The Basics of Genetic Disorders

The role of genes and gene mutations in human health and disease is a vast and complex scientific subject which is still being researched so that treatment developments and innovations can hopefully be found.

This brief breakdown describes the types of gene mutations, when they occur, and the types of disorders which may occur as a result.

Firstly, some basics:

• DNA – deoxyribonucleic acid (DNA) is the hereditary material which carries the genetic instructions for development, functioning, growth and reproduction. It is a long molecule that contains each person’s genetic code.
• Genes – the basic units of heredity, a gene is a section of DNA that controls how a body will grow and function. Each human cell contains two sets of genes (one from the father and one from the mother) the genes are bundled into 46 parcels called chromosomes.
• Chromosomes – bundles of tightly coiled DNA found in the nucleus of almost every cell. Humans have 23 pairs of chromosomes (one each from the mother and father).

When a gene’s DNA sequence changes permanently it creates something different within a human’s genetics and this is known as a genetic mutation. Not all genetic mutations are bad and, in fact, the occurrence of genetic mutations are how humans have been able to evolve.

A genetic abnormality may not be physically apparent at birth and may only manifest later in life.

The following are the different types of genetic abnormality and the causes:

Inherited mutations
Genetic mutations can be inherited from parents (who may not even know they are carrying a gene mutation) and passed down from one generation to the next through conception.

Constitutional mutations
These occur pre-fertilisation, with the mutation present in the sperm or the egg at conception. The genetic mutation will be present in every cell of the body. Also known as germline mutations, these are the DNA mutations most typically associated with genetic abnormality and disease.

Each generation could inherit the same mutation which causes a particular condition or predisposition to disease.

Constitutional mutations may also be De novo variants i.e. when a sperm fertilises an egg and a new mutation occurs. Such De novo mutations are the reason why an offspring may possess a gene mutation in every cell, whereas the parents do not.

Acquired mutations
Also known as a somatic mutation, an acquired gene abnormality may occur at any time over the course of a person’s life and is only present in certain cells.

A single gene or group of genes may mutate at random due to a DNA replication mistake occurring during cell division. Acquired mutations may occur immediately following conception or later in life as a result of environmental exposure.

Chromosomal disorders
These are caused by structural changes to chromosomes or an excess or deficiency in the amount of genes within a chromosome.

Crucially, the likelihood of abnormalities occurring will increase as the woman ages. So, there is an increased risk of chromosome disorders in the offspring the older a woman is when she chooses to have a child.

The three main categories of disorders

There are thousands of potential combinations of genetic mutations which may occur; however, they are broadly categorised as:

• Monogenic disorders: a single gene mutation
• Multifactorial disorders: mutations in multiple genes
• Chromosomal disorders: the number or structure of DNA-containing chromosomes has changed

For more detailed information see our page: Types of Genetic Disorders and the Conditions Caused