Final answer:
An alternative form of a single gene is known as an allele. Alleles can be either dominant or recessive, determining the phenotype of an organism, and while any individual carries only two alleles for a gene, many alleles may exist within a population.
Step-by-step explanation:
An alternative form of a single gene is known as an allele. Alleles are variations of a gene that may cause different traits to be expressed in an organism. When individuals possess two copies of the same gene, these alleles may either be identical, in which case the organism is homozygous for that trait, or different, making the organism heterozygous for that trait.
A classic example provided by Mendel's experiments involves flower color, where a gene can have two alleles, one for violet flower color and one for white. However, many genes have multiple possible alleles within a population though any individual can only have two of these multiple alleles.
Furthermore, when considering gene expression, if only one allele is expressed over the other, the expressed allele is termed the dominant allele and the unexpressed is the recessive allele. The genotype of an individual, which is the combination of alleles inherited, determines the phenotype, or the physical representation of those traits.