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.
