Genetics cross/mutation question.?

Question

Consider the results of the following cross: RR x rr

(R is dominant and results in a wild type phenotype.)

The allele for wild type is said to be dominant, so why is it, for some alleles, the answer is not wild type? How can a defective allele present in one copy along with a wild type one, show the phenotype of a homozygous defective individual?

Answer 1

In some cases, when the dominant and recessive alleles are combined, and produces an allele not like the dominant allele, AKA incomlete dominance

Answer 2

I think you might have the answer in your question. It sounds as if a mutation must have occurred.

If one parent is RR, then all of the gametes from that individual should be R. However, if there is a mutation, almost all the gametes would be R and the gamete that gets the mutation would be r.

Then crossing the RR parent with rr, a mutated allele from the first parent (r) and a nonmutated gene from the second parent (also r) would produce an offspring with rr - homozygous recessive or homozygous defective.

Because your question specifies that the first parent is RR, that rules out the possibility that the parent was heterozygous, leaving mutation as the only explanation.