What does heredity have to do with psychology? Certain mental defects have long been known to be present at birth. Several forms of severe retardation are now known to be genetic defects. Mongolism (Down's syndrome), for example,is caused by the presence of an extra chromosome. There is also much evidence that a tendency to schizophrenia is inherited. Intelligence, too, seems to be at least partly a function of the intelligence of one's parents. Behavior is dependent to a large extent on the physical structure of the nervous system,which is definitely inherited.The fertilization of an ovum by a sperm marks the first moment of existence for every human being. Each ovum contains 23 chromosomes which are the carriers of hereditary material. These chromosomes are randomly derived from the 23 pairs in the mother's body. There are over eight million different sets of 23 chromosomes that could be produced by any given woman's body. Each sperm also contains 23 chromosomes. These, too, could be in any one of over eight million different combinations. The fertilized ovum, or zygote, combines these two random sets into 23 pairs of chromosomes, the normal number for human being. Thus it would be extremely unlikely for any two persons to have exactly the same heredity. How, then, do identical twins happen? A zygote grows by cell division. Sometimes the first cell division also causes separation. Then the separate cells grow and develop into genetically identical twins. The separation can occur more than once, resulting in identical triplets, quadruplets, and so on. This is the only way two or more persons can be genetically identical. Identical twins are very useful in psychological studies because of their genetic equality. Only identical twins triplets, and so on) can possibly have exactly the same genetic endowment Fraternal twins develop from two separately fertilized ova. Since two sperm and two ova are involved, these twins are no more similar genetically than ordinary siblings. Like ordinary siblings, fraternal twins may be of the same or opposite sexes. They, too, have furnished psychologists with valuable data. Chromosomes are the carriers of hereditary material. The actual hereditary material is DNA (deoxyribonucleic acid), which comes in long, complex molecules. These molecules contain thousands of genes, the determiners of traits.The genes are located in specific positions on specific chromosomes. Thus they can be in as many different combinations as the chromosomes can, if not more.With so many possible combinations from any two parents, it is not surprising that no two offspring (except identical twins) are ever genetically identical. The offspring are similar, however, since all of the genes were derived from the same two parents.
Most traits exist in degree, indicating the cumulative action of several genes. Some traits are either present or absent, indicating the presence or absence of one gene
Most traits are determined by the interaction of several genes. This is especially true of traits that show a wide range of~variation, such as intelligence.Some traits, however, do seem to be controlled by a single pair of genes, one gene of which is dominant and one recessive. In man these traits include eye color, curly or straight hair, color blindness, and hemophilia.
Normal running in mice is also controlled by a single pair of genes. Detailed study has been done on this trait of normal running versus a waltzing movement in mice (Dunn, 1932). Figure 5 illustrates the inheritance of this trait. The gene for normal running is dominant over a gene that causes a waltzing type of movement. Any mouse with both genes for waltzing will move in a waltzing way.Any mouse with both genes for normal running will run normally. Which trait will be inherited by the offspring of a male "waltzer" and a female who has two genes for running? Every sperm will have a gene for waltzing (r), and every ovum will have a gene for running (R).* Thus, every zygote will have one gene of each variation (Rr). Since the gene for running is dominant, all the offspring of these two mice will run normally. The gene variation for waltzing is recessive; it does not affect the animal's behavior or appearance unless there is no dominant gene present. Figure 5 shows this condition.
The breeding of two mice, each of which has both variations of the gene (Rr), creates the situation shown in Figure 6. The offspring of two Rr mice will be 1/4 RR (runners), 1/2 Rr (runners), and 1/4 rr (waltzers). Here it is clear that a recessive gene is expressed in an individual only when no dominant gene is present. If both parents have both types of gene, the recessive trait does not show up in them. One fourth of their offspring, however, will have two recessives and show the accompanying trait.
Traits that depend on the cumulative effect of many genes are the most difficult to study
Most traits are much more complex than the simple dominant-recessive controlled trait. It is much more difficult to study the interactions that contribute to the inheritance of such factors as intelligence or height. The effect of inheritance on intelligence, for example, has been studied in some detail. The similarity of IQ between identical twins was found to be significantly greater than between fraternal twins (Newman, et al/., 1937). These data indicate that heredity is probably involved in determining intelligence, although there are not enough precise data to determine the extent or degree of involvement. Note: geneticists follow the convention of using capital letters to denote dominant genes and lowercase letters to denote recessive genes.
Now test yourself without looking back.
1. What is the basic hereditary material?___________________________________________
2. Define briefly:
a. A person with two recessive genes for a trait has offspring by a person with one recessive and one dominant gene for that trait. What fraction of the offspring will have the following distributions of genes?
a. Two recessives
b. One recessive and one dominant
c. Two dominants
4. The dominant trait is expressed in an individual when he has:
a. two dominant genes.
b. one recessive gene and one dominant gene.
c. two recessives.
d. one parent who had two dominants.
5. Most traits are determined by:
a. a single pair of chromosomes.
b. a single pair of genes.
c. several pairs of genes.
d. one gene.
6. Which of the following describe identical twins?
a. They develop from an ovum fertilized by two sperm.
b. They have exactly the same heredity.
c. They may be of different sexes.
d. Differences are due to environmental influences.
7. Give a genetic reason for the resemblance between a child and his parents.
ANSWER KEY PAGE102
6 OR MORE CORRECT Module 4
FEWER THAN 6 CORRECT - review
PROGRESS CHECK 2