Conception and your baby's genes

Your baby’s physical and mental development will be affected by genes passed down from both biological parents.

Genes are like a set of building blocks for our growth and development. We all inherit genes from our parents. They come in pairs, so we get one set from each biological parent.

Our genes determine things like our blood type, eye and hair colour, and even what our personalities might be like. They can also affect our risk of some medical conditions.

Some medical conditions are caused by genes alone, but most are caused by a combination of genes and environmental (life) factors.

Whether your child has a medical condition will depend on several factors, including:

  • What genes they inherit from both biological parents.
  • Whether the gene for that condition is dominant (they only need one copy of the faulty gene to have the condition) or recessive (they need two copies of the faulty gene; if they only have one, they will be a carrier but they will not have the condition).
  • Your child’s environment, including any treatment they may get to prevent developing a condition.

If you or your baby’s other biological parent have a high risk of passing a genetic condition to your child, you may be offered genetic testing. This will help you to find out more about the risks for your child.

Genetic conditions

Our genes are made up of DNA. Genetic mutations occur when DNA changes, which can affect how that part of the DNA works. Some medical conditions are caused by a mutation in a single gene. This could have been passed on to a child by their parents.

These genetic conditions can be inherited in 3 main ways:

The type of inheritance affects how likely it is that biological parents will pass the condition on to the children.

Here’s more information on these types of inheritance:

Autosomal recessive inheritance

Some conditions can only be inherited in an ‘autosomal recessive’ pattern. ‘Recessive’ means the condition can only be passed on to a child if both biological parents have at least one copy of the faulty gene. ‘Autosomal’ means that the faulty gene can be found in both biological males and biological females.

If the child only inherits one copy of the faulty gene, they'll be a ‘carrier’ of the condition, but will not be affected by the condition. If they inherit two copies, they will have the condition.

If both parents carry one copy of the faulty gene (they are ‘carriers’), there's usually a:

  • 1 in 4 (25%) chance of each child they have inheriting the genetic condition
  • 2 in 4 chance (50%) of each child being a carrier
  • 1 in 4 chance (25%) of each child neither having the condition, nor being a carrier

If one parent has two copies of the faulty gene (they have the condition), and the other parent has one copy (they are a carrier), there’s usually a:

  • 2 in 4 chance (50%) of each child having the condition
  • 3 in 4 chance (75%) of each child being a carrier

If both parents have two copies of the faulty gene (they both have the condition), each child will usually have the condition. 

Examples of genetic conditions inherited in this way include:

  • Cystic fibrosis – a condition in which the lungs and digestive system become clogged with thick, sticky mucus
  • Sickle cell disease – a condition where red blood cells, which carry oxygen around the body, develop abnormally
  • Thalassaemia – a group of conditions where the body doesn’t produce enough haemoglobin, which helps red blood cells to carry oxygen around the body
  • Tay-Sachs disease – a condition that causes progressive damage to the nervous system

Autosomal dominant inheritance

Some conditions are inherited in an ‘autosomal dominant’ pattern. ’Dominant’ means that only one parent needs to carry the mutation for the condition to be passed on to the child.

If one biological parent has the condition, there's a 2 in 4 (50%) chance it will be passed on to each child the couple has.

If the child does not have the condition, they will not be a ‘carrier’, and will not pass the condition on to their own children.

If both biological parents have the condition, each child will inherit two copies of the mutated gene. This usually causes a more serious version of the condition. Sadly, it’s often so serious that a pregnancy cannot survive.

Examples of genetic conditions inherited in this way include:

X-linked inheritance

Some conditions are caused by a mutation on the X chromosome (one of the sex chromosomes). These conditions don't usually affect biological females, who have two X chromosomes. One of these chromosomes will almost certainly be normal and can usually make up for the changed chromosome.

Females who inherit one changed chromosome will become carriers.

Biological males have an X chromosome and a Y chromosome. They can't inherit X-linked mutations from their father because they receive a Y chromosome from them. But male children may develop the condition if they inherit it from their mother. This is because males don't have another X chromosome to compensate as a female would.

When a mother is a carrier of an X-linked mutation:

  • each male child they have has a 2 in 4 chance (50%) of developing the condition
  • each female child they have has a 2 in 4 chance (50%) of becoming a carrier

When a father has an X-linked condition, his sons won't be affected because he'll pass on a Y chromosome to them. Any daughters he has will become carriers.

Examples of genetic conditions inherited in this way include:

  • Duchenne muscular dystrophy – a condition that causes the muscles to gradually weaken, resulting in an increasing level of disability
  • Haemophilia – a condition that affects the blood's ability to clot
  • Fragile X syndrome – a condition that usually causes certain mental, facial and bodily characteristics such as learning difficulties, a long face, large ears and flexible joints

New genetic mutations

Although genetic conditions are often inherited, this isn't always the case.

Some genetic mutations can occur for the first time when a sperm or egg is made, when a sperm fertilises an egg, or when cells divide after fertilisation. This is often known as a de novo mutation, sporadic mutation, or spontaneous mutation.

Someone with a new mutation won't have a family history of a condition, but they may be at risk of passing it on to their children.

They may also have, or be at risk of developing, a form of the condition themselves.
Examples include some types of muscular dystrophy, haemophilia and type 1 neurofibromatosis.

Chromosomal conditions

Some conditions aren't caused by a mutation on a specific gene, but by an abnormality in a person's chromosomes. This happens when a person has too many or too few chromosomes, instead of the usual 23 pairs.

Examples of conditions caused by chromosomal abnormalities include:

While these are genetic conditions, they're generally not inherited. They are usually a result of a problem before, during or soon after the fertilisation of an egg by a sperm.

How our environment affects our health

Very few health conditions are only caused by genes. Most are caused by a combination of genes and other factors, including diet and exercise.

The whole set of genes that a person has is known as their ‘genome’. Studies of the whole genome in large numbers of people show that tiny changes may increase or decrease a person's chance of having certain conditions.

Each variant may only increase or decrease the chance of a condition very slightly, but this can add up across several genes.

In most people, the gene variants balance out to give an average risk for most conditions. But in some cases, the risk is significantly above or below the average.

It's thought it may be possible to reduce the risk for many diseases by changing environmental and lifestyle factors.

For example, coronary heart disease – when the heart's blood supply is blocked or interrupted – can run in families. However, a poor diet, smoking and a lack of exercise can also increase your risk of getting the condition.

Consanguinity and genetic conditions

A consanguineous relationship is one where the couple are blood relatives. A common form is between first cousins.

Consanguinity is a common cultural practice in many societies around the world, particularly in the Middle East, parts of Asia and North Africa, as well as among emigrants from these communities now residing in North America, Europe and Australia.

Most babies born to couples who are blood relatives are healthy, but it does double the risk of a birth defect from 3 in 100 to 6 in 100.

The closer the parents’ relationship, the higher the risk of having a baby with a genetic disorder. For example, first cousins have a higher risk than second cousins.

This could be in part because close relatives are more likely to have genes that are alike. So, if one parent is a carrier for a genetic condition, there’s a higher chance that the other parent will be, too. This, in turn, means there’s a higher chance of their child having that condition.

Getting tested for genetic conditions

There are a number of different testing options if you’re at risk of carrying or developing a genetic condition.

You may be offered genetic testing if:

  • your doctor thinks you could have a genetic health condition
  • someone in your family has a genetic condition
  • you or your partner has a genetic health condition that could be passed on to your children

If you’re worried about genetic conditions, speak to your GP. You may be offered testing, such as a blood test, genetic testing or pre-implantation genetic testing. Read more about these below.

Blood testing

There is a blood test you can have to find out if you are a carrier of sickle cell disease or thalassaemia. Anyone can ask their GP or local specialist sickle cell and thalassaemia centre for a free test, at any time.  You can find your nearest sickle cell and thalassaemia centre here.

If you or your partner is concerned you may be a carrier, it's a good idea to get tested before you start a family, but the tests are a routine part of early pregnancy screening.

Genetic testing

Genetic testing can be used to find out whether a person is carrying a specific altered gene (genetic mutation) that causes a medical condition. 

A genetic test can help to:

  • diagnose a person with a genetic condition
  • work out the chances of a person developing a particular condition
  • find out whether a person is a carrier of a condition that could be inherited by any children they have

You'll need a referral from your GP or a specialist doctor for genetic testing. Speak to your doctor if you think you may need it.

What does genetic testing involve?

Genetic testing usually involves having a sample of your blood, saliva or body tissue taken. The sample will contain cells containing your DNA.

In some cases, genetic testing can be carried out during pregnancy, to find out whether a baby is likely to be born with a certain genetic condition.

This is done by testing samples of the fluid that surrounds the baby in the womb (amniotic fluid) or cells from the placenta (chorionic villus). These are usually taken from the womb using a needle.

Learn more about amniocentesis and chorionic villus sampling.

Depending on the condition(s) being checked for, the fluid or cell samples will be sent to a genetics lab for analysis.

In some cases, an entire set of genes can be checked for mutations using a process called whole genome sequencing. This has to be done with great care, and can take a long time compared with most other hospital lab tests.

Depending on the reason for the test, it can take weeks or even months to get the results.

It isn’t always possible to give definite answers after genetic testing. Sometimes it’s necessary to wait to see if the person being tested, or their relatives, do or don’t develop a condition. Other tests may need to be performed.

Genetic counselling

If your doctor offers you genetic testing, you may be referred for genetic counselling as well.

Genetic counselling is a service that provides support, information and advice about genetic conditions.

It may involve:

  • learning about a health condition that runs in your family, how it’s inherited, and which family members may be affected
  • an assessment of the risk of you and your partner passing an inherited condition on to your child
  • support and advice if you have a child affected by an inherited condition and you want to have another child
  • a discussion about genetic tests, including the risks, benefits and limits of genetic testing
  • getting help understanding the results of genetic tests and what they mean
  • information about patient support groups

You’ll be given clear information so you can decide what’s best for you.

Your appointment will usually take place at your nearest NHS regional genetics centre. The British Society for Genetic Medicine has details for each of the genetics centres in the UK.

Pre-implantation genetic testing

For couples at risk of having a child with a serious genetic condition, pre-implantation genetic diagnosis (PGD) may be an option.

PGD involves using in-vitro fertilisation (IVF), where eggs are removed from the ovaries before being fertilised with sperm in a lab.

After a few days, the resulting embryos can be tested for a certain genetic mutation. If there are embryos that are free from the mutation, one of these is transferred to the womb.

PGD avoids having to terminate pregnancies because of serious conditions. But pregnancy is not guaranteed, and it can be harder to get pregnant with IVF.  Plus, PGD isn’t always available on the NHS, so there may be costs involved.

Talk to your GP if you think PGD could be helpful for you.

Hair and eye colour

Many parents-to-be wonder if they can predict their baby’s hair colour, eye colour or other physical characteristics. These are all to do with the genes your baby inherits from both parents, but can be hard to predict.

There are many genes involved in a person’s hair and eye colour. This means that a baby might not always have the same characteristics as their parents.

For example, if both parents have brown eyes, their baby may be more likely to have brown eyes. However, there’s still a small chance that the baby’s eyes could be a colour other than brown.

The only way to tell for sure is to wait and see what colour your baby’s hair and eyes are as they grow!

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Review dates
Reviewed: 22 January 2024
Next review: 22 January 2027