Genetic tests

There are many reasons why people decide to have genetic tests. A geneticist explains what genetic testing involves, the reasons for having or not having it, and how to deal with a positive test result.

Media last reviewed: 12/03/2014

Next review due: 12/03/2016

Male and female sex chromosomes

Your sex is determined by chromosomes. You inherit two sex chromosomes  one from your mother and one from your father.

During early pregnancy, all unborn babies are female because only the female sex chromosome (the X chromosome, which is inherited from the mother) is active.

At the eighth week of gestation, the sex chromosome inherited from the father becomes active, which can either be an X chromosome (female) or a Y chromosome (male).

If the sex chromosome inherited from the father is X, the unborn baby will continue to develop as female. If the sex chromosome inherited from the father is Y, the baby will develop as male.

Therefore, a female baby has XX sex chromosomes and a male baby has XY sex chromosomes.

Genetics is the branch of science that deals with how you inherit physical and behavioural characteristics.

Your genes contain the genetic information that controls characteristics such as the colour of your eyes and hair.

Genes are single units of genetic material found inside chromosomes (long, threadlike structures in each of the body's cells that contain DNA, see below).

However, characteristics are not just due to genes alone environment also plays an important role. The way a child is brought up will have a direct impact on their development.

For example, a child may inherit 'tall genes' from their parents, but if their diet does not provide them with necessary nutrients, they may not grow very tall.

Read more about how genes are inherited


Each cell in the body contains 23 pairs of chromosomes. These carry the genes you inherit from your parents.

As well as inheriting characteristics such as eye and hair colour, you can also inherit a health condition, or a tendency to develop a particular condition.

One of each pair of chromosomes is inherited from each parent, which means that with one exception, there are two copies of each gene in each cell. 

The exception is with the X and Y sex chromosomes. These determine the sex of a baby. Babies with an X and a Y chromosome will be male (XY). Those without a Y chromosome, but two of the same kind of X chromosome, will be female (XX). This means that males only have one copy of each X chromosome gene.

Each chromosome contains hundreds or thousands of genes. There are about 21,000 genes in total, all written into the DNA contained in the chromosomes. The whole set of genes is known as the genome.


Deoxyribonucleic acid (DNA) is the long molecule that stores genetic information. This information is written in the DNA using an alphabet of four 'letters', or smaller molecules: A, C, G and T.

Each gene consists of a precise sequence of letters. This is the genetic code. Each gene contains the instructions to make a particular protein, in a particular cell, at a particular time. Proteins are complex chemicals that make up:

  • blood
  • muscles
  • tendons (fibrous cords that join bones to muscle)
  • skin
  • nerves
  • organs

DNA is made of two strands coiled together, each one a mirror image of the other. Because of this, DNA can be easily split when a cell divides and the genetic code is passed on exactly. This DNA copying process when a cell divides is known as replication.  

There are around 3 billion letters of DNA code in the 23 chromosomes.

The Human Genome Project

The Human Genome Project is an international scientific project that involves thousands of scientists around the world.

The initial project ran from 1990 to 2003. Its objective was to map the immense amount of genetic information found in every human cell.

As well as identifying specific human genes, the Human Genome Project has enabled scientists to gain a better understanding of how certain traits and characteristics are passed on from parents to children.

It has also led to a better understanding of the role of genetics in inherited conditions such as:

  • muscular dystrophy  a condition that gradually causes the muscles to weaken over time, leading to an increasing level of disability
  • Down's syndrome  a condition that affects a child's normal physical development and causes learning difficulties
  • cystic fibrosis  a condition that causes  internal organs to become clogged with thick, sticky mucus

Genetic testing and counselling

Genetic testing can be used to find out whether you are carrying a particular genetic mutation (altered gene) that causes a medical condition.

Testing usually involves taking a blood or tissue sample. The sample will contain cells containing your DNA. It can be tested to find out whether you are carrying a particular mutation and are at risk of developing a particular genetic condition.

Read more about genetic testing and counselling.

Page last reviewed: 30/07/2012

Next review due: 30/07/2014


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