Final answer:
A normal cell in a human has two versions of each gene, known as alleles. While multiple alleles exist in a population, an individual cell carries only two, which can be identical or different. Gene expression varies in different cell types, leading to diverse functions within the organism.
Step-by-step explanation:
A normal cell in a diploid organism like a human carries two versions of each gene, one from each parent. These different versions are known as alleles. Although there can be multiple alleles at the population level, a single cell will have only two alleles for each gene, which may be the same or different versions that contribute to the organism's genotype and phenotype. Gregor Mendel's work initially suggested there were only two alleles for a gene, this is now known to be a simplification as many alleles can exist for any given gene within a natural population, even though individuals carry just two.
In cases where a single gene influences a single trait, the genetic variation and expression can be profoundly important. The presence of different alleles accounts for the variety in characteristics observed within a population. Furthermore, not all genes are always 'turned on' in every cell. Gene expression is cell-specific, explaining why different cell types perform various functions despite having the identical set of genes.