What does it mean for chromosomes to have alleles that do not necessarily cause the same expression of a trait?

The concept of alleles that do not necessarily cause the same expression of a trait relates specifically to the genetic makeup of organisms. Heterozygous alleles refer to a scenario where an individual has two different alleles for a particular gene—one inherited from each parent. Because these alleles can express different traits or characteristics depending on whether one is dominant or recessive, the result is that the individual may express a trait that differs from those expressed by individuals with either identical alleles (homozygous) or both alleles being dominant or recessive.

In contrast, identical alleles would indicate that the individual has two copies of the same allele, leading to a uniform expression of the associated trait. Codominance refers to a situation where both alleles in a heterozygous individual contribute equally and visibly to the phenotype, which does not align with the idea of differing expression. Lastly, homozygous alleles indicate uniformity in allele expression, either both being dominant or both recessive, hence not leading to variation in trait expression. Heterozygous alleles, therefore, clearly demonstrate that differing genetic information can lead to a range of expressed traits in an organism.