Kid's Hair Color

So, it's early in the morning and I can't sleep because pregnancy has made my usual insomnia way worse. And I got to obsessing over genetics. Specifically, what color will my child's hair be?

I'm blond. Which means that I have two recessive blonde alleles - let's call that bb. My Mom and brother are also blond, so they must have the same alleles. My Dad is dark-haired, but since he had two blond children, he must carry the blond gene, so let's call his genotype Db.

The sperm donor is dark-haired. His profile says so, and his childhood photo confirms it. But, the question is, does he carry a blond gene?

If he has any blond children, then he definitely does. But although the Donor Sibling Registry says he has four registered donor offspring, I haven't paid for a membership yet, and the information I can see for free tells me nothing more than that they exist and some of their genders. So, up until I get around to signing up for that, I can't say anything about my kid's diblings.

If he has a blond parent, he also must 100% be a carrier of the blond gene. Blond people are bb, remember? So a blond parent always passes on a blond allele to their child. But, according to his donor profile, both his parents are dark-haired.

So, could he still carry a blond gene? Sure, if one or both of his parents carry a blond gene. He lists one sibling, also dark-haired, but of the grandparents listed, two are blond. Now, the profile doesn't say which grandparents are on which side. But since both his parents are dark-haired, we know that both must have at least one dark-haired parent. So the two blond grandparents must be on opposite sides, meaning that both of his parents are carriers of the blond gene.

Two dark-haired carriers of the blond gene can have children with the following alleles and phenotypes:

DD - dark-haired, 25%

Db - dark-haired, 50%

bb - blond, 25%

So, a dark-haired child of two known Db parents has a 2/3rds chance of carrying a blond gene.

If the donor's genotype is DD, then my kid's genotype and hair color is 100% dark-haired Db. That's the only possible outcome.

But, if the donor's genotype is Db, then my kid has a 50% chance of either being Db (dark-haired) or bb (blond). They're definitely inheriting a blond allele from me, but the donor could give them either allele.

So, given that the donor has a 2/3 chance of being a carrier, there's a 1/3rds chance my child will be dark-haired because the donor is DD, a 1/3rds chance my child will be dark-haired from a Db donor, and a 1/3rds chance my child will be blond. So, that's a 2/3rds chance of dark-hair, and a 1/3rds chance of blond hair.

It'll be interesting to see my kid's hair color!