In biology class we just finished the classical genetics unit. I ot this question wrong on the test,

 

Cross a Homozygous Black (BB), Homozygous Running (RR) mouse with a brown (bb) walzting mouse (rr).

 

My answer looked like this

 

RB RB RB RB

 

rb RrBb RrBb RrBb RrBb

 

rb RrBb RrBb RrBb RrBb

 

rb RrBb RrBb RrBb RrBb

 

rb RrBb RrBb RrBb RrBb

 

It was marked wrong because I can't but the same thing on the top row. (RB)

But wouldn't it make sense that since one parent is Homozygous dominant for both traits that they would all at least express the dominant traits? Then the other parent has no dominant genes so as far as I can see that should be right. If I'm wrong can someone explain it to me so I don't get this wrong on the final?

If you have a pure homozygote dominant crossed with a pure homozygote recessive, it'll be het for both genes, yes.

 

I think the thing is that your instructor didn't want the full punnett square, but rather for you to simply say that all the offspring would be RrBb. Unless there's some aspect of the question you've not told us.

 

Mokele

No, the question was worded like this.

 

Cross a Homozygous Black Homozygous running with a brown walzting mouse. Draw a punnet square to support your answer.

 

Ah well. If I get screwed on the final I'll complain.