Tag Archives: genetics

Cousins or Siblings?

Imagine two sets of identical twins: Alice & Amelia, and Bob & Benjamin. Now imagine that Alice marries Bob, and Amelia marries Benjamin, and that both couples have a child: Alice & Bob give birth to Charlie, and Amelia and Benjamin give birth to Catherine.

By law, Charlie and Catherine are first cousins (i.e. the children of your parents’ brothers and sisters are your first cousins). Genetically, however, the situation is very different: Alice and Amelia have the same genes, as do Bob and Benjamin. Charlie and Catherine will get fifty percent* of their genes from each parent, but as their parents are genetically identical they will share fifty percent of their genes with each other (rather than the normal 12.5%), making them genetically brother-and-sister. Also, because their uncle and auntie are genetic clones of their parents, Charlie and Catherine will share 50% of their genes with each of their uncle/auntie, so genetically speaking they would be indistinguishable from parents or siblings.

In the UK, marriages between cousins have been permitted since the time of King Henry VIII. If Charlie and Catherine are legally cousins, but genetically siblings, should they be permitted to marry?

* I should point out that it won’t be exactly 50% in any of these cases, but considering the huge number of genes and the population as a whole the average will be 50%.


A telomere is a section of repetitive DNA* found at the end of the chromatids that make up a chromosome. Unlike normal DNA, telomeres do not code for the production of proteins – they are non-coding DNA. The purpose of telomeres is to protect the important coding DNA during the DNA copying process; without telomeres the ends of each chromatid would be lost during cell division. Dr Elizabeth Blackburn, who won the 2009 Nobel Prize in Medicine or Physiology for her work on telomeres likened them to the plastic caps on the ends of shoelaces that prevent them from fraying.


Human chromosomes with telomeres highlighted in yellow.

Imagine that cell division works like photocopying, with each division making a perfect photocopy of the original. However, this photocopier is not perfect, and always loses the last five pages in the copying process, making the copy five pages shorter than the original. Adding telomeres to the end of chromatids is like adding many blank pages to the end of your original before the copying process starts. Although five pages are lost during each copy, there are many, many blank pages that can afford to be lost before important (i.e. coding DNA) pages begin to be lost.

During the aging process telomeres become shorter and shorter, and the shorter your telomeres are the more susceptible you are to disease and the shorter your lifespan is likely to be.

* In humans telomeres code the sequence “TTAGGG” over and over again.

Consanguinity and the coefficient of relationship

Or What percentage of your genetic code do you share with your second cousin?

Two people who share some of their genetic code (their DNA) are said to be consanguineous (“co” meaning shared and “sanguis” being Latin for blood) and the extent to which their DNA overlaps is known as the coefficient of relationship and is measured as a percentage.

Bearing in mind that you get 50% of your genes from each of your parents, it’s relatively easy to calculate the percentage of your DNA that you share with any relative you choose.

The closer to white a relation is, the lower the amount of DNA shared with that person.

To explain where these figures come from let’s take an example: How much of my DNA do I share with my sister Caroline?

Every one of your genes is split into two parts called alleles; you get one of these two alleles from each of your parents to make up each of your genes. If we take as an example a random gene, OCA2 (a gene that controls for eye colour) it will make it easier to understand the calculation process.

I got half of my OCA2 gene from half of my Dad’s OCA2 gene and the other half of my OCA2 gene from my Mum’s OCA2 gene; the same is true for my sister. I’ll call my Dad’s OCA2 alleles ED1 and ED2 and my Mum’s OCA2 alleles EM1 and EM2. There are therefore four possibilities for my and my sister’s OCA2 genes:

  • ED1EM1
  • ED1EM2
  • ED2EM1
  • ED2EM2

With four possible choices for the OCA2 gene there is a 25% chance of us sharing 100% of our DNA, a 50% chance (25%+25%) of sharing 50% of our DNA and a 25% chance of us sharing 0%. If you sum these percentages across all of our genes you get:
(25% × 100%) + (50% × 50%) + (25% × 0%) = 50%.

To quickly calculate the percentage of DNA you share with a relative simply count the number of (vertical) steps you have to take to get to them, and raise one-half to that power. For example, to find the percentage of DNA shared with a second cousin once removed you have to go three steps up and three steps down (the step between grandparents and great-grandparents is not counted twice) giving six steps in total. One half to the power of six is 0.015625 or 1.5625%; this is shown on the diagram below.

All of these calculations assume that none of your distant ancestors interbred with each other: i.e. that you have four unique grandparents, eight unique great-grandparents, sixteen unique great-great-grandparents and so on. Any half-relationships (e.g. half-brother, step-mother) cannot be included and of course I’m ignoring the fact that we are all related if you go back far enough.

Dating a common ancestor

Time Tree is a website that allows you to search for the point in time at which the genetic code of two organisms diverged; the time at which their last common ancestor lived.

Investigating human beings is quite fun: the last common ancestor that we shared with chimpanzees lived 6.3 million years ago and we shared an ancestor with gorillas 8.6 million years ago. We are much closer to cats and dogs (95.2 million years ago) than we are to ducks (292 million years ago).

If we start to look at more obviously different organisms we find much older most recent common ancestors: we shared an ancestor with jellyfish 892 million years ago and with the northern red oak tree 1.43 billion years ago.

It isn’t just humans that you can find the most recent common ancestors for. Horses and camels shared an ancestor 84.2 million years ago and cats and dogs a mere 55.7 million years ago.

Lactose tolerance

Because most in the western world are able to digest dairy products it is often assumed that this condition (lactose persistence) is the norm. But if you are able to tolerate lactose then you are actually in a minority: ninety-eight percent of Southeast Asians and ninety-five percent of Chinese are lactose intolerant. Between them these two ethnic groups make up more than 28% of the world’s population.

The graph below shows some of the ethnic groups with over 50% of the population lactose intolerant: