Table of Contents
- Overview
- Prevention
- Images
Human beings have cells with 46
Genes are sections of DNA. The location of the gene is called the locus. Most genes carry information that is necessary to make a protein.
The pairs of autosomal chromosomes (one from the mother and one from the father) carry basically the same information. That is, each has the same genes. However, there may be slight variations of these genes. These slight differences occur in less than 1% of the DNA sequence and produce variants of a particular gene called alleles.
If a gene is abnormal, it may lead to an abnormal protein or an abnormal amount of a normal protein. Since the autosomal chromosomes are paired, there are two copies of each gene, one from each parent. If one of these genes is defective, the other may make enough protein so that no disease is seen. This is called a recessive disease, and the gene is said to be inherited in a recessive pattern.
However, if only one abnormal gene is needed to produce a disease, it's called a dominant hereditary disorder. In the case of a dominant disorder, if one abnormal gene is inherited from mom or dad, the child will likely show the disease.
A person with one abnormal gene is termed heterozygous for that gene. If a child receives an abnormal recessive disease gene from both parents, the child will show the disease and will be homozygous for that gene.
If two parents each have one copy of a recessive disease gene, then each child has a 25% (1 in 4) chance of showing the disease. If one parent has two copies of the disease gene and the other has one copy, then each child has a 50% (1 in 2) chance of being homozygous.
GENETIC DISORDERS
Almost all diseases have a genetic component. However, the importance of that component varies. Disorders in which genes play an important role (genetic diseases) can be classified as:
- Single-gene defects
- Chromosomal disorders
- Multifactorial
A single gene disorder (also called Mendelian disorder) is caused by a defect in one particular gene. Single gene defects are rare. But since there are about 18,000 known single gene disorders, their combined impact is significant.
Single-gene disorders are characterized by how they are passed down in families. There are six basic patterns of single gene inheritance:
Autosomal dominant Autosomal recessive X-linked dominant X-linked recessive - Y-linked inheritance
- Maternal (mitochondrial) inheritance
The observed effect of a gene (the appearance of a disorder) is called the phenotype.
Images
Review Date: 04/26/2010
Reviewed By: Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of
Pediatrics, University of Washington School of Medicine. Also
reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)
