Genetics degree of relationship

Marriages between people who are related is more common than you might think. Unlike what many people think, their offspring are not doomed to birth defects or medical problems.

In fact, unless they both carry the same gene mutation, the couple’s chance of having a healthy child is almost as high as any other couple. Let’s examine why, through the story of one couple.

And baby makes three

Maria is planning a pregnancy with her partner Max. They visit their doctor to discuss family planning and pregnancy health, and during questioning, they disclose that they’re first cousins. Their doctor refers them to a clinical genetics service for further advice.

What risks do they face, if any? And what information would be requested by the clinical genetics service?

The first of these relates to their exact genetic relationship. Genetically speaking, the closer one is to a family member, the more genes will be shared. Monozygotic (identical) twins have the same genetic make-up and share 100% of their genes. A parent and child share half their genes, as do siblings. An uncle and his niece, or an aunt and her nephew (a second-degree relationship) share a quarter of their genes.

Maria and Max, being cousins (a third-degree relationship), share an eighth of their genetic make-up. This being the case, what are Maria and Max at risk of?

Receiving the same faulty genes from both parents puts a child at risk of having a genetic disease. Ryan Croson

As they share a significant proportion of their genes, the couple are at risk of having a child with an autosomal recessive condition. This kind of condition is caused by having a “double dose” of a faulty gene.

We have two copies of every gene (for most genes) – one inherited from our father, and one from our mother. For many genes, our body can cope with just a single working copy, but when both copies are faulty, the person gets an autosomal recessive disease. Examples of such diseases include cystic fibrosis, thalassaemia (diseases of the blood), and spinal muscular atrophy.

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