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Originally Posted by Gilda
I hate to nitpick, but there's a 25% chance a son would have it. That's a 12.5% chance a child would have it. This doesn't, however, affect the rest of what you had to say, it just reduces the likelihood it would come into play.
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I know that's what lurkette said, but it's not what she meant. Warning: Science Content Ahead. Warm up your Punnett Squares.
She has one affected X chromosome and one unaffected X chromosome (her mother was a carrier, her father was not). That makes her a carrier too, but since the HED gene is recessive, it's blocked by the healthy, dominant X chromosome. Girls can't have this disease unless both parents are carriers, and the gene ends up on both X chromosomes.
There's a 50% chance she'll supply the affected chromosome. In which case, there's a 50% chance that I'll supply a Y chromosome, which wouldn't have the corresponding healthy gene to block the expression of the X-linked HED gene, resulting in an affected male child. So, 50% of our male offspring would, statistically, have the disease, or 25% of our children overall. By the same logic, there's a 25% chance we'd have a carrier girl (the category lurkette and her mother are in), and a 50% chance we'd have a completely unaffected child of one gender or the other.