At the time of conception, two living germ cell—the sperm from the father and the ovum, or egg, from the mother—unite to produce a new individual. The male and female germ cells are known technically as gametes, and the single cell they form at the moment of conception is called the zygote. Both the egg and the sperm contain tiny thread-like structures called chromosomes. These have been called, somewhat poetically, the “threads of life.”
In 1962, the Nobel Prize for Medicine and physiology was awarded to James Watson, Maurice Wilkins, and Francis Crick for the discoveries in the field of the chemistry and mechanics of heredity. Each chromosome (threads) is made up of long molecules of deoxyribonucleic acid, called DNA for short.
The atoms of a DNA molecule are arranged in two long strands twisted together in a complex double spiral, along each spiral are ultramicroscopic areas called genes, which are the bearers of heredity. The order of the atoms which make up a particular gene serves as a blueprint or master code which govern the formation of the parts of the body for which that gene responsible. The branch of biology concerned with the mechanism of heredity is called genetics.
The all-important function of DNA as the chemical basis for the inheritance of characteristics has been proved in the case of one-celled organisms. The investigators observed that DNA, taken from one species of bacteria and mixed with that of another closely related species, produced structural changes that were passed along to succeeding generations.
The human zygote contains forty-six chromosomes—twenty-three from each parent. Recent studies have found an abnormal number of chromosomes (a number other than forty-six) associated with structural abnormalities. For example, some women who lack ovaries are found to have forty-five chromosomes, and Mongolian idiots have been found with forty-seven.
When the zygote is formed, the chromosomes from two parents “line up” and function as pairs. Then, as the zygote divides and redivides into more cells, the chromosomes divide similarly, so that there are the same numbers in each new cell. Check also this article about receptors (also referred to as the vestibular receptors or labyrinthine) that have the function to keep you informed at all times about your position in your environment–whether you move right or left, up or down, or forward or backward.
Dominance and Recessiveness of Genes
The phenomenon of dominant and recessive genes also helps explain how heredity can produce differences as well as resemblances among the members of a family. Sometimes the genes given to the individual by both parents have the same effect, as when the child receives only genes for black hair. But in other cases, the child receives genes that are determiners of different characteristics–as, for instance, genes for blond hair and genes for black hair. A gene that takes effect in the child whether or not it is paired with a like one is said to be dominant; a recessive gene is one that is ineffective unless it is paired with one just like itself.
An example of the operation of dominant and recessive genes is shown in the determination of blood type. There are four blood groups or types: A, B, AB, and O; determined by three genes: A, B and O. Each person receives a pair of such genes, one from each parent. A and B genes are equally expressive and both are dominant over the O gene. Therefore, a person can have type O blood only if he has received an O gene from each parent. If one parent has the blood genes AO (manifested as type A blood) and the other parent has the genes BO (type B blood), all four blood type may be represented among their children.
At the present time, we do not know exactly how many cases of dominance occur in the human being. Many Traits, however, are definitely known to be dominant. Among these are some types of skin and brain cancers, drooping eyelids, cataracts, certain muscular troubles, white forelock (“blaze”), baldness (dominant in men only), and dwarfism. Many less dramatic traits are probably also dominant, but they are less easily observed and traced.
There are also many recessive conditions, such as taste blindness, some types of visual and hearing defects, certain kinds of paralysis, some neuritis, albinism (“dead white” skin with pink eyes and white hair), possibly certain types of mental disease and epilepsy, and amaurotic family idiocy, a rare form of mental retardation.
When an organism has two different genes controlling a trait, it is said to be heterozygous for that trait; when two genes are the same it is said to be homozygous. In general, it appears to be better for an organism to have two different genes in a pair than two identical ones. Farmers, as well as scientists, have long observed “hybrid vigor” –the tendency of mixed plants to be stronger than purebred ones.
Interestingly enough, there is evidence that the genes themselves tend to produce heterozygosity just as the body tends to preserve advantageous conditions of temperature, chemical balance, and so on. Read also: Understanding Memory, Metamemory, and Language after Brain Injury