In Blood Will Tell I talked about the theory that Henry VIII had a Kell positive blood type, but in my quest to prevent innocent readers from slipping into an over-science coma I didn’t go into deep detail about what it is exactly and how it is transmitted. This post will provide a tad more information.
To have Kell positive blood means you have at least one of the Kell antigens on your red blood cells. This is in addition to the ABO blood groups that most people know about. There are apparently quite a few antigenic things that can be running around on the surface of your red blood cells. Who knew?
The funny thing about a Kell positive blood type is that the Kell antigen is passed on by a dominate gene, but even though it is dominate it is rare; less than 10% of the people in the UK have it. How can that be? Wouldn’t dominate genes … dominate?
Not necessarily, as it turns out.
Do you remember the Punnett Square from middle school biology classes? Okay, let’s call the Kell positive allele “K” and the Kell negative gene “k”. Everyone would have two of these genes, so people with Kell positive blood are either “KK” or “Kk”, and people with Kell negative blood are “kk”. Now, what would happen if a Kell positive person reproduced with a Kell negative person? Their children have the potential genotypes of:
We know two things from these squares and the fact Kell positive blood is rare. First, people who are Kell positive are most likely to be Kk instead of KK because there is a more than 90% chance that one of their parents was Kell negative, and thus had only k genes to bring to the party. Secondly, when Kell positive people with the typical Kk genotype reproduce with one of the Kell negative (kk) majority, each child has only a 50/50 chance of being Kell positive.
Say a Kk/kk couple has four children. Now, we’ve all met families where all the kids were the same gender, so you know 50/50 doesn’t mean it will always split down the middle between two outcomes. Thus, all four children could be kk and Kell positive blood would disappear from their genetic line. Unless on of the offspring reproduces with a Kell positive partner (which is a less than 10% chance) no grandchildren will be Kell positive, and so on forevermore.
But what if some or all of the children get the K gene? If all four children are girls, and they each are Kk, then Kell positive blood is very likely to show up in the grandchildren. However, if all four children are boys then the Kell gene is much less likely to turn up in the grandchildren, because every pregnancy after the FIRST one will only survive if it is Kell negative (kk). So if every boy grew up, married, and had 4 kids himself … then no more than 4 out of the 16 grandchildren could possibly have the Kell positive gene. Since there is only a 50/50 of a first born getting the K gene, maybe only 2 out of the 16 hypothetical grandchildren will be Kell positive. Or maybe none of them will be.
Basically, every time a Kk person has a male child, the odds of the K gene making it to the next generation is lessened. (Well, it used to be like that before modern medical treatments that will keep a Kell positive fetus alive inside a Kell negative mother. Kell may become more common now. Ask me again in 150 years.)
Moreover, random chance can make a dominate gene go bye-bye even when the dominate gene doesn’t cause the death of many offspring who inherit it. Take brown eyes and blue eyes, for example. For simplicities sake we’ll say brown eyes are “B” and blue eyes are “b”. This means we can use the Punnett square again and see the genotypes of the offspring of a brown-eyed person (BB or Bb) and a blue-eyed person (bb).
Let’s say mom has brown eyes of BB and dad has blue. Every one of their kids will have brown eyes because of the B, but they will all be carrying the blue b as well. If mom’s brown eyes are Bb, then some of the kids might be blue-eyed, but the brown-eyed kids are all going to be carrying a recessive too. See? Once you get a bb in the mix those little b genes are lurking in every gonad. If those offspring marry blue-eyed (bb) people, then more of the grandkids will be blue-eyed too. Once a person is blue-eyed, there are NO dominate genes to pass on to their kids. Dominate browns can easily be hiding secret blue genes (insert blue jean joke here) but the brown B is gone once the recessive have the field. If two couples, let’s name them Ted (BB) & Alice (bb) and Bob (bb) & Jane (Bb), have children who marry each other then it is possible that all their grandchildren can have blue eyes in spite of the fact that two grandparents have the dominate brown-eyed gene. Of course, all of them could have brown eyes – it depends on the roll of the genetic dice when Mr. Sperm meets Ms. Egg.
There you have it. A very simplified version of how a dominate gene (especially one that is harmful under certain conditions) can wind up being in only a small minority of a population.
It really was just bad luck that Henry (in theory) had a Kk genotype. How do we know Henry was Kk and not KK? Because the New Year’s Boy survived for a few weeks and Mary lived to adulthood. If the King had not had a “k” gene to give them, they would have died in utero or a few hours after their birth like their poor siblings did. QED – Henry had a Kk genotype.
Next post – how did Henry get that Kell positive gene?