If gene is expressed in someone.. they are NOT a carrier. Right?

[Ryan71]

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Doesn't really make much sense to me but it appears to be the case.

Also, a bit confused about this Kaplan question.

What's the likelihood that the offspring is a carrier?

RR 25%
Rr 50%
rr 25%

It says the answer is 2/3. I would have thought (before being told that ones not a carrier if its expressed) that 3/4 would be carries.. and the 1/4 would be from the rr.

I understand everything else from these types of problems.. just don't understand what's going on here.

Thanks!

 

[RogueUnicorn]

this is when either RR or rr is lethal. while 2/4 conceived zygotes are carriers, since only 3 genotypes can make it to birth, the denominator drops to 3. thus, 2/3

 

[fuzzypanda4u]

Yeah that is a little confusing. I'm not sure if my interpretation is right, but I did a problem like this in my Genetics class.
Say if you are looking at a family tree for a autosomal recessive disease that is. Those individuals that are affected will be indicated on the tree (black filled). But if you look at a person normal for the gene, you can't tell that he or she is homozygous dominant or if he or she is a carrier, however you do know that he/she is not affected. So looking the ratios, the genotype for rr will not be considered in the ratio for determining carriers. Now you are looking at only the choices of RR, Rr, and Rr. 2/3 is the ratio