Dominant and Recessive Traits

Dominant and recessive traits exist when a trait has two different forms at the gene level. The trait that first appears or is visibly expressed in the organism is called the dominant trait. The trait that is present at the gene level but is masked and does not show itself in the organism is called the recessive trait.

In order to understand the concept of dominant and recessive traits, it is necessary to know what is meant by the word "allele." Alleles have to do with genes, and genes are the carriers of information that determine an organism's traits. Our height, hair color, blood type and overall looks are but a few examples of traits that are the result of the chemical activities directed by our genes. Every human being is produced by sexual reproduction and therefore receives twenty-three gene-containing chromosomes (coiled structures in the nucleus of a cell that carries the cell's DNA) from each parent, resulting in a full complement of forty-six chromosomes. When the chromosomes pair up to form a new and unique individual (since chromosomes always exist in pairs), they do so in a very particular way so that the same trait is always carried on the same place or position on the chromosome. In other words, since the offspring receives information on each trait from both its parents, there are corresponding pairs (or two genes) that match together for each trait. Sometimes these are the same (when a person inherits a gene for blood type O from both its parents), and sometimes these are different (when the person inherits blood type O from the father and blood type A from the mother). When these forms of the same type of gene are different or alternative versions, they are called "alleles." Therefore, alleles are different forms of a gene for a particular characteristic. However, more and more the word allele is used interchangeably for gene.

Most often when an individual receives two different alleles for a given trait, one allele is expressed and the other is not. For example, a person may receive one allele for a straight hairline and another for a widow's peak (when the hair comes down to a point in the middle of the forehead). In such a case, the person will have a widow's peak since that allele is "dominant" or is the one that is able to express or show itself. In the same way, the allele for brown eyes is known to be dominant over the allele for blue eyes. Conversely, the allele that is masked or is not able to show itself (despite being there) is called "recessive."

Austrian monk Gregor Mendel (1822–1884) made the first detailed investigation of inherited traits in the 1860s. Since Mendel's ground-breaking work, the rule has been that when two organisms showing different traits are crossed, the trait that shows up in the first generation is considered the dominant trait. A dominant trait could be compared to an athlete who dominates a game or a person who dominates a conversation. Each of these people monopolize things to the point where others have no chance to express their ability or ideas. It is in this way that a dominant allele expresses itself and suppresses or masks the activity of the other allele for that trait. Although the masked allele is not expressed, it is still there and remains part of the person's inherited package. This means the recessive allele can still be passed on to the next generation. Masked or recessive traits can only express themselves when the individual has a matching recessive allele (totaling two alleles for that trait).

Although Mendel did not know exactly what the gene and the allele were, he knew very well that they existed in some form (he called then "factors"), and that they followed certain laws. He was therefore able to formulate what became known as the law of dominance. This law states that when a dominant and a recessive form of a gene come together, the dominant form masks the recessive form. Thus, even though the recessive allele (or member of the gene pair) is still present, it is not visible.

[See alsoGene; Inherited Traits; Mendelian Laws of Inheritance ]