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Genetic Definitions

Genetics - Basics

The human body is made of cells; all cells contain chromosomes , which contain allmost all of the cell's Deoxyribonucleic acid (DNA). DNA is the genetic material, or genes , which provide the genetic blueprint for all offspring produced. Most cells of the human body contain 23 pairs of chromosomes, half of which are inherited from each parent. Only the human reproductive cells, the sperm cells in males and the ovum in females, have 23 individual chromosomes, not pairs. Scientists identify these chromosome pairs as the XX pair , present in females, and the XY pair , present in males, and number them 1 through 22.

When the reproductive cells, the sperm and ovum , combine at fertilization , the fertilized egg that results contains 23 chromosome pairs and the combination of the genetic blueprint from the mother and father. A fertilized egg that will develop into a female contains chromosome pairs 1 through 22, and the XX pair. A fertilized egg that will develop into a male contains chromosome pairs 1 through 22, and the XY pair. [ 1 ]

Genetic disorders may be influenced by several factors. When a disease runs in a family, faulty genes are passed from parent to child. Or a change in the genetic material, a mutation, may occur during formation of an egg or sperm cell. Mutations may also appear during fetal development. In these cases, children with genetic abnormalities may be born to parents without genetic disorders. The major types of genetic disorders are described below: [ 2 ]

Chromosomal disorders are abnormalities in the number or structure of the chromosomes. These disorders are often severe and include mental retardation and a variety of physical deformities. Examples include Down syndrome , Turner syndrome, and Klinefelter syndrome . [ 2 ]

Single-gene (or Mendelian) disorders are caused by a defect in a single gene or pair of genes. Most of these are rare, but together they cause significant illness. They may vary greatly in severity and may not produce symptoms until late in life. These conditions include the following:

  1. Autosomal dominant disorders , such as Marfan syndrome and Huntington disease, which occur when one gene is abnormal and the corresponding one is normal.
  2. Autosomal recessive disorders , such as cystic fibrosis, sickle-cell anemia, and Tay-Sachs disease, which usually occur only when both genes in a pair are defective. While children in these cases may suffer genetic disease, both parents will be healthy because their defective genes are paired with normal copies. [ 2 ]

X-linked disorders , such as hemophilia, colour blindness, and some muscular dystrophies, which are caused by genes located on the X chromosome. Women are less prone to X-linked disorders because if they have an abnormal X chromosome, they usually have another normal X one to compensate for it. However, a man who inherits an abnormal X chromosome with a defective gene will develop the disease even if the gene is recessive, since his Y chromosome can't compensate for it. The woman will, however, be a carrier of the disorder. [ 2 ]

Multifactorial (or polygenic) genetic diseases are the most common but the least understood of all genetic diseases. Although a person inheriting one or more of these genes has an increased risk of a particular disorder, environmental factors will determine whether the disease manifests itself. This category includes such common chronic disorders as coronary heart disease, diabetes mellitus, schizophrenia , and cleft palate. [ 2 ]

Somatic gene disorders are conditions in which gene abnormalities develop only in certain cells. Genetic defects responsible for these disorders are not inherited; rather, they occur in the developing foetus or at some time after birth. For example, cancer is usually a somatic gene disorder in which only genes in the diseased tissue are aberrant. [ 2 ]

Sex-linked disease . If a mutant gene is part of the X chromosome (a sex chromosome; females have two X chromosomes per pair, and males have an X chromosome and a Y chromosome), the disease is sex-linked. All male offspring are affected because the Y chromosome of the XY pair does not have a compensating normal gene. Because the mutation is on the X chromosome, however, and males transmit only the Y chromosome to their sons during fertilization, males do not transmit the disease to male offspring but to female offspring. [ 3 ]

The information in this page is presented in summarised form and has been taken from the following source(s):
1. United States National Institutes of Health, National Institute of Child Health and Human Development:
2. The Yale University School of Medicine Patient's Guide to Medical Tests, Barry L. Zaret M.D., Senior Editor, published by Houghton Mifflin. Online:
3. The Encycloædia Britannica© Online:

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Contact Last modified: Oct 20 2004