Distribution of Genes in Human Beings 89 



made when dealing with genes in autosomes. Thus the off- 

 spring of the Fi and the curved parent are 1 wild type : 1 curved 

 no matter whether the Fi or the curved fly is used as the female. 



Distribution of Genes in Human Beings 



In Chapter 3 a number of genes were mentioned which are 

 known to produce certain inherited characters in human beings. 

 These genes act on the developing organism in apparently the 

 same manner as genes in other animals and in plants. They are 

 also repeated in succeeding generations in exactly the same 

 way as genes in other organisms. Since, however, human families 

 are normally smaller than most other animal families, and since 

 large numbers of offspring are never produced comparable to 

 families of plants, it is not always so easy to determine the 

 mode of inheritance of genes in human beings. 



An interesting example of the transmission of a human trait 

 is shown in Fig. 35a, page 102. A man who was born with crooked 

 little fingers married a woman with normal fingers. From this 

 marriage six children were produced. Since four had crooked 

 fingers and two did not, it appears very probable that this trait 

 was inherited. From this information we can infer either that 

 crooked fingers are the result of a dominant gene and that the 

 male parent is heterozygous or of a recessive and that the mother 

 is heterozygous. Either way, a family in which four had crooked 

 fingers and two had normal ones could be produced. One daugh- 

 ter with crooked little fingers married a normal man and pro- 

 duced two children with crooked little fingers. A son with the 

 trait married a normal woman and produced two normal children. 

 Another daughter with the character married a normal man and 

 had one normal daughter and one with crooked little fingers, 

 and the other affected child of the original mating had three 

 daughters, all of whom had crooked little fingers. 



If we assume that crooked little fingers are produced by a 

 dominant gene, all the Fi individuals with crooked little fingers 

 would be heterozygous. Therefore, when four of them married 

 normal recessive individuals, each family should have affected 

 and normal people in equal numbers. Since, however, all families 

 had only two or three children, it is quite within the limits of 

 probability for all the children of any one family to be either 

 affected or normal. None of the four families disagrees with 



