What are dominant and recessive alleles?

Dominant and recessive alleles are terms used in genetics to describe the expression of certain traits or characteristics.

To understand these terms, it helps to have some background knowledge about genes and chromosomes. Genes are segments of DNA located on chromosomes, which are found inside the cells of living organisms. Each gene carries the instructions for a specific trait, such as eye color or blood type.

An allele is a specific version or variant of a gene. For example, the gene for eye color may have different alleles like "blue" or "brown." Alleles can be either dominant or recessive.

A dominant allele is one that is always expressed or observed in the phenotype, which is the physical manifestation of the gene. In other words, if an individual carries one dominant allele and one recessive allele for a particular trait, the dominant allele will determine the trait's expression. It overrides the presence of the recessive allele.

On the other hand, a recessive allele is only expressed or observed in the phenotype when an individual has two copies of it (one from each parent). If an individual carries two copies of the recessive allele, the recessive trait will be expressed. However, if an individual has one dominant allele and one recessive allele, only the dominant trait will be expressed.

To determine whether an allele is dominant or recessive, geneticists perform experiments called crosses or breeding studies. By observing the offspring of these crosses, they can determine the pattern of inheritance and identify which alleles are dominant and which are recessive.

In summary, dominant alleles always express their trait, even if only one copy is present, while recessive alleles require two copies to be expressed.