GENETICS 591 



continuity of the germplasm rules out the inheritance of acquired char- 

 acters, cut off the tails of generation after generation of mice. The tails 

 of the nineteenth generation of mice, however, were just as long as 

 those of the first. In an even more searching experiment, Zeleny raised 

 250 successive generations of fruit flies in total darkness, yet the charac- 

 ter of the eyes remained unaltered. 



292. Human Inheritance 



The results of many studies have shown that the inheritance of 

 human traits follows the same laws as those of other animals and plants. 

 Human traits may be controlled by multiple factors, multiple alleles, 

 sex-linked genes, and so on. The study of human inheritance offers 

 special difficulties, not only because test crosses cannot be made, but also 

 because human beings have so few offspring per generation, because 

 the large number of years between successive generations means that 

 records for only a few generations are available, and because human 

 beings are heterozygous for many traits. Careful examination of pedigree 

 records has revealed that the inheritance of several hundred human 

 traits is determined by single dominant or recessive genes. 



In the last forty years statistical methods have been developed which 

 enable investigators to pool the results of similar matings and calculate 

 the relative frequency of dominant and recessive alleles. Many of the 

 methods of such investigations of population genetics are derived from 

 the Hardy-Weinberg Law. The study of L. H. Snyder of the inheritance 

 of the abdity to taste phenylthiocarbamide provides an example of the 

 application of these principles. Snyder tested 3643 people and found that 

 70.2 per cent reported that this substance has a bitter taste and 29.8 

 per cent found it to be completely tasteless. If this trait is inherited by a 

 single pair of genes, with "tasting" dominant to "nontasting," the meth- 

 ods of population genetics permit one to calculate that 12.4 per cent of 

 the children of marriages of tasters with tasters will be nontasters and 

 35.4 per cent of the children of marriages of tasters with nontasters will 

 not be able to taste phenylthiocarbamide. In Snyder's survey the per- 

 centages actually found were 12.3 per cent and 33.6 per cent, respectively; 

 the close agreement of the theoretical and observed values indicates 

 that the original assumption is correct, and that tasting and nontasting 

 are inherited by a single pair of genes. 



It is important to realize that not all of the characters present at 

 birth are inherited and that, conversely, not all inherited traits are evi- 

 dent at birth. A condition present at birth is said to be congenital; some 

 congenital traits are inherited, others are the result of environmental 

 influences acting during development. For example, if a woman has 

 German measles during the first three months of pregnancy she is very 

 likely to give birth to a blind, deaf or deformed child. Many inherited 

 traits are not evident at birth, but develop some time later. Amaurotic 

 idiocy becomes expressed during childhood and Huntington's chorea 

 may not develop until a person is 40 years old. 



