Image from International Redhead Day 2011

Getting physical

Small genetic changes can mean big physical differences

We are fascinated by difference, but genetically humans are really rather similar. The latest figures suggest we are all 99.6 per cent similar (identical twins excepted). Many human characteristics vary continuously – there are people at many different points on the scale. Height is a typical example, and recent analysis suggests at least 400 different gene regions (as well as environment) affect how tall a person grows.

A few characteristics of appearance vary more simply. Red hair, for example, is associated with having two altered copies of a gene known as MC1R, which is involved in making the pigment melanin. (Having just one ‘red’ allele may affect skin tone and freckling.) There is some evidence for a number of the other characteristics often said to be associated with red hair, such as a lower tolerance for pain. The MC1R gene encodes a receptor that seems to interfere with pain circuits in the brain.

However, redheads’ reputation for ‘fiery’ temperament probably has more to do with the associations of the colour than with anything in the DNA. Less obvious personal differences, such as having wet or dry earwax, have been tied to a specific gene.

99.6%

How genetically similar all humans are (identical twins excepted), according to latest figures

Wet earwax is harmless, but the same genetic variation has been linked with a slightly increased risk of breast cancer. However, dry earwax is much less common, so it’s difficult to find large enough groups to compare.

Meanwhile, genomics is uncovering more details of the small differences in genes that can make a big difference to your appearance. For example, the International HapMap Project helped to pin down a single gene, out of 170 thought to be involved in hair growth, that makes an important contribution to hair thickness in eastern Asian populations. The gene EDAR comes in a thick-hair variant, which must have appeared after ancestral Asian and European populations divided. The gene seems to affect the activity of a  factor – one of the many ways one gene can alter the regulation of another. This factor is important in fixing the final thickness of hair fibres.

Lead image:

International Redhead Day 2011

QSimple, Memories from the Future/Flickr CC BY NC ND

References

Further reading

About this resource

This resource was first published in ‘Genes, Genomes and Health’ in January 2010 and reviewed and updated in December 2014.

Topics:
Physiology, Genetics and genomics
Issue:
Genes, Genomes and Health
Education levels:
16–19, Continuing professional development