals; however, in birds, butterflies, and in some fish there 

 is another type of sex determination known as the WZ type. In this, 



Fig. 99. — A cross involving another type of sex-linked character found in bii'ds. 

 Here the characters are carried on the Z chromosome; two are found in the male 

 and one in the female. 



the male has two Z's while the female has the WZ combination. The 

 characters are carried on the Z chromosome. This means that sex-linked 

 characters will be just reversed in this type of inheritance from that 

 seen in the XY type. The male will have two genes, and the female 



