Final answer:
In a population, alleles can show complete dominance, incomplete dominance, or codominance. Incomplete dominance results in a blended phenotype, while codominance shows both traits simultaneously. Multiple alleles contribute to genetic diversity within a population.
Step-by-step explanation:
The alleles in a population can exhibit different forms of dominance. Complete dominance occurs when the presence of a single dominant allele masks the effect of a recessive allele. Incomplete dominance happens when the phenotype of the heterozygotes is a blend of the two homozygous phenotypes. Codominance is demonstrated when both alleles in the heterozygote are fully expressed, with neither being dominant over the other, resulting in a phenotype where both traits are displayed simultaneously.
For example, in the ABO blood group system, the A and B alleles exhibit codominance. An individual with both A and B alleles will have type AB blood, showing a mix of A and B antigens. With incomplete dominance, if a red-flowered plant is crossed with a white-flowered one, the offspring may have pink flowers, which is intermediate between the phenotypes of both parents.
At the population level, multiple alleles may exist for a given gene, although any individual, being diploid, can only have two of those alleles. This contributes to the genetic diversity of a population.