330 ELEMENTS OF BIOLOGY 



of human offspring from a mating is extremely limited, the prob- 

 ability of occurrence of such an individual becomes somewhat 

 remote. 



No Dominance. Still other facts complicate the problems of 

 inheritance. Earlier it was stated that Mendel's law of dominance 

 and recession does not always hold. In some cases neither gene 

 of a pair is dominant or recessive; they appear to exert equal in- 

 fluence. In which case both are apparent in the Fi generation. 

 For example, blue Andalusian fowls are the Fi generation of a 

 cross between blacks and whites splashed with blue and are a 

 mixture of white and blue areas, giving a mosaic effect. Such 

 fowls do not breed true for color; in order to obtain them a cross 

 between black and white splashed with blue is necessary. 



Multiple Genes. Another complication arises from the fact 

 that in many characters the activity of more than one pair of 

 allelomorphic genes is involved, some of which are truly dominant 

 or recessive, but others may show incomplete dominance, or no 

 dominance. This is said to be the case in Man in crosses between 

 the White and Black races. More than one pair of genes for color 

 are involved; the result in the Fi generation is a pigmentation 

 that is neither White nor Black but a blend of the two, due pre- 

 sumably to incomplete dominance of some of the multiple genes 

 involved in determining pigment of the skin. 



Sex-limited Inheritance. Heretofore it has been assumed in 

 the examples given that the genes are located on autosomes, chro- 

 mosomes that do not have to do with sex determination. But the 

 X chromosomes, in addition to their role in determining the sex 

 of the individual, also carry genes. The peculiarity of certain types 

 of inheritance in which the female does not show the character but 

 her sons frequently do is readily explained when it is understood 

 that the genes for such characters are located on the X chromo- 

 somes and are recessive to the usual character, or normal. For exam- 

 ple, White eye is a character that occurs in certain flies which have 



