Every so often, news stories about the endangerment of the redhead population appear on the internet. The reasons suggested for this are diverse, from climate change to the rejection of redheads as sperm donors. Where does the truth lie? These types of stories usually crop up just to stir a commotion.
Let's begin at the end: the likelihood of the redhead gene’s disappearing is nearly zero. Even if one day a deadly virus would wipe out the entire redhead population on Earth, this would not eliminate the redhead gene, and sooner or later, a new freckled ginger baby would be born. To understand the reason for this, we have to delve into the depths of natural selection and heredity.
Every property in our body, such as the color of our hair, shape of our nose and even sensitivity to fava beans, is encoded in a genetic unit called a gene. Every gene exists in two similar “copies,” or alleles, in our body – one inherited from each parent.
Alleles can be dominant or recessive and, just like people, the dominant ones are expressed at the expense of the recessive ones. The allele encoding red hair is recessive, so if I inherited it from my mother and a dominant allele for black hair from my father, I would have black hair, since that is the allele that would be expressed. Only if I were to inherit two ginger alleles from my parents, they would be expressed and I would have red hair.
In the course of the evolutionary process, certain traits undergo natural selection. Some disappear, while others thrive and get passed on to future generations. Selection affects only the expressed traits; therefore dominant traits undergo natural selection more simply, and can thus disappear more easily. Recessive traits, in contrast, may remain “dormant” for generations, affecting an individual's reproductive success only in the rare cases in which they are expressed.
Let's return to the example of the theoretical virus that only attacks redheads: The virus basically applies selective pressure on the ginger trait – only people expressing this trait would be afflicted and would not be able to pass on their alleles to their offspring. In contrast, those who are not redheads but carry the ginger allele (e.g., those with one ginger allele and one black allele) would be immune to the disease and could pass on the ginger allele to the next generation. In this manner, the ginger allele remains protected inside the DNA of its carriers.
When selection is applied against dominant traits that are invariably expressed, there is a higher chance that the trait will become extinct. In contrast, if all people expressing a recessive allele die, the allele may become very rare in the population – but will almost never disappear altogether.
In the case of redheads, this evolutionary mechanism works to ensure that the ginger trait does not disappear from the face of the Earth. Yet this same mechanism presents a problem when it comes to genetic diseases: Recessive diseases are much more difficult to eradicate, as there are families, sometimes entire communities, in which a part of the members are disease carriers who do not manifest any symptoms, and may thus pass it on to their offspring.
Translated by Elee Shimshoni