Re: what kind of dominant recessive cross produces an 8 :1phenotype ratio

Well, this is actually a pretty tricky problem. I've delayed my response a 
bit, on the grounds that this sounds like a homework problem, so I'd 
rather not give the answer until after the due date. Matter of principle.

I'll assume you haven't covered recombination yet. If you have, then one 
possibility is that you're dealing with two genes that have a combined 
effect (both must be mutated to make the plant albino, for example), but 
they're on the same chromosome. In that case, using Punnett squares to get 
the ratio of offspring might not work, because the genes won't obey 
Mendel's Law of Independent Assortment, which basically says that a given 
plant's odds of getting gene A and its odds of getting gene B are 
independent.

Now, If I actually saw a weird ratio like that in the lab, and I knew one  
gene was involved, I'd suspect incomplete penetrance. What incomplete 
penetrance means is that mutations don't always show their effects. A 
plant might have two copies of a recessive mutation that causes, say, 
reduced height, and yet look normal. Environmental effects are one reason 
for this.

But a better explanation, I think, is this: Do you know if all your 
progeny survived? What if you try drawing a 4-by-4 Punnett square (16 
total progeny), and then finding combinations where 7 out of 16 progeny 
die, and 1 out of the 9 remaining progeny has the albino phenotype?

(Another solution, that doesn't involve lethality, involves thinking in 
terms of interactions between THREE genes, and looking for cases where 1/9 
= 7/64 progeny are albino. This is better done by multiplying fractions by 
hand, rather than using a Punnett square.)

I hope that this helped. Perhaps there's some explanation that should be 
blindingly obvious to me, but that I've missed.

Paul