The dominant and recessive alleles affect the phenotype by their combination.
Phenotype
The phenotype is the organism observable characters combinaison. The phenotype is the result of the effects of both living environment and genotype.
Genotype
The genotype is the genetic information held by an organism. Because of alleles and non-codant genome mutations we all have a different genotype even the twins from a same egg.
What’s an allele ?
An allele is a version of a gene. For examples if you have a different hair color from your friends its because you have different hair color gene alleles.
Homozygous and Heterozygous
Because of the fact that some organisms owns two ore more chromosoms each gene is held several times by a same organism. So it’s possible for an organism to hold differents alleles for a gene (heterozygous) or the same alleles (homozygous)
Dominant and recessive allele
Some alleles are told dominants because in case of heterozygosity only their information will be shown by the phenotype. For example the brown eye color.
Some alleles are told recessives because their information will be shown by the phenotype only if the organism is homozygous for this gene. For example the O blood type.
If two dominants alleles are held by the same organism the phenotype will be the result of the both they’re codominants. For example the AB blood type is the result of codominance between the A and B blood type allele.
So the dominant and recessive alleles affect the phenotype by their combination.
Hoping it’s helpful !