heterozygote defective genes

[pohely]

Lets say we cross r+/r+ x r/r. With r+/r+ being the wildtype fly and r/r being the defective phenotype for the fly. by doing this cross we get r+/r with a wildtype phenotype. but there are cases when r+/r will not give a wildtype phenotype, but instead it'll give the phenotype of a homozygous defective individual. my question is : How can a defective allele present in one copy along with a wild type one, show the phenotype of a homozygous defective individual?

I understand that in cases where r+/r has a wildtype phenotype its because the product of the cross is heterozygote, therefore has half the enzyme activity of wild-type flies. Apparently having half the XDH activity of a wild-type fly is sufficient to allow heterozygote to have normal (wild-type) colored eyes. i also know that the protein product of most genes is made in excess so that the heterozygote tends to have the same phenotype as the homozygous wild type. but can someone please explain the cases when the r+/r does not give the wildtype phenotype. I would greatly appreciate it, because my exam is in 2 days and im really confused about this.


Thanks in advance!

[mith]

...sounds impossible to me. Where did you get this question?

[blcr11]

What about dominant mutations? There are such things and are usually some sort of gain-of-function change. Heterozygotes will exhibit the dominant mutant phenotype rather than the recessive wild type. I don't know if this type of mutation can be found for r+/r-, but Gla is an example of a dominant mutation affecting Drosophila eye color--to name one.

[mith]

Does that mean r+/r represents two kinds of mutations?

[pohely]

i looked up dominant negative mutation and it says:

Dominant negative mutations (also called anti-morphic mutations) have an altered gene product that acts antagonistically to the wild-type allele. These mutations usually result in an altered molecular function (often inactive) and are characterised by a dominant or semi-dominant phenotype. In humans, Marfan syndrome is an example of a dominant negative mutation occurring in an autosomal dominant disease. In this condition, the defective glycoprotein product of the fibrillin gene (FBN1) antagonizes the product of the normal allele.

So when asked this question on my exam can I say: that the r+/r can express the phenotype of a homozygous defective individual, if the r allele kind of overpowers the effect of the r+ allele through dominant negative mutation, making the r+ unexpressed and therefore the wildtype phenotype wont be expressed?

[pohely]

Just wanted to say thx guys for helping me! I got the question correct on the exam

[Ultrashogun]

So actually 'r' are at least two different allels, one dominant and one recessive?