Recessive alleles are best defined as the least frequently expressed allele in a given population a weakened or attenuated allele only expressed in present in a double dose a deleterious or disease causing allele

Only expressed in present in a double dose

The different types of a gene are called alleles and it is described as either dominant or recessive. Recessive alleles only expresses in present in a double dose, which means that it only show their effect if the person has two copies of the allele or not expressed unless they are present in homozygous condition.

Recessive alleles are best defined as alleles that are expressed only in a double dose

Explanation:

According to Mendelian genetics there are 2 different alleles that cause the expression of certain characters in an individual. Mostly one comes from the maternal parent and the other comes from the paternal parent.  

If it is dominant then it can get expressed even if a single copy of the gene is present whereas if it is recessive it required 2 copies to be present to express the disease.

b would be the right answer