What does it mean if there are two different alleles for a trait ?

The trait is heterozygous.

Explanation:

Heterozygous: "Having two different alleles of a particular gene or genes."

A heterozygous genotype looks like: Rr. The capital R is a dominant allele, and the lowercase r is a recessive allele. These two alleles are not the same (not two dominant alleles or two recessive alleles) meaning that the trait is a heterozygous trait.

The opposite of a heterozygous trait is a homozygous trait, in which both alleles are the same (both dominant or both recessive) and can look like RR or rr (genotype-wise).

answer; polycythemia is the correct answer;