You may have heard of a few genetic disorders, like cystic fibrosis, Down's syndrome and haemophilia - but it's unlikely you'll know of them all, as around 6,000 of these often devastating conditions affect more than half a million people in the UK.
To support children and families affected by these conditions, Genetic Disorders UK (www.geneticdisordersuk.org) runs the annual fundraising campaign Jeans for Genes Day (www. jeansforgenesday.org), which falls on September 21 this year.
To mark the event, Anna Whaite, a Genetic Disorders UK genetic counsellor, shares eight things everybody should know about genetic disorders...
1. Genes and genetic disorders vary wildly
We all carry thousands of genes (made of DNA and carried on chromosomes inherited from our parents), which are instructions for how our bodies grow and function.
"The exact code of our genes varies from person to person, which is why we're all different," Whaite says. "If someone has an alteration which disrupts a gene, that can cause a genetic disorder."
While some genetic disorders are quite common, others are incredibly rare and can range from being mild to devastating and life-threatening.
2. We can't change our genes
Once a genetic disorder has been diagnosed, there may be treatments and interventions available to help manage the symptoms, but sometimes there's no medication that can help.
Whaite says: "We can't simply switch a gene off, take it out, or mend it if it doesn't work properly, although gene therapy and other research continues to improve options for the future."
3. Chromosome changes can cause some genetic disorders
While many genetic disorders are caused by changes in single genes, others are due to changes to chromosomes, the structures that carry genes. For example, Down's syndrome is caused by having an extra copy of chromosome 21, instead of the usual two.
Other chromosome changes can mean parts of a chromosome are deleted, duplicated, or swapped around. This can have a significant effect on development and can sometimes be detected during pregnancy scans.
4. Gender matters
Some genetic disorders affect males and females differently, particularly if they're X-linked (caused by a change in a gene found on the X chromosome). This is because females have two 'X' chromosomes and males have one 'X' and one 'Y' chromosome.
If there's a genetic change in a gene on the 'X' chromosome, for example with Duchenne muscular dystrophy - an X-linked condition - a male would show symptoms of this, but a female with the same gene change may have only mild symptoms or no problems at all.
5. Some inherited disorders are still a mystery
While many genetic tests are available to make or confirm a diagnosis, there are some inherited disorders that can't yet be detected by a gene test.
Whaite says: "Huge scientific efforts, like The Human Genome Project and related schemes, have completed a map of the entire human genome (all of the genetic material on a living thing's chromosomes), but it will take many more years to find out what each gene does and how they interact with one another."
6. Genetic disorders aren't always inherited
Some genetic disorders run in families and have been around for generations, like Huntington's Disease. But others happen when two healthy carriers have a child with a genetic disorder -cystic fibrosis, for example.
Some occur due to a new spontaneous change in the DNA sequence of a gene, called a de novo mutation, which can happen around conception.
7. Chance has no memory
When someone with a genetic disorder has children, the chance the disorder will be passed on is the same for each pregnancy. If a recurrence risk of a genetic disorder is 50%, this risk is the same each time a baby is conceived.
"Some families can seem to be 'lucky', others less so, but the risks remain the same each time - chance has no memory," Whaite stresses.
Different conditions have different recurrence risks, which can be discussed during genetic counselling.
8. Help is available
GPs can refer patients to NHS genetics clinics, which provide testing and genetic counselling.
Support is also available for children and families through disease-specific support groups and charities such as Genetic Disorders UK, which runs a free helpline on 0800 987 8987.