Plausible cause of a family only giving birth to females?

Consider: there are conditions that are passed on by X-linked recessive inheritance. The most famous is haemophilia. Here's how it works: mutation BadNews is recessive, and carried on the X chromosome. The mother is a carrier. A daughter would get a healthy X chromosome from the father, and either a healthy or a mutated X chromosome from the mother - she is either unaffected, or a carrier - healthy either way. The son only gets one copy of the X chromosome, from his mother, so he is either healthy, or sick.

Now, imagine that mutation BadNews is so lethal, that the foetus fails to develop, resulting in a miscarriage. Mutations that are so lethal exist, and they could conceivably be X-linked. It's possible that there is an actual disorder that would fit those requirements.

But that still leaves the possibility of a healthy boy, you say. 50% of boys conceived would be healthy, resulting in a 2:1 girl:boy ratio. True. There are several ways you can address that, depending on the way you plan your story.

• The family is aware of the condition, and having to resort to IVF for unrelated reasons, pick only female foetuses. (Picking only females is much easier and cheaper than picking healthy males).
• A really unlikely but theoretically possible situation: the mother carries not one, but two different X-linked recessive lethal conditions: BadNews and TerribleNews, one on each X chromosome. The boys get either one and die, the girls get either one + a healthy X chromosome from the father.
• The family could have in fact had a healthy son, but with chances being 2:1 in favour of a girl, the dice just fell this way.

posted over 6 years ago

Galastel‭

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