Last night on Loveline, Dr. Drew’s guests were Steve-O and Wee Man of “Jackass” fame. Wee Man had an announcement to make: his girlfriend is pregnant! In the ensuing discussion, Wee Man, who is a dwarf, said that the child has a 25% chance of sharing his condition. Dr. Drew did not object; I think he even affirmed the claim.
But I’m fairly certain they’re both wrong – which is weird, given that Wee Man presumably knows a lot about his own condition, and Dr. Drew seems to know a lot about everything medical. Achondroplasia (the technical term for dwarfism) is caused by a single gene that is functionally dominant: you only need one copy to exhibit dwarfism. And in fact, if you have two copies, you’ll die pretty quickly. Now, I don’t know if Wee Man’s girlfriend is a dwarf or not. But if she is not, then their child has a 50% chance of inheriting the gene from the father; otherwise the child will be normal-sized. If she is a dwarf, then the child has a 50% chance of being a dwarf who lives, a 25% chance of dying quickly, and a 25% chance of being normal-sized. No matter what assumption we make about the girlfriend, there’s no way you get only a 25% chance of achondroplasia.
I suspect that both Wee Man and Dr. Drew were assuming that achondroplasia is caused be a recessive gene; it would take two recessive genes to cause the trait to appear. Drew even mentioned the possibility of Wee Man’s girlfriend being “a carrier,” which supports my hypothesis. But achondroplasia is not caused by a recessive gene; if the girlfriend were a carrier, she would display the condition.
(By the way, I use the term ‘dwarf’ because I can’t stand the currently popular term ‘little person.’ But I don’t wish to offend. If anyone knows a better term [‘achondroplasic,’ perhaps?], I’ll be happy to use it.)