504 - Heredity and Evolution 



plants that practice inbreeding (self-polli- 

 nation) on a natural basis. 



It is also important tc realize that most 

 genes affect several or many characteristics in 

 the organism. The white-eye gene (w), which 

 was used to exemplify sex linkage, received 

 its name because its eye color effect is more 

 dramatic and easier to follow than its other 

 effects. Careful study, however, reveals that 

 the white (w) gene has many other effects. 

 Flies that are homozygous for this gene lack 

 pigment in many of the internal organs; they 

 develop more slowly and display less vigor 

 than red-eyed flies; and homozygous white- 

 eyed flies produce fewer offspring than their 

 wild-type relatives. And what is true of this 

 gene is probably true of all the genes in every 

 organism: each gene has various effects upon 

 many different parts and functions. Thus, to 

 speak of the "gene for white eyes," or the 

 "gene lor vestigial wing," is merely to iden- 

 tify a certain gene by its most conspicuous 

 and obvious effect. 



HEREDITY IN MAN 



Great difficulties have beset the scientific 

 study of human inheritance. The impossi- 

 bility of controlled experimental breeding, 

 particularly as to the technique of inbreed- 

 ing; the long period between generations; 

 the relatively small number of offspring; and 

 finally the large number of man's chromo- 

 somes — all have combined to hamper the 

 progress of a detailed analysis of human 

 heredity. In fact, human geneticists have had 

 to depend largely upon family pedigrees; 

 and sometimes such data are not completely 

 reliable. 



Despite these difficulties, a great deal has 

 been learned about human inheritance. Re- 

 liable statistical methods for the analysis of 

 human populations have been developed, 

 and it is now evident that man and other 

 organisms possess essentially similar genetic 

 processes. Other mammals — especially mice, 

 rats, guinea pigs, and rabbits — are much 

 more favorable than man for genetic studies; 



and a vast amount of data prove conclu- 

 sively that mammalian heredity proceeds 

 along the same lines as were first mapped 

 out in Drosoplula. 



Population Genetics. The currently active 

 field of population genetics was opened up 

 in 1908 as a result of the mathematical and 

 statistical studies of Hardy and Weinberg. 

 The Hardy-Weinberg method of analvsis, 

 based on an extension of the binomial equa- 

 tion, makes it possible to determine the 

 genetics of a given trait on the basis of its 

 statistical occurrence in a population, pro- 

 vided that the population and sampling are 

 large enough to yield valid probabilities. 



A few of the results of human population 

 analvsis are indicated in Table 26-2. Take, 

 for example, the case of the recessive gene of 

 albinism. The homo/vgous condition, which 

 produces an overt albino, occurs in only 1 in 

 20,000 individuals; the heterozvgous combi- 

 nation, however, which vields a carrier, is 

 found in f of every 70 people. In addition to 

 the several examples cited in Table 26-2, the 

 inheritance of a tasting capacity (ability in 

 man to taste a special chemical, namely 

 phenylthiocarbamide) deserves fuller consid- 

 eration. This classic case was analyzed very 

 completely by L. H. Snyder of the University 

 of Hawaii. 



Snyder tested the tasting capacitv of some 

 3500 subjects and found that 29.8 percent 

 were insensitive to the bitter taste of phenyl- 

 thiocarbamide (PTC). Then, working on the 

 assumption that the "taste gene" is dominant 

 to the "nontaste gene" and that PTC-tasting 

 is determined by a single pair of genes, an 

 analysis was made of the occurrence of sen- 

 sitivity among children derived from a num- 

 ber of taster X taster and taster X nontaster 

 marriages. The actual values derived from 

 these unions were 12.3 percent and 33.6 per- 

 cent of nontasters, respectivelv. These values 

 corresponded very closely with the mathe- 

 matically predicted values of 12.4 percent 

 and 35.4 percent, which proves that the 

 original assumptions were valid. 



The importance of the Snyder analysis, 



