What is genetics?
Genetics is the area of biology that studies heredity and spontaneous changes in genetic material. Heredity refers to the passing of features, such as eye colour, from parent to child.
In health care, genetics is important because some diseases, or the risk of getting some diseases, can be passed from parent to child. In other cases, diseases are caused by a spontaneous change (mutation) in genetic material.
For more information, 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 many features of a living thing.
Every person has a unique set of genes. This is known as the genotype of that person. Phenotype refers to the features and qualities of a living thing that come from the genotype influenced by the environment.
DNA is organized in chromosomes. Each chromosome is a single long DNA molecule.
Cells are the basic building blocks of living things. Within each cell, there are 23 chromosome pairs. One chromosome in each pair comes from the mother, and the other comes from the father. Twenty-two of these chromosome pairs, numbered from 1 to 22 based on their features, are the same for males and females. The 23 rd 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 in the same order, but one copy of each gene comes from the mother and one comes from the father.
Another structure in each cell, the mitochondria, also contains DNA. Mitochondria are structures that make energy for the cells by way of a chemical called adenosine triphosphate (ATP). Genes in the mitochondria, as well as in the nucleus of the cell, direct the cells to make the enzymes needed for ATP production. The DNA in mitochondria is inherited only from the mother. Mutations in this DNA have been related to some hereditary disorders.
What do genes do?
Genes control the making of a protein or help control the activity of other genes.
Proteins are very important molecules for living things. There are many types of proteins that do many things. 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 feature. In some cases, one gene of the pair will control the feature. In this case the gene that controls the feature is called the dominant gene, and the other gene in the pair is called the recessive gene. If there are two copies of the recessive gene, the feature is controlled by the recessive gene.
The activity of a gene is also affected by the environment around it. When a baby is growing during pregnancy, the protein controlled by a gene can control a particular feature. Some features are controlled by one gene, but many features are controlled by a number of genes acting together under the influence of the environment at many levels.
Many genes have one or more variations (alleles). Most of these alleles do not cause problems, but some can cause disease.
Many of the genes that cause disease are recessive. If a baby has only one copy of a recessive gene that causes a disease, she will not have the disease. But if she has two copies of the recessive gene, she will have the disease. If the disease is caused by a dominant gene, one copy is sufficient to produce the disease.
What is a mutation?
Mutation refers to a change in the structure of DNA. Some mutations that cause disease have been present in a family for many generations, but many happen spontaneously. All the causes of mutation are not known, but aging, and some things in the environment like toxins or radiation can promote or cause mutations. Mutations can happen in a single gene or in a group of genes.
Gene mutations may cause genes to not function properly and may result in genetic disease. However, some mutations will have no effect on the individual at all if they don't occur in an important part of the gene. In other cases, the body is able to repair the damaged cell.
What kinds of inheritance patterns are seen in genetic disorders?
There are 5 general inheritance patterns seen with genetic disorders:
- Single gene or Mendelian disorders. These result when a mutation 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 mutations in a number of genes, often coupled with an environmental influence. Environmental factors include things like alcohol or drug use, maternal infections, 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 mutations in DNA found in mitochondria. If there is a mitochondrial gene mutation, 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.
- Epigenetic disorders. These are disorders related to changes in the activity of genes, rather than a mutation in the structure of the DNA.
Are some people more at risk for genetic disorders than others?
People with a family history of a given genetic disorder may be at higher risk than people in the general population. As women age, the chance for them to have a child with a chromosomal disorder (such as Down syndrome) also increases.
Chromosomal disorders
What causes a chromosomal disorder?
Researchers don't exactly know what causes chromosomal disorders, but they do know that problems tend to occur with an egg or sperm cell prior to conception. Disorders can also just happen spontaneously. Parents may wonder if they did something to cause a problem, but chromosomal disorders are not the result of something a parent did or did not do before or during pregnancy.
What kinds of chromosomal disorders are there?
Chromosomal disorders include:
- deletions: a chromosome may be missing a small section
- microdeletions: an even smaller amount of the chromosome may be missing, potentially just affecting one gene
- translocations: one part of a chromosome may be attached to another
- inversions: a piece of the chromosome is upside down
- aneuploidy: a change in the number of chromosomes
Down syndrome is one of the more common, and better known, chromosomal disorders. A child who has Down syndrome has an extra copy of chromosome 21. Technically, Down syndrome is called trisomy 21, which means that there are 3 copies of chromosome 21 instead of the usual 2.
Children with Down syndrome have an increased chance of being born with a congenital heart defect.
In a very small percentage of cases (less than 5%), Down syndrome can be inherited in a family. For people with a relative with Down syndrome, testing is available to determine whether or not it is the inherited type.
Another example of a chromosomal disorder is Turner syndrome, which affects only girls. In this syndrome, one of the two X chromosomes normally found in females is missing or altered. Turner syndrome causes short stature and infertility. A portion of these girls also have heart defects.