Noah's Y chromosome
I just had the distinct displeasure of encountering someone who is a Biblical literalist (and who actually believes that all evidence supports the fairy tale of Noah's Ark). Let's review the story, briefly:
God decides that the human race is sinful and wants to destroy it. But Noah is virtuous and God warns him about the upcoming catastrophe and tells him to build an Ark (a big ship) and to load it with breeding stock of animals, so as to repopulate the earth. Noah does this, and with his wife and three sons and their wives embarks on the ship. God then covers the earth with water for 40 days and 40 nights, drowning all animals. (It's not completely clear how this would harm whales and seals, but let that go.) Then the water begins to recede and Noah's Ark grounds on Mount Ararat. He releases his animals and the world is repopulated.
This is ecological nonsense. If the whole flora of the world was released from a single point, most of it wouldn't survive and they sure wouldn't be found where they are today.
It's also genetic nonsense. There are a lot of ways of showing this, but one is particularly unambiguous: human Y chromosomes. Nearly all the genetic information each of us carries comes from both our parents. Humans have 23 pairs of chromosomes and one of each comes from our mothers and one from our fathers. One pair controls the sex of the child. One of these is normal sized and the other is extremely small (relatively speaking), and they're called "X" and "Y" respectively. (But the Y chromosome still carries millions of codons.)
In mammals, XX is female and XY is male. (This is not universal. In birds, XX is male and XY is female. In some insect species, females are diploid and males are haploid. And when you look at plants, sex isn't determined by genetics at all.)
Each human parent contributes 23 chromosomes to the baby, in egg and sperm. The egg always has an X chromosome and the sperm may have an X or a Y (unless there's an abnormality, about which more in a moment). If the sperm has an X then the resulting fertilized egg is XX, thus female. If the sperm has a Y then the fertilized egg is XY, male.
So the sperm cell determines the sex of the child (in mammals). But sometimes when the egg or sperm are formed there's a mistake, and they get more or less than 23 chromosomes. This can happen with any of the chromosomes. When it happens with the sex chromosomes you get individuals who may have three or one instead of the normal two.
If the egg forms wrong and has no X, and if the sperm cell has a Y, then you get a fertilized egg with a Y chromosome but no X chromosome. This egg isn't viable. The X chromosome contains genetic information without which a baby can't develop; the result is a miscarriage (so early, in fact, that the mother may not realize she's been pregnant).
The other three abnormal situations all result in children. If there's only one X chromosome ("Turner's syndrome") then the person looks female. XXY ("Kleinfelter's syndrome") is male. XYY (sometimes called the "supermale") is also male.
The development of male features is controlled by the presence of testosterone in the baby in the womb. This in turn is stimulated by the presence of the Y chromosome. There is, however, an extremely rare condition where the genes which describe the testosterone receptors are damaged, and in such an individual testosterone will be ignored even if it is present. The genes describing those receptors are not on the sex chromosomes but it's possible for them to be present in a person who has a Y chromosome, and if this happens the resulting person will look female. However, such a person will also be sterile, because no ovaries form.
The point is this: if a person is fertile and has a Y chromosome, then that person will be male.
I am male. I have a mother and a father. I got my X chromosome from my mother and my Y chromosome from my father.
Each of them also had two parents, so I have two grandfathers. My father's father gave his Y chromosome to my father. My mother's father gave his X chromosome to my mother.
I got my Y chromosome from my father and he got it from his father. I carry my paternal grandfather's Y chromosome. He got it from his father, and from his paternal grandfather, etc.
My mother got one of her two X chromosomes from her father and one from her mother. There's a 50% chance that the X chromosome I carry came from my maternal grandfather, but no chance whatever that I carry his Y chromosome. (He gave his Y chromosome to my mother's brother, but I'm not descended from my uncle.)
I have many male ancestors but my Y chromosome only came from one of them. I have many male ancestors but only one by strict patrilineal descent. They're the same person. That doesn't mean I have no genetic information from any of my other male ancestors; there are 22 other chromosomes to talk about and I may have gotten some of them from other men way back when. But the Y chromosome itself can only have come from one place, and it is from my strict patrilineal ancestor. My lineage from all my other male ancestors includes at least one intervening woman, and at that step their Y chromosomes were not passed on.
If any two men have the same strict patrilineal ancestor, no matter how far back, then those two men will have the same Y chromosome.
Which brings us back to Noah. According to the story, the ark carried 8 people: Noah, his wife, his three sons and their wives. Genetically only five are important, because the three sons carried no genetic information not present in Noah and his wife.
So if the story is true then the entire human race is descended from just five individuals. And four of them were women; Noah is the only man among the five. 10 sex chromosomes, and nine of them are X. Of the ten, only one was a Y. Noah carried it and passed it on to his three sons.
And they passed it on to their sons, and their grandsons, and great grandsons, and ultimately to all living men. So if the Noah story is true then every existing Y chromosome in men should be identical because they'll all be copies of the one carried by Noah.
And they aren't. Human Y chromosomes have been tested many times and they are not all the same. There is enormous variety among them. It is impossible for all human men to have the same strict patrilineal ancestor.
Therefore the Noah story is not true.
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