Understanding the concepts of genotype and phenotype begins with the distinction between dominant and recessive alleles. Alleles are different versions of specific genes, and they can be classified as either dominant or recessive. A dominant allele exerts its effects whenever it is present, symbolized by a capital letter. In contrast, a recessive allele has no effect if a dominant allele is present and is represented by a lowercase letter.
For example, in Gregor Mendel's studies of pea plants, the allele for yellow peas is dominant over the allele for green peas. This means that the yellow allele (Y) will mask the effect of the green allele (y) when both are present. The dominant allele (Y) is visually represented as strong and prominent, while the recessive allele (y) appears subdued and is only expressed when no dominant allele is present.
In summary, the dominant allele can overshadow the recessive allele, leading to the expression of the dominant trait in the phenotype, which is the observable characteristic of an organism. The recessive trait will only manifest in the phenotype when both alleles are recessive. This foundational understanding of alleles sets the stage for exploring the broader concepts of genotype, which refers to the genetic makeup of an organism, and phenotype, which refers to the physical expression of that genetic makeup.