Write down symbols for the alleles. (These may be given in the problem.) When represented by single letters, the dominant allele is uppercase and the recessive is lowercase.

Answer: A (uppercase letter) is used to refer to dominant alleles.

a (lowecase letter) is used to refer to recessive alleles.

Explanation:

Alleles are different forms of a gene, and they can be dominant or recessive. Each gene codes for a specific trait.

Diploid organisms, such as human beings, have two alleles for each gene. If an organism is heterozygous for a certain trait, it means it has one dominant and one recessive allele. And since the dominant allele masks the effects of the other one, the dominant trait is expressed in the phenotype. So, dominant alleles show their effect even if the organisms only has one copy of it.

If an organisms is homozygous, it has two dominant alleles or two recessive alleles. Then it can be homozygous dominant, where both are dominant alleles; or homozygous recessive where both are recessive. Recessive alleles only show their effect if the organisms has two copies of it.

For example, if we want to study the gene that codes for hair color, we could say that dominat allele "A" codes for brown hair, and recessive allele "a" codes for blonde hair. If a person has brown hair, the genotype could be either AA or Aa, becuase AA is homozygous dominant and it has two copies of brown color. And Aa is heterozyogus, but the person only needs one copy of the allele to be expressed. On the other hand, if the person has blonde hair, the genotype can only be aa because recessive alleles only show their effect if there are two copies of the allele.