Final answer:
An allele is a variant of a gene which determines traits. The genotype is the set of alleles an individual has, while the phenotype is the expression of these alleles. Population genetics studies the distributions and changes of these alleles in populations over time.
Step-by-step explanation:
An allele is a variant form of a gene that may exist in multiple versions within a population, each of which can determine different traits associated with that gene. For example, the ABO blood type is influenced by three alleles: IA, IB, and i. The combination of alleles an individual inherits is known as their genotype, which can be either homozygous (same alleles) or heterozygous (different alleles).
The physical expression, or phenotype, of an organism, like pea color in plants or hairline in humans, is the result of the interaction between its alleles. In cases of Mendelian inheritance, the phenotype shows the dominance pattern, where the dominant allele masks the presence of a recessive allele. This can be observed through Mendelian ratios, which Gregor Mendel discovered through his work on pea plants.
Population genetics further explores the distribution and frequency of alleles within a population, examining how evolutionary processes such as natural selection influence genotype and phenotype frequencies over time.