Final answer:
Alleles are different forms of the same gene, contributing to genetic variation. Individuals can be homozygous or heterozygous depending on whether they inherit identical or different alleles. These alleles, such as those determining blood type, can be dominant or recessive.
Step-by-step explanation:
Alleles are different forms of the same gene, and they play a critical role in determining the genetic variation within a population. In diploid organisms, like humans, each individual has two alleles for each gene, one inherited from each parent. These alleles can be identical, in which case the individual is homozygous for that gene, or they can be different, in which case the individual is heterozygous.
When it comes to phenotype expression, if two alleles differ, one (the dominant allele) is usually expressed over the other (the recessive allele). An organism's observable characteristics, or phenotype, are the result of the interaction between its genotype and the environment.
There are instances where multiple alleles exist for a gene within a population, such as the alleles that determine the ABO blood type (IA, IB, and i). Each person can have only two of the multiple possible alleles for a given gene.