In some family trees, affected children are born to unaffected parents.?

Question

However, the parent actually had the allele. How to explain this phenomenon?

Answer 1

I didn't understand what u mean by affected....that is affected by what disease....specify it.

In general for any hereditary disease a defective gene might remain dormant for about 1 generation or so.
Sometimes but very rarely they can remain dormant for many generations and then resurface later.
Thus it could be difficult to trace the gene source.

It can also be a Carrier case.
For example in hemophilia females are carriers that is their sons may possess the disease ,but they themselves are seldom affected.

Thus as you said sometimes affected children are born to unaffected parents because of resurfaction of a dormant gene.

Answer 2

I believe what you are trying to say is the domiant and recessive traits. Sometimes when a certain person is affected, it requires both gene from the corresponding of each chromosome to be recessive. If such condition is not fulfilled, the trait would not be shown.

There is a 25% chance that both parents being carrier of the gene (carrier means consist of 1 recessive and 1 dominant gene) to give birth to a child that is affected.

Answer 3

The unaffected parent had the gene for the particular condition in genotype form. The gene was recessive. However in the next gen. the gene became phenotype for that trait, that is the gene and its allele (partner) are the same

Answer 4

Are u talking about aids?