Question: Why do some genetic things e.g hair colour, the need for glasses ect skip generations?

Kevin O'Dell answered on 14 Jun 2014:

Good question! What you say is absolutely correct that inheritance of characteristics is a bit weird in that some characteristics are passed down families from grandparents to parents to children but appear to skip generations. Some geneticists have written entire books on this, but I’ll try and keep my answer a little bit shorter!

As you probably know humans have about 25000 genes, which are the instructions to make you. In fact everyone has two sets of genes, one full set of 25000 from their mum and one full set of 25000 from their dad.

Perhaps the best example of characteristics skipping generations is eye colour (though I should stress it is a bit more complicated than I’m going to describe here). In people of European decent there are two different versions of the eye color gene (a gene that geneticists call OCA2). There is a brown version and a blue version. The best way to think about this is that the brown version of the eye colour gene makes a brown pigment, but the blue version of the eye colour gene doesn’t work (some of you will know that we call different versions of the same gene alleles). So if both versions of your eye colour gene are the brown type you’ll have brown eyes, and if both copies of your eye colour gene are the blue type you’ll have blue eyes.

The way this helps answer your question is that people with one brown version and one blue version of the eye-colour gene (we call these people heterozygotes) always have brown eyes. This is because the one brown version makes the brown pigment whilst the blue version does nothing, so the person who is genetically blue/brown will always have blue eyes. Geneticists (who are unfortunately obsessed with using complex words to describe things) say that at the eye colour gene brown is dominant to blue. In families that have the blue version of the gene, blue eyes often disappears for a generation or two. This is because individuals may inherit the recessive blue version of the gene, but it is masked by the dominant brown version of the gene. I hope this is making some sense!

In principle inheritance the two characteristics you mention, hair colour and eyesight, works in exactly the same way, except that there won’t be just one gene involved but several (for hair colour) and possibly hundreds of genes for something as complex as vision. For example at one of the many genes that influence hair colour there is a recessive version that leads to having ginger hair. So you quite often see ginger hair skipping generations as the recessive ginger version of the gene is masked by other versions (alleles) of that hair colour gene.

Going back to our story about blue-eyed Europeans. Recent research has shown that the first person with blue eyes probably lived in modern day Romania around 8,000 years ago. All blue-eyed Europeans living today are descended from this one person (and their special friend!). Which means that about half of the people reading this share the same 8,000 year-old ancestor to me.