8th grade science

Question

Andalusian chickens show incomplete dominance for feather color. A cross between a white bird and a black bird produces offspring that have blue feathers. A cross between two F1 blue chickens produces mostly blue chickens, but also some white chickens. Are the blue chickens purebred? Explain

Answer 1

Hi Kaelyn S.,

Congratulations to your science teacher for giving your some questions requiring thought. So you know what incomplete dominance is, right: and you might initially suspect that the cross was BB x bb -> Bb, then Bb x Bb -> (1/4) BB + (1/2) Bb + (1/4) bb . But you didn't see exactly that, did you.

So therefore you didn't have what you thought you did -- a single gene with only two alleles, and incomplete dominance. So what was the true case? You definitely have at least two alleles, but possibly more! Or, maybe multiple genes are controlling. In either case, you don't have "purebred" blues, and not even uniform heterozygous ones, necessarily. ("purebred" meaning, when crossed they produce the same phenotype, and conversely, produced by the crossing of that same phenotype). That's for the problem as stated.

P.S. If you look up actual data, you find that black x white Andalusians actually give heterozygous grey ones. So, this is an artificial problem, in a sense. (grey, heterozygous) Andalusians only breed "true" 1/2 of the time. Get over it. That's the chickens, not their eggs, which are white, regardless (and probably came first?!).

-- Cheers, -- Mr. d.