This page explains genetics concepts, including the role genes play in our body, the types of inheritance patterns, and how changes in genes are linked to health and disease.
What is genetics?
Genetics is the scientific study of genes and heredity, and how changes in our genetic material contribute to health and disease. Heredity refers to the passing of traits, such as eye colour, from biological parent to child.
In health care, genetics is important because some conditions, or the risk of developing some conditions, can be passed from biological parent to child. A genetic condition can also be caused by a spontaneous change (variant) in genetic material.
For more detailed information on cells, chromosomes, genes, and inheritance patterns (autosomal dominant, autosomal recessive, X-linked, etc.), please see: How the Body Works: Genetics.
What are genes?
Genes are the basic units of heredity. They are made of lengths of a complex molecule called deoxyribonucleic acid (DNA). DNA carries the genetic code, the information that will determine our specific traits.
Every person has a unique set of genes. This is known as their genotype. Phenotype refers to the traits and qualities of an individual that come from the genotype (examples: blood type, eye colour).
DNA is packaged in chromosomes. Each chromosome is a single long strand of DNA.
Cells are the basic building blocks of living things. Within each cell of a person, there are 23 chromosome pairs. One chromosome in each pair comes from the biological mother, and the other comes from the biological father. Twenty-two of these chromosome pairs, numbered from 1 to 22, are the same for males and females. The 23rd pair of chromosomes is called sex chromosomes. These are different in females and males:
- Females have two X chromosomes (XX).
- Males have an X chromosome and a Y chromosome (XY).
Except for the sex chromosomes in males, both chromosomes in a pair contain genes with the same function; but one copy of each gene comes from the biological mother, and one comes from the biological father.
Another structure in each cell, the mitochondria, also contains DNA. Mitochondria are structures that make energy for the cells. The DNA in mitochondria is inherited only from the biological mother. Certain genetic variants in the mitochondrial DNA have been linked to some hereditary disorders.
This animation goes inside the cell to show the chromosome pairs described above. This animation is supplemental and repeats information from the body text.
What do genes do?
Genes provide the instructions to make proteins. Proteins are very important molecules for living things. There are many types of proteins in our body. Some form the parts of the body, while some help control functions of the body.
Genes are always found in pairs that control the making of the same protein, or the same traits. Some examples of traits that are determined by genes inherited from our biological parents include dimples, freckles, and green/red colour blindness.
It is estimated that humans have between 20,000 and 25,000 genes.
What is a genetic variant?
A genetic variant in a gene refers to a change in the DNA code that makes up the gene. Some genetic variants (pathogenic variants) cause the gene to not function properly while other genetic variants (benign variants) do not. Genetic variants that are linked to diseases were previously called mutations.
Some genetic variants that cause disease have been present in a family for many generations, but many happen spontaneously. Genetic variants can occur in a single gene or in a group of genes.
We all have many gene variations which make us individuals. Some of these variants may increase our chances to develop a health condition.
What kinds of inheritance patterns are seen in genetic disorders?
There are four general inheritance patterns seen with genetic disorders:
- Single gene or Mendelian disorders. These result when a genetic variant causes a single gene to be altered or missing. Single gene disorders are typically described as inherited in families, since they are passed from one generation to the next.
- Multifactorial or complex disorders. These are related to changes in a number of genes, often coupled with an environmental influence. Examples of environmental factors include alcohol or drug use and exposure to hazardous materials. These disorders tend to run in families, although a pattern of inheritance is often difficult to identify.
- Mitochondrial disorders. These disorders result from changes in DNA found in mitochondria. If there is a genetic change in a mitochondrial gene, energy production is affected. The DNA in mitochondria is inherited only from the mother.
- Chromosomal disorders. These disorders result when entire chromosomes or parts of chromosomes are missing, duplicated or changed. Chromosomal disorders usually occur spontaneously; however, on rare occasions they are inherited.