NATURE AND NURTURE
It is well known that we cannot pass along our college education through the channels of inheritance. On the other hand, we do know that color blindness in males is inherited and dominant, but in females it is recessive; that'is, the woman will probably not be color blind, but she could carry the characteristic on to her son. Genetics st'..idies the mechanics of heredity. We speak of genes as the unit of inheritance; they are housed, so to speak, in chromosomes. These structures are found in pairs and are responsible for our heredita..-y traits. Half of an individual's chromosomes come from the mother, half from the father. If both genes in a pair are dominant, the individual wiil display a given trait. If one is dominant and the other recessive, the dominant will still prevail, although the recessive will be passed on arId may show up in a later generation. In other words, we can think of the dominant gene of an opposing pair as one that takes over to produce a hereditary trait.
Phenotypes and Genotypes
Characteristics that are obviously displayed, such as eye color, are called phenotypes to distinguish them from traits that are carried genetically but not displayed. These latter we call genotypes. If both brown-eye and blue-eye genes are present in a baby, he will have brown eyes because brown eyes are dominant and blue eyes are recessive. Among recessive characteristics are blond and red hair, baldness, and susceptibility to poison ivy. Some dominant characteristics include dark hair, curly hair, and even immunity to poison ivy.
Behavior Genetics
What about the things we do? Did Sally inherit her mother's pleasing personalitY? Did Bill inherit his father's aggressive tendencies? Such questions relate to behavior genetics, and we find example~ such as mating chickens in terms of their tendency for fighting or not fighting. One investigator bred aggressive cocks and hens and then
compared them to chickens bred for their timid behavior. After only four generations, offspring of these two groups were still consistently different from each other although they had been given no chance to observe the aggressive or timid habits of their parents.
Selective breeding has been studied over many generations in cattle, horses, and other animals, and, of course, in an indirect and less controlled way in man. Vie speak of the behavioral characteristics of "high-strung" horses or of animals with "hot blood" as a description of their desire for sexual behavior.
It is a generally accepted fact that in human beings, an individual's phenotypical or obvious level of ability results from a developmental process that involves the interaction of inherited traits and the environment. Since "selective breeding" experimentation in human breeding is impractical, we can only turn to indirect data, for example, studies of resemblances of behavior between blood relations. Identical twins are much more alike than ordinary siblings. Cousins are shown to have very little similarity. In studies of identical twins reared apart, the resemblance of intelligence remained higher than for fraternal twins raised together. But, interestingly, the intelligence relationships between identical twins reared apart were lower than for identical twins reared together. ,
Parent-child resemblances are greater with blood parents than with foster parents, whether or not the child has grown up with his blood parents.
Another indirect way of getting at human inherited problems is to look at disease. For a long time we have known that hemophilia is inherited, .and although there are treatments for such "bleedtrs" as individuals, the potential for the disease stays with-the blood line. Let us look at another disease example.
Disease Example. One inherited disease now being studied is sicklecell anemia, so-called because the red blood cells take on a sickleshaped form. The disease is characterized by periodic attacks of acute pain, weakness, jaundice, and leg ulcers. In some people it is recessive, causing them to be carriers; in others it is dominant. The disease is not infectious and cannot be caught from others by contact. Just as some diseases appear to be limited to Caucasians (thalassemia, for example, a blood disease), sickle-cell anemia has been found for the most part in Negroes. It is estimated that two out of twenty-five American Negroes carry the sickle-cell trait, while the disease appears about once in 400 births.
Heredity and Environment Together
It is a generally accepted fact that human beings are a product of heredity and environment. Out of the product come both our individuality and our individual differences. If a person with high hereditary potential is placed in a poor environment, his measured ability will probably be low. If hereditary potential is low, even the best environment will be able to take the person ahead only so far.
One may conclude that heredity provides a potential, which may or may not be developed by environment. Let us look at some animal studies on heredity and environmental stimulation since good control studies are so difficult using human subjects.
It has been found possible to breed rat strains with different amounts of inherited learning ability. In one laboratory situation, the learning ability of a large number of rats was studied by compelling them to run mazes. The animals were divided into two groups. Group 1 was composed of animals that made few errors and were .mated together. Group 2, composed of those that made many errors, were likewise bred together. Succeeding generations in each group were tested for learning ability and selectively bred for intelligence. By the seventh generation, there was little or no overlap in the scores of the bright and dull groups.
In other studies rats equal in heredity were placed in twa groups by using the split-litter method. One group was reared in a very restricted environment. Each animal was isolated in its own cage with no opportunity to socialize with other rats. The cage was bare, and there was no opportunity either to seek out or to solve problems. The animals in the other groups were raised in a stimulating environment, called by ooe psychologist an "amusement park for rats." Here the subjects were given objects to play with; barriers and pathways provided opportunities for exploration.
When the animals grew up, they were tested for their problemsolving ability. The group that had grown up in isolation was found to be inferior to those raised in the richer environment. Furthermore, the superiority of the second group was found to be lasting. Similar studies with other animals has shown the same sort of results. However, this superiority in performance did not carry over into the heredity strain for succeeding generatio:ls.
To conclude, nature and nurture work together, but one of them may play a larger role than the other in producing some given behavior.
