2023-10-23 23:46:02
These last days has jumped to the press the case of a person from Galicia who allegedly committed a rape more than ten years ago and who, finally, was able to be identified and arrested due to his condition as a redhead. Although the background of the news is terrible (the raped woman was murdered) it is, in my opinion, an excellent opportunity to refresh some basic concepts of genetics that can help us all to correctly interpret this case and many others similar.
In the case cited, the rapist left just a few drops of his own biological fluids on the victim’s body that were used to obtain his DNA, his genome, and to analyze it. Investigators quickly realized that the criminal had to be redhead. And they began to look for families in the area with a red-haired member until they found the person now detained.
MC1Rthe gene for redheads
There are relatively few genes whose mutations have a direct impact on a person’s external appearance. Many genes regulate the functioning of our organs but do not manifest observable external characteristics. But there are others whose presence is obvious, among them precisely the reason why there are redheads.
Specifically, it is the gene MC1Rwhich encodes the melanocortin type 1 receptor. In the human genome it is located on chromosome 16. The protein it encodes is anchored in the membrane of melanocytes, the pigment cells of our body present in the skin, hair, iris, choroid, etc. It can be activated by the hormone alpha-MSH and then the cell produces black melanin or eumelanin. But when the protein is mutated, it is only capable of producing another pigment, a yellowish-reddish-orange melanin that we call pheomelanin.
Therefore, when a person has red hair we can say that it is most likely that they have the MC1R gene mutated (although there are other genes which can give rise to red-haired people, although their participation is much smaller, very insignificant). And vice versa, if by analyzing a genome we discover that a person has the mutated MC1R gene then we can conclude with absolute certainty – here it is irrefutable – that it is a person with red hair.
Knowing this is very useful in forensic genetics, since it allows us to rule out or assign observable external characteristics as notable as being or not having red hair. And thus narrow the search for criminal suspects.
In European countries like Ireland or the United Kingdom, redheads can make up up to 30% of the population. In Spain, however, it is unusual to have red hair: it only happens to 1-2% of the population3% on the Atlantic and Cantabrian coasts due to their maritime interaction with the British Isles.
Redheads hidden for generations
For a person to be born with red hair, they must have both copies of the MC1R gene mutated, both the one they inherit from their father and the one they inherit from their mother. This is because we are dealing with a recessive mutation, which does not manifest its effect if it is in the presence of at least one correct functional copy. That is why there are many people who may be carriers of mutations in the MC1R gene, who will have inherited them from their ancestors, but who are not redheads.
When two people carrying mutations in the MC1R gene are a couple and decide to have offspring, we know, from the Gregor Mendel’s laws of inheritance, that in each pregnancy they will have a 25% chance of having a red-haired son or daughter, that is, inheriting only the mutated copies of the MC1R gene. However, it is possible that neither of the parents has red hair, not even the grandparents, and that the mutated gene has been passed from parents to children for generations, without manifesting itself. That is why sometimes a red-haired person appears in a family in which neither the father nor the mother is aware of having a red-haired ancestor.
Family tree of a family showing the ancestors of the mother and father up to the generation of the great-great-grandparents of the two children (daughter and son) shown at the bottom of the diagram. The circles represent women. The squares represent men. The orange circles or squares represent red-haired people. White circles or squares represent non-redheads. For each person, their genotype (the two copies of the MC1R gene) is indicated. The M indicates the functional MC1R gene. The m indicates the mutated MC1R gene. The purple Xs indicate the carriers who have transmitted the mutation in the MC1R gene through each generation. Diagram: Lluís Montoliu.
This graph illustrates a supposed case of a couple (at the bottom of the diagram) who has two children: a non-redhead daughter and a redhead son, who no one knows where they came from and the family wonders what their origin. In this family I have placed a red-haired great-great-grandmother on the maternal side of the couple. And on the paternal side of the couple a red-haired great-great-grandfather. They were the ones who introduced the mutations into their respective families, mutations that began to be passed from parents to children through successive generations. But as long as two carriers did not coincide in a couple, no other red-haired person could be born. And that’s why so many generations went by without redheads that they even forgot about this trait.
Going back to the graph (and Mendel), if we identify the intact MC1R gene with a capital “M”, and the mutated MC1R gene with a lowercase “m”, then we will have three types of genotypes, three types of people:
On the one hand we will have the “MM”, who have inherited intact copies of the MC1R gene from their father and mother and will not be redheads (nor will they be able to have offspring who are).
Secondly, we will have the carriers, “Mm”, who will have inherited an intact copy of the MC1R gene and a mutated copy. This single copy is insufficient to produce an effect and these carriers will still be non-redheads, but it leaves the door open for offspring with red hair.
Finally we will have the “mm” people, those who have inherited both mutated copies from both the father and the mother. These will be the red-haired people.
There is no cat in the bag, no magic. It is, plain and simple, basic genetics.
#Learning #genetics #redheads