I was spurred on by Noreen Olson’s article last month to address an issue that often comes up in family history research: how closely are we related to each other? I used to teach college anthropology, and genetics required many hours of instruction, so this topic will probably spill over into several columns in some way or other. An understanding of the basics can help you in your family history studies and may even save you some cash if you are thinking about having DNA testing done (more on that another time).

Today’s topic: Cousins and their many varieties –

I have discovered that several generations ago, in my own family, second cousins married each other. As you will recall from Noreen’s explanation, this means they were the children of first cousins. This revelation may seem shocking, however, it is not such a big concern.

Firstly, it was not uncommon in the past for relatives to marry, even closer relatives than second cousins. For example, Egyptian pharaohs would marry their siblings to preserve the divine royal bloodline (not recommended!). The royal families of Europe were also very closely related, for similar reasons. England’s Queen Victoria and Prince Albert are a famous example of marriage between royal first cousins. Not only the upper classes, but also the commoners would marry relatives. It has even been suggested that 80% of all marriages in history have been between second cousins or closer, for the simple reason that men went courting within a five-mile distance, which was as far as they could walk out and back on their day off, and most people had settled near relatives to begin with. So, there was no social stigma involved in marriage between cousins.

Secondly, from a genetic perspective, inbreeding is a concern in the case of inheritance of recessive genes, but this effect is reduced as the genetic distance between relatives increases.

To explain, in our cells we carry all the instructions for the building and functioning of our bodies. These instructions are found on a structure called DNA (which stands for deoxyribonucleic acid). A portion of DNA that governs a particular physical characteristic or trait is called a gene. We have many traits; some are controlled by one gene, but most are controlled by several genes working in consort. Let’s look at the simplest case: a trait controlled by one gene.

We get our genes from our parents. Some genes come only from our mom (a more complicated case to discuss another time), some come from only our dad (if we are male), but in most cases we get a copy of each gene from each of our parents, and these two copies work together in the expression of the trait. The way they work together can be complex or simple. Let’s look at the simplest case. If you recall the discussion of Gregor Mendel’s study of pea plants from your high school biology class, this will sound familiar.

Each gene (that expresses a trait) can have different variant forms. The classic example here is the trait for pea colour having two forms: green and yellow. The green colour is more likely to occur and is therefore called dominant. That colour (green) will be expressed whether the plant has a green copy of the gene from only one parent, or from both parents; all it takes is one copy of the green colour gene for that variant (green) to be expressed. Yellow colour is less likely to occur and is thus called recessive. If a plant has only one copy of the yellow variant, that colour will not be expressed; two copies are needed, one from each parent, for the recessive trait (yellow colour) to be expressed physically.

Some human diseases operate this way, as a recessive trait, controlled by a single gene. Therefore, two copies of that disease-inducing gene are required for the disease to occur in the body. An example of such a disease is phenylketonuria (PKU), where a specific enzyme that breaks down an essential dietary protein is not produced. This results in a buildup of that protein fragment in the urine and blood, which causes mental retardation and behavioural abnormalities. A test for this genetic disease was invented in 1960, which diagnoses this disease within three days after birth, allowing diet to be altered and therefore more normal development of the child.

In the general population, recessive genes are not as common as dominant genes, thus, the likelihood of having a genetic disease of this kind is fairly small. However, recessive genes, like dominant genes, are inherited. The danger occurs when individuals with only one copy of the disease-inducing recessive gene mate with someone from the same family. People from a family that carry the variant for the disease are more likely to have a child with the disease if they marry each other, than if they each married someone from the general population. Even if they both carried a gene for the disease and they married each other, every child such a couple would have is not doomed. A child would need to inherit the recessive gene from both parents; since there is a 50% chance that they would get a recessive gene from each parent, their chances of getting a recessive gene from both parents is 25%. The percentage of risk drops with increasing genetic distance between the parents; each parent is less likely to be a carrier of the disease, the more ancestors they have from the general population, rather than from within the family that carries the disease-inducing variant of the gene.

So, to summarize, yes, second cousins married, but this was fairly common and not terribly risky genetically. I hope this little background in social customs and in genetics reassures you, should you find a similar situation in your own family history.

This article was originally printed in the Bergen News and is being reprinted with permission.