What happens if a person has one dominant gene and one recessive gene for the same trait in his or her DNA? (PLS HELP ASAP)

Only the dominant allele will be expressed in the individual's phenotype.

Explanation:

Only the dominant allele (version of that trait/gene) will be expressed in the individual's phenotype (physical characteristics).

Dominant alleles always 'mask' recessive alleles whenever they are present (for the same trait/gene). This means if an individual has at least one dominant allele, they will always display the dominant version of that trait.

The only way for an individual to express the recessive gene in their phenotype for that trait, is if they are homo-zygous recessive (have both recessive alleles) as there are no dominant genes to mask the recessive ones, so they can be expressed.

Hope this helped!