When one allele of a gene masks the presence of the other allele, what form is masked?
The masked allele, is called RECESSIVE and the allele that expresses itself is called DOMINANT.
Alleles are different forms of same gene. One person can have only two alleles of the same gene.
In presence of dominant allele the recessive character can not be expressed, i.e. masked. When recessive allele is present in double dose, only then the recessive character is expressed.