Originally posted on 24 December 2012 when my now 5 year old was 4.
Tis the season…for my 4 year old to ask me to sing Rudolph the Red-Nosed Reindeer repeatedly during any car trip longer than 30 seconds. My apologies to anyone who gets caught in the crossfire. My singing does not get better with repetition.
My kids also love the Rankin/Bass stop animation classic film Rudolph the Red-Nosed Reindeer. As you probably have come to expect, I have had a lot of time to wonder about how two seemingly normal reindeer could have a child with a glowing nose. Classic genetics is well-equipped to deal with this problem.
Both Santa and we should be very concerned about the genetics of red reindeer noses. According to Wikipedia, the Rudolph story dates back to 1939. There have probably been quite a few foggy Christmas Eves in the intervening years. According to the Pittsburgh Zoo, reindeer typically live for 10 years in the wild. While we can expect that Santa’s reindeer do a bit better than those in the wild, it is clear that Rudolph alone would not be able to “guide Santa’s sleigh” today. Given their success breeding flying reindeer, it is not hard to imagine that Santa’s elves could generate a stable of red-nosed reindeer. How they would go about doing so would depend on how, genetically, Rudolph wound up with that first Red Nose.
For the following analysis, I’m going to use Rudolph’s family tree from the Rankin/Bass Rudolph the Red-Nosed Reindeer. In the film, Rudolph is the son of Donner (or Donder, or Dunder) and “Mrs. Donner” – my four year old assures me with great confidence that her name is Bertha. For the purposes of illustrating how different genetic causes of the Red Nose will play out in future generations of reindeer, I’m assuming that Rudolph’s youthful romance with Clarice develops into a fruitful, long term relationship.
The following is not exhaustive, but covers some of the simplest genetic ways you could wind up with a Rudolph the Red-Nosed Reindeer using only one gene to control the trait.
1. Red Nose is a dominant, novel mutation.
This is essentially what the traditional view of Rudolph as a “freak” implies. In this case the gamete from one of Rudolph’s parents carries the mutation, while none of that parent’s somatic cells would be mutated. I would suggest that Donner, his dad, is the most likely source of such a mutation due to elevated ionizing radiation exposure during Christmas Eve flights.
In this scenario, half of Rudolph’s offspring would carry the Red Nose allele, or gene variant, and have “very shiny” noses themselves.
Though it is unlikely, future siblings of Rudolph might also exhibit the Red Nose trait, if Donner’s sperm is the source of the mutation. Reindeer typically mate between September and November. If the mutation occurred during a Christmas Eve flight (the time of maximum ionizing radiation exposure), an affected sperm cell would be unlikely to survive an 8-10 month wait. To pass on the mutation event to his son, the mutation would have to be in one of Donner’s sperm precursor cells, not an individual sperm cell.
If the mutation was in one of Bertha’s eggs, there would be no chance of Rudolph’s siblings having the Red Nose trait. Given that only a minuscule fraction of Donner’s sperm would contain the Red Nose mutation, Donner would have to sire an extremely large number of offspring to have any chance of distinguishing, empirically between the two hypotheses.
2. red nose is a recessive allele for which both Rudolph’s parents are heterozygotic.
In this scenario, both copies of a reindeer’s nose color gene would need to have the red nosemutation in order to have a red nose. Both of Rudolph’s parents would have one copy of the normal nose color gene and one copy of the red nose mutation. Because it is recessive, neither of his parents would have a red nose. Rudolph would have inherited one copy of the red nosemutation from Donner and one copy from Bertha.
Because Rudolph seems to be the first reindeer with a Red Nose that anyone at the North Pole has ever seen, we have to assume that the recessive red noseallele is extremely rare. Therefore, if we assume that Clarice does not carry the recessive red noseallele, we can conclude that none of their offspring would have a Red Nose. Eventually, another Red Nose reindeer would be born. In a domesticated animal husbandry situation, it is even likely that the elves would resort to inbreeding between reindeer to produce the useful Red Nose trait.
If Clarice was a heterozygote, then 25% of their offspring would have a Red Nose.
2a. red nose is a sex-linked, recessive allele for which Rudolph’s mother is heterozygotic.
A sex-linked, recessive allele is a lot like the recessive allele discussed above. In the typical case, described in #2, there is nothing special about having two copies of the recessive allele. What is crucial is not having any copies of the dominant, normal allele.
In a sex-linked trait, the gene is located on the sex chromosome – the X chromosome in this case. Male mammals technically have two sex chromosomes, the X and Y (females are X and X). The Y is a pretty degenerate little chromosome and does not contain the same genes as the X. If the Red Nose gene is located on the X chromosome, female reindeer will have two copies of the Red Nose gene, but male reindeer will only have one.
If a female reindeer inherits an X chromosome with the red nose allele, she will not have a Red Nose because she still has a normal allele on her other. A male reindeer that inherits a copy of the recessive red nose allele on his X chromosome has no normal copies and will have a Red Nose. This is how the most common form of color blindness in humans works and is why men are
so much more likely to be color blind than women.
In this scenario, Bertha was the parent carrying the red nose allele. She could have inherited that allele from her mother or a mutation in the sperm cells of her father (like Donner in #1, as we know her father did not have a Red Nose.
Assuming that Clarice is not a heterozygotic carrier of the recessive red nose allele, none of Rudolph’s children will have a Red Nose. None of his sons will be carriers. Their single copy of the Red Nose gene will have been inherited with their X chromosome from Clarice (their Y chromosome necessarily comes from their father). All of his daughters will be heterozygotic carriers. Their sons will have a 50% chance of having a Red Nose.
3. Red Nose is a multigenic trait.
A multigenic trait is what it sounds like. It is a trait controlled by multiple genes – each making some contribution to the final value. Usually this means that the trait will have some sort of continuous distribution across man values. For Red Nose, this would mean a spectrum of noses between “shiny & red” and standard reindeer. This is certainly not the case among Santa’s reindeer. Rudolph is not the most extreme individual on a continuum. He is a total aberration.
Occasionally, a trait can have a multigenic basis, but manifest itself as discreet phenomena. A situation like that would require some wild speculation about the physiology of glowing reindeer noses, and that is a hard question. How hard? Wait until next year hard.