heredity

Chromosomes and Genes. Inside the nucleus of each germ cell are structures called chromosomes, composed of deoxyribonucleic acid (DNA) on a framework of protein. genes are segments of the DNA molecule; there are thousands of them in each cell, each carrying a specific hereditary trait, which may be physical, biochemical, or physiologic. Thus genes affect not only the physical appearance of an individual but also the physiologic makeup, the tendency to develop certain diseases, and the daily activities of all the cells of the body.

The human ovum and the human sperm each contain 23 chromosomes. Aside from the pair determining the sex, each chromosome in the sperm is similar in shape and size to one in the ovum. When the sperm penetrates the ovum, the fertilized ovum thus contains 23 pairs of chromosomes, or 46 chromosomes in all. The fertilized ovum (zygote) then begins to reproduce itself by dividing (mitosis). The original cell divides and forms two cells, each of these divides and forms a total of four cells, and so on until a many-celled embryo begins to take form. In the process of cell division, the chromosomes in the nucleus have the ability to make duplicates of themselves. They do not split in two, but instead each one produces another chromosome exactly like itself. When the two cells are formed from one, the chromosomes are divided so that each cell contains the same number and kind of chromosomes as the original. For this reason, all the cells in the developing embryo and in the human body, except the ovum and sperm, contain identical sets of 46 chromosomes.

The ovum and the sperm are formed by a special process of cell division (meiosis) in which each sperm or ovum receives only one member of each chromosome pair. If this were not true, and sperm or ova contained the full complement of 46 chromosomes, the cells of the offspring would have 92 chromosomes, their offspring would have 184, and so on. As it is, the amount of hereditary material in the body cells remains constant from generation to generation.

In the formation of the germ cells, it is a matter of chance which member of each pair of chromosomes goes to a given ovum or sperm. It is also purely a matter of chance which sperm fertilizes an ovum. All in all,, there are about 70 trillion possible combinations of chromosomes that a child could inherit.

Inherited Traits. Although many details of human heredity are not known, we know that the child receives a set of genes from the parents. These genes (hereditary determinants) develop into characteristics reflecting those of the parents, grandparents, and other ancestors. Before birth these inherited traits are influenced by conditions within the mother's body; after birth they can be shaped by environmental influences such as diet, training, and education.

Some specific aspects of human heredity are well understood. One member of a chromosome pair is contributed by one parent and the other by the other parent. A gene in one chromosome acts on the same trait as a gene in the same position on the other chromosome. It has been found that one gene may be more powerful in its influence than the other gene that acts on the same trait. The more powerful gene is called a dominant gene and the other is called a recessive gene.

Sex-Linked Traits. Certain hereditary traits are known as sex-linked because they are carried on the X chromosome. Color blindness is an example. This condition, in which colors appear as varying shades of gray, is rare in females but appears in about 8 per cent of the male population. The genes for color vision are located on the X chromosomes, and the gene for normal vision is dominant to that for color blindness. A female having one gene for normal vision on one X chromosome and one for color blindness on the other will have normal vision, since the color blindness gene is recessive. A male, however, having only one X chromosome, will be color blind if that chromosome has the recessive gene, since there is no corresponding dominant gene to suppress it. It is possible for a female to be color blind, if she has two of the recessive genes, but it is quite rare that these two genes come together in one person.

Another characteristic associated with sex is baldness. The gene for baldness is dominant in males and recessive in females. Thus a male need have only one gene for baldness for the trait to be expressed, but a female must have two.

Hereditary Diseases. These should be distinguished from congenital birth defects. A congenital defect is one that the infant is born with, such as a cleft lip, a birthmark, or congenital syphilis, but the defect can arise during conception or pregnancy and not be related to heredity. Hereditary diseases, on the other hand, are passed from generation to generation by genes. Some diseases, such as cystic fibrosis, are transmitted by recessive genes.

Role of Mutation. Mutation is the term used for a spontaneous change in a chromosome or gene. Normally chromosomes duplicate themselves exactly during cell division. Occasionally, however, the new cells contain an altered gene or chromosome. If the mutation occurs in an ovum or sperm involved in reproduction, the new trait will be expressed in the offspring.

Many mutations are so minor that they have no visible effect. A mutation that is very harmful will usually result in the death of the fetus and spontaneous abortion. Occasionally a mutation is beneficial. Favorable mutations gradually tend to spread through a population. The accumulation of mutations over millions of years has contributed to evolution.