Genetics Explained

About genes

Your genes are a set of instructions for the growth and development of every cell in your body. For example, they determine characteristics such as your blood group and the colour of your eyes and hair.

However, many characteristics aren't the result of genes alone – environment also plays an important role. For example, children may inherit "tall" genes from their parents, but if their diet doesn't provide them with the necessary nutrients, they may not grow very tall.

Chromosomes

Genes are packaged in bundles called chromosomes. In humans, each cell in the body contains 23 pairs of chromosomes – 46 in total.

You inherit one of each pair of chromosomes from your mother and one from your father. This means there are two copies of every gene in each cell, with the exception of the sex chromosomes, X and Y. The X and Y chromosomes determine the biological sex of a baby. Babies with a Y chromosome (XY) will be male, and those without a Y chromosome will be female (XX).

This means that males only have one copy of each X chromosome gene, rather than two, and they have a few genes found only on the Y chromosome that play an important role in male development. Occasionally, individuals inherit more than one sex chromosome. Females with three X chromosomes (XXX) and males with an extra Y (XYY) are normal, and most never know they have an extra chromosome.

The whole set of genes is known as the genome. Humans have about 21,000 genes on their 23 chromosomes, plus 37 genes in their mitochondria.

Mitochondria are small structures inside cells that allow the cell to get energy from sugar and fat, and they're only passed on within eggs. This means mitochondrial DNA is only inherited from mothers.

DNA

Genes are made up of deoxyribonucleic acid (DNA). DNA is a long molecule made up of a combination of four chemicals: adenine, thymine, cytosine and guanine, represented as letters A, T, C and G. These "letters" are ordered in particular sequences within your genes. They contain the instructions to make a particular protein, in a particular cell, at a particular time.

Proteins are complex chemicals that are the building blocks of the body. For example, keratin is the protein in hair and nails, while haemoglobin is the red protein in blood.

Collagen

Collagen is the major structural protein of connective tissue and therefore one of the most abundant structural proteins in our body. Collagen is a major structural component of blood vessel walls, the bowel, lungs uterus tendons and also our skin. In addition, collagen plays a large and fundamental role in wound healing after an injury to the skin.

COL3A1 gene is the instruction for making collagen type III a tough fibre-like protein that makes up a third of our body protein. When this gene is altered, it causes a lack or deficiency of this collagen making the connective tissue less effective, particularly in skin, lung, uterus, intestine and the vascular system.

Genes and medical conditions

As well as determining characteristics such as eye and hair colour, your genes can also directly cause or increase your risk of developing a wide range of medical conditions. Although not always the case, many of these conditions occur when a child inherits a specific altered (mutated) version of a particular gene from one or both of their parents.

There are also many conditions that aren't directly caused by genetic mutations. These conditions can occur as the result of a combination of an inherited genetic susceptibility and environmental factors, such as a poor diet, smoking and a lack of exercise.

Genetic testing

Genetic testing can be used to find out whether you're carrying a particular genetic mutation that causes a medical condition. This can be useful for a number of purposes, including:

diagnosing certain genetic conditions
predicting your likelihood of developing a certain condition
determining if any children you have are at risk of developing an inherited condition

Testing usually involves taking a blood or tissue sample and analysing the DNA in your cells. Genetic testing can also be used to find out whether a foetus is likely to be born with a certain genetic condition. A sample of cells from the womb is extracted and tested.

Please note: Current lead times for diagnostic genetic testing results in the NHS is approximately 12 months, in some instances results are available sooner so we advise to stay with the NHS for genetic testing.

Read more about genetic testing and genetic counselling.

Page last reviewed: July 2023