Heredity and Inherited Diseases

Understanding Heridity and Inherited Diseases
To understand how some disorders can be passed from one generation to another, one must first understand the role played by the genes in determining the form and function of each cell in the body.

The genes are the basic units that determine the hereditary characteristics of an organism. Genes, which are composed of molecules of deoxyribonucleic acid (DNA), can be thought of as chemical instructions. Each gene, by virtue of the particular structure of its DNA molecule, contains the code for a specific trait, determining both what a cell is and how it works (as if a computer program not only told the computer what to do but helped to form the machine itself).

Within each cell, thousands of genes are linked in a specific order, like beads on a necklace, to form structures called chromosomes, which are, in effect, continuous strands of DNA. It has been estimated that each cell contains about five feet of coiled DNA strands and that each strand is made of about 100,000 genes.

The particular composition of the genes and their arrangement on the chromosomes are what constitute the genetic blueprint for each individual. Cells that develop into liver tissue, rather than blood cells or nerve fibers, for example, do so because that is what their genetic coding dictates. In this way, the cells of the body are programmed to create a person with a certain color of eyes and hair, as well as the thousands of other characteristics that make each human unique.

Sex Cells

Each cell in the human body contains 46 chromosomes. The only exceptions are the sex cells (the ovum and the sperm), each of which contains only 23 chromosomes. When these sex cells unite in the fertilization of the ovum by a sperm, the result is a full complement of 46 chromosomes, with genes donated by both parents. Since each parent contributes only 23 chromosomes (half the genetic coding that makes each parent a unique individual), the genetic makeup of their offspring is a blend of components of both parents' genetic material.

Dominant and Recessive Traits

The traits that genes give rise to may be either dominant or recessive. A recessive gene produces a certain trait only if its effects are not overridden by those of a dominant gene.

Eye color provides a relatively straightforward illustration of how inheritance of traits works. The gene for brown eyes is dominant; the gene for blue eyes is recessive. The child of a brown-eyed parent who has two brown-eye genes and a blue-eyed parent (who must have two blue-eye genes) will have brown eyes because the brown-eyed parent has only dominant brown-eye genes to contribute to the child's genetic makeup. However, if the brown-eyed parent has a dominant brown-eye gene and a recessive blue-eye gene, the child has a fifty-fifty chance of receiving a blue-eye gene from both parents and thereby having blue eyes. (Actually, inheritance does not always work with such textbook simplicity -- sometimes the child of a brown-eyed parent and a blue-eyed parent will have green or hazel eyes.)

The union of two blue-eyed persons, because it involves only recessive blue-eye genes, will always produce blue-eyed offspring.

The offspring of two brown-eyed persons who each have a recessive blue-eye gene have a one-in-four chance of receiving a blue-eye gene from both parents and, as a result, having blue eyes. (This final combination illustrates how recessive genes can be present but unsuspected, allowing a trait to appear unexpectedly after skipping generations.)

Mutation

Genes are normally transmitted unchanged from one generation to the next. Sometimes, however, mutations occur -- that is, the structure of the gene itself is changed, perhaps due to the effect of a toxic substance, an infection, or exposure to radiation. Offspring that receive a mutated gene will exhibit a characteristic that is not present in either parent.

The discovery of the structure of the DNA molecule opened a new era in medical research. Scientists in the new field of genetic engineering are exploring ways of artificially creating mutations in genes so that someday it may be possible to correct the errors in genetic coding that are responsible for causing various disorders.

Genetic Counseling

As the medical profession has learned more about inherited diseases, it has been able to offer genetic counseling to couples concerned about the possibility of having a child with an inherited disease or abnormality. A genetic counselor or specialist in genetic disorders can estimate the likelihood that a couple's offspring will be afflicted with a problem due to an inherited trait or to the age of the parents. For some genetic disorders, tests can determine whether one or both parents are carriers or can detect whether a defect is present in a fetus.