. Adventures with animals and plants. Biology. 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
. Adventures with animals and plants. Biology. 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 separat
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