A few weeks ago I was contacted by a journalist who is writing a story about relationships between first cousins. She was curious to get a genetic counselor’s view on the issue. When two people have children together who are related, this is called consanguinity. This word comes from the Latin meaning from the same blood. In the Australian culture which heralds from the British Isles, marriage between first cousins is not common, is not encouraged and has a certain stigma surrounding it. However in other cultures, first cousin relationships are not only okay, but they are also actively encouraged. Having children with a relative like a cousin can even have advantages. “Keeping it in the family” so to speak. Families are already aware of what “they are getting into”. They understand each other’s background and financial backgrounds.

However, most people are cognizant of the fact that there can be a higher chance of having a child with an illness when one has children with a relative. This is due to what is known as recessive inheritance. Some conditions, such as Down syndrome, are linked to the mother’s age. The older the mother is at conception, the higher her chance of having a child with a chromosome problem. Dominant conditions can spontaneously occur, due to a fault or mutation in a gene in the egg or the sperm. The chance of this happening actually increases with the age of the father. However, when two people are related, they have a higher chance of both being carriers of the same recessive condition. All of our genes come in pairs; one from each parent. With recessive genetic conditions, an affected person has two copies of the same gene that is not working properly. Their parents who have one working and one non-working copy of the gene are known as carriers. Our relatives are more likely to be carriers of the same genetic conditions and this is why the rate of genetic conditions is higher in people who are born to first cousins.

What most people don’t realize though is that this risk still isn’t really high. The chance of any two unrelated people having a child with a medical problem is about 3%. When two people who are cousins have a child, their chance of having a child with a medical condition is doubled to 6%. So more than 90% of the time there are no problems. This is explained in more details in the fact sheet written by The Centre for Genetics Education, which I have shared the link below. Genetic counsellors are equipped with the knowledge and skills to help people who are seeking information regarding first cousin marriages. We provide this medical information in a non-judgmental way free from stigma. However, consanguinity is not to be confused with incest, which is sexual relations between close relatives such as siblings or parents and children. This is illegal. Consanguinity isn’t something that a lot of people this a lot about. But people in consanguineous relationships often think about it a lot. There can be guilt and shame associated with the stigma. I hope knowing a bit more about this topic can help diminish this.

Fact sheet