514 Organ'mns Are Products of Heredity and Enviroimieiit unit ix 



Fig. 463 These identical twins have the same gene viake-up. What striking similarities 

 and what differences do you find? Can you explain? (science illustrated) 



not to mention all the people you know, 

 even the people in your town whom you 

 never speak to. A person's job also makes 

 a great difference in the kind of life he 

 leads, not only because the washes will 

 vary with the particular kind of work, 

 but also because the kind of work he does 

 influences his opinions. The housewife 

 often has opinions different from those 

 of the office worker or the factory hand. 

 And the kind of job may cause physical 

 differences too. See Exercise 6. 



Slight differences can cause great 

 changes. It is difficult to realize that even 

 the slightest difference in the environ- 

 ment may cause great differences in peo- 

 ple. You may think that two children in 

 the same family have the same environ- 

 ment. Studies on identical twins show 

 that this is not true. Identical twins have 

 the same genetic make-up. They are 

 formed from a single fertilized o.^^. In 

 the early stages of cleavage, two equal 

 masses of cells separate; each gives rise 



to a child. There are other tAvins (fra- 

 ternal) which develop from two sepa- 

 rate eggs fertilized by t\vo separate 

 sperms; these are no more alike than any 

 two brothers and sisters. But identical 

 twins are really "identical" in genetic 

 make-up. Yet identical twins, even when 

 brought up together, do not grow up to 

 be exactly alike. Evidently, environments 

 which would seem to be the same are 

 never exactly the same. In Exercise 7 

 you will find examples of differences. 



Identical twins in different environ- 

 ments. In one study, biologists came as 

 close as they ever can come to a con- 

 trolled experiment in human heredit)' 

 when they did the following. They suc- 

 ceeded in locating 19 pairs of identical 

 twins who, for one reason or another, 

 had been separated shortly after birth 

 and were thus exposed to different en- 

 vironments. Since identical twins start 

 with the same gene make-up, any differ- 

 ences found later between two twins 



