Chapter 11 



STUDIES OF HUMAN TWINS 



1 



N THE preceding Chapter it was 

 concluded that, in general, pene- 

 . trance and expressivity may be 

 modified by the environment or by the geno- 

 type, or both. In organisms other than man, 

 it is possible to standardize conditions experi- 

 mentally, so that a standard genotype exposed 

 to different environments would show to 

 what extent environment was responsible for 

 phenotypic variability, whereas a standard 

 environment to which different genotypes 

 were exposed would reveal to what extent 

 these genotypes produced different pheno- 

 types (cf. p. 6). Since neither the environ- 

 ment nor the genotypes of human beings are 

 subject to experimental control, the question 

 may be asked, how can it be determined to 

 what extent a particular human trait is con- 

 trolled by genotype (nature) and by environ- 

 ment (nurture)? Fortunately, this nature- 

 nurture problem can be studied using the 

 results of certain naturally occurring experi- 

 ments. What are these? 



An individual contains many different 

 parts, all of which can be presumed to have 

 the identical genotype. Accordingly, as men- 

 tioned in the last Chapter, one can attribute 

 to nurture any phenotypic differences in ex- 

 pressivity or penetrance found among parts 

 which are essentially duplicates of each other. 

 So, for example, a Polydactyly heterozygote 

 with six fingers on one hand and five on the 

 other illustrates the extent to which environ- 

 ment can affect this trait. When, however, 

 a trait appears which involves the entire 

 individual, or which occurs either in several 

 74 



nonduplicated parts of the body or in a single 

 part, the contribution of nurture can be 

 learned only from comparisons of different 

 individuals who have identical genotypes. 



What is the probability that following sex- 

 ual reproduction of human beings two indi- 

 viduals of identical genotype will be produced? 

 On the assumption that the members of each 

 pair of chromosomes in the two parents are 

 genetically different in one respect, then, the 

 chance of two offspring being genically identi- 

 cal is )r^ X }^", or K'*^. This is so because 

 the chance a gamete will carry the same geno- 

 type as another gamete of that individual is 

 }<2^^, since chromosome pairs segregate inde- 

 pendently, and because gametes fertilize at 

 random. Since each human individual is 

 heterozygous for numerous genes, the chance 

 of obtaining genetic identity in two siblings 

 (brothers and/or sisters of the same parents) 

 is, in effect, infinitely small. 



However, two or more siblings with identi- 

 cal genotypes may be produced as a conse- 

 quence of asexual reproduction in man. This 

 kind of reproduction occurs in the following 

 manner. A single fertilized egg starts its 

 development normally by undergoing a series 

 of mitotic cell divisions. At some time, how- 

 ever, the cells produced fail to adhere to each 

 other, as they would normally do to form a 

 single developing unit, but instead become 

 separated into two or more parts, each of 

 which may be capable of forming a complete 

 individual. Each individual produced this 

 way is genetically identical to all others 

 formed from the same fertilized egg. The 

 separation referred to may occur at the two- 

 cell stage or it may occur later, at which time 

 the number of cells may be unequal in the 

 two or more groups formed. It is even possi- 

 ble for these separations to occur twice, at 

 different times in the development of a partic- 

 ular zygote. Individuals produced this way 

 are called identical or monozygotic twins, trip- 

 lets, quadruplets, etc. We need only consider 

 identical twins here, since multiple births of 



