Step 1: Understand the statements.
The assertion says that even though a parent has two alleles, the pair separates during gamete formation. The reason says segregation happens in all sexually reproducing organisms. We check both.
Step 2: Recall Mendel's Law of Segregation.
Each parent carries two alleles for a trait. When gametes form, these two alleles split so that each gamete gets only one allele. So the assertion is true.
Step 3: Why does this matter.
This clean split keeps the number of alleles steady from generation to generation. When two gametes join, the offspring again gets two alleles.
Step 4: Check the reason.
Segregation is seen in every organism that reproduces sexually, because all of them make gametes through meiosis. So the reason is true.
Step 5: Link reason to assertion.
Since segregation is universal during gamete formation, it directly explains why the alleles of a pair separate. So the reason explains the assertion.
Step 6: Final decision.
Both are correct and the reason explains the assertion.
\[ \boxed{\text{Both (A) and (R) are correct and (R) is the correct explanation to (A)}} \]