Genes are the instructions or recipes made of DNA that tell our bodies how to build and repair themselves. We inherit our genes from our parents. Our genes come in pairs and we inherit one copy in each pair from our mum and the other from our dad.
Each of the two different copies of a gene can have tiny differences in their genetic code. It is these subtle differences in our genes that make us all different and unique. Sometimes a change in the genetic code of a gene can prevent the gene from working properly.
An individual with a recessive genetic condition has a change or genetic misprint (which scientists call a mutation) in both copies of a particular gene. They therefore have no working copy of the gene and so will develop signs and symptoms of the condition.
They will have inherited a changed copy of the gene from both parents. If neither parent has the condition themselves then both would be expected to be ‘carriers’ of the condition. Carriers of recessive conditions have one changed copy of the gene and one working copy. They are not affected by the condition themselves because they have a working copy of the gene which compensates for the changed non-working copy.
When two people are both carriers of the same recessive genetic condition there is a possibility that any children they have MAY inherit the condition. Each time two carriers have a child there is a 1 in 4 or 25% chance the child will inherit the condition and a 3 in 4 or 75% chance they will not inherit the condition. This chance is the same whether the child is a boy or a girl. The chance also remains the same whether the child is first born, second born, third born etc. Some families will have three children who are all affected by the condition and others will not have any children with the condition. It is chance and probability, none of us can control which genes we pass on to our children.
Recessive inheritance. Photo: US Nat. Library of Medicine
The diagram above shows how recessive genetic conditions are inherited.
This YouTube video from UCD Dublin, National Children’s Research Centre Ireland explains how recessive inheritance works https://www.youtube.com/watch?v=_oVvTkjDm6g
There are around 4,000 known inherited conditions that are caused by a difference in a single gene and many of these follow a recessive pattern of inheritance.
The most common recessive genetic disorder in the UK is Cystic Fibrosis (CF). CF affects the internal organs, primarily the lungs and digestive system, by clogging them with thick, sticky mucus.
About 1 in 2,500 babies are born with Cystic Fibrosis in the UK each year which equates to around five babies born each week. They will have inherited two changed copies of the CF gene causing them to have the condition. The CF gene is located on chromosome 7 and controls the way the body’s cells handle sodium.
Around 1 in 25 people in the UK are carriers of Cystic Fibrosis, having one working copy of the CF gene and one copy with a genetic misprint.
For more information about Cystic Fibrosis visit the Cystic Fibrosis Trust http://www.cysticfibrosis.org.uk/
Other examples of recessive genetic disorders include Spinal Muscular Atrophy, Sickle Cell Anaemia, Tay-Sachs disease and Phenylketonuria (PKU).
Here’s a video we made for schools explaining What ‘Genes’ Means: https://www.youtube.com/watch?v=nBNeENaDA3s
Other Useful Links
National Genetics and Genomics Education Centre – Autosomal recessive inheritance https://www.genomicseducation.hee.nhs.uk/modes-of-inheritance/autosomal-recessive-inheritance
The Virtual Genetics Education Centre – Patterns of Inheritance http://www2.le.ac.uk/departments/genetics/vgec/highereducation/topics/inheritancepatterns/patterns-of-inheritance