Phenol y pic Effects of Gene Action 



75 



ferences in expressivity or penetrance found 

 among parts that are essentially duplicates 

 of each other. For example, a heterozy- 

 gote for Polydactyly with six fingers on one 

 hand and five on the other illustrates the ex- 

 tent to which environment can affect this 

 trait. When, however, a trait involves the 

 entire body, or only one or several different 

 nonduplicate parts of the body, the contri- 

 bution of nurture can be learned only by 

 comparing different individuals which have 

 identical genotypes. 



Since each human individual is hetero- 

 zygous for a relatively large number of genes, 

 the chance of obtaining genetic identity in 

 two siblings (children of the same parents) 

 is very small indeed. However, two or more 

 siblings with identical genotypes can be pro- 

 duced in man by asexual reproduction, 

 which occurs in the following manner. A 

 single fertilized egg starts development nor- 

 mally by undergoing a series of mitotic divi- 

 sions. At some time, however, the cells 

 produced fail to adhere to each other, as 

 they would normally do, and separate into 

 two or more groups, each of which may be 

 able to develop into a complete individual. 

 Each individual thus produced is, barring 

 mutation, genetically identical to all others 

 formed from the same fertilized egg. The 

 separation referred to may occur at different 

 stages of early development, and the num- 

 ber of cells in the two or more groups formed 

 may be unequal. Separation may even 

 occur more than once, at different times in 

 the development of a particular zygote. The 

 individuals produced in this asexual manner 

 are identical or monozygotic twins, triplets, 

 quadruplets, etc. We need consider only 

 identical twins here, since multiple births 

 of greater number are usually too infre- 

 quent to be useful for a general study of 

 the nature-nurture problem. 



Multiple births can also be produced di- 

 rectly by sexual reproduction. When twins 

 are produced in this way, they start as two 



separate eggs, each fertilized by a separate 

 sperm. Such twins are genetically different 

 — being, in this respect, no more similar 

 than siblings conceived at different times — 

 and arc nonidentical or dizygotic (fraternal) 

 twins. 



These two kinds of twins provide natural 

 experimental material for determining the 

 relative influence of genotype and environ- 

 ment upon the phenotype. Barring muta- 

 tion, monozygotic twins furnish the identical 

 genotype in two individuals, and both kinds 

 of twins share similar environments before 

 birth and, when raised together, after birth. 



The phenotypic differences between iden- 

 tical twins reared together are essentially the 

 consequence of environment alone (Figure 

 6-6). One can compare the average dif- 

 ference between such identical twins with 

 the average difference between identical 

 twins who, for one reason or another, were 

 reared apart. This comparison yields in- 

 formation regarding the influence of greater, 

 as compared with lesser, environmental dif- 

 ferences upon the phenotype. Since non- 

 identical or identical twins reared together 

 are exposed to environments which, on the 

 average, vary to the same extent, a com- 

 parison of the average difference between 

 identical twins and the average difference 

 between nonidentical twins will give an index 

 of the genotype's role in causing the differ- 

 ences observed. In order to collect valid 

 data from twin studies, it is essential that 

 one be able to recognize in each case whether 

 the twins are monozygotic or dizygotic in 

 origin. 



The best way to identify nonidentical 

 twins is to compare the siblings with refer- 

 ence to a large number of traits known to 

 have a basis in those genes which are 100% 

 penetrant and of fairly uniform expressivity 

 —such traits as sex, eye color, ABO, MN, 

 Rh. and other blood group types. Naturally, 

 only traits for which at least one parent is 

 heterozygous are of use in testing the dizy- 



