CHAPTER 15 



Human Genetics 



A. THE METHODS OF HUMAN GENETICS 



The inheritance of hereditary characters in man does not differ in 

 principle from heredity in other organisms, but the study of human 

 genetics is so important and differs so much from animal or plant 

 genetics in the methods which it employs that it must be given separate 

 consideration. Since controlled breeding is impossible, genetic analysis 

 has to be carried out by special and rather roundabout methods, which 

 will be discussed in this chapter. We shall first consider the study of 

 rare mutant genes with relatively clear-cut effects and then pass on to 

 the analysis of the genetic differences which exist within a population 

 of what we should call "normal" men and women. 



I . The Identification of Genes^ 



The first task of himian genetics is to find the genes. We can expect 

 that some genes will have clear-cut, noticeable and constant effects on 

 development; they have good expressivity. Two well-known examples 

 are the genes which determine the different blood groups; the two 

 dominant allelomorphs A and B specifically cause the presence of the 

 isoagglutinins A and B, the compound xiB having both and the homo- 

 zygous recessive neither. Another similar example is the dominant gene 

 which confers the capacity to taste thiophenylurea as a bitter substance 

 (to homozygous recessives it is nearly tasteless). ^ Rarer examples are 

 the genes for haemophilia, albinism, etc. 



In other cases we find, and indeed must expect, complications. These 

 are of two sorts. In the first place, there may be two or more different 

 genes which produce identical effects. Thus there seem to be at least 

 two different forms of retinitis pigmentosa, one determined by an 

 autosomal dominant, and one by an autosomal recessive, while there 

 may be a third due to a sex-linked factor. In some cases the discrimina- 

 tion of two different methods of hereditary transmission of apparently 

 similar pathological conditions has led to a more complete examination 



^ General references: Baur, Fisher, and Lenz 193 1, Blacker 1934, Gates 1929, 

 Hogben 1931, 1933, Holmes 1934. " Blakeslee and Fox 1932. 



