Simple dominance, also known as complete dominance, describes the most straightforward way a trait can be passed from parent to offspring. This pattern of inheritance is the foundational rule of classical genetics, first identified by Gregor Mendel in the mid-19th century. In simple dominance, the presence of just one version of a gene is enough to fully determine a physical characteristic, completely overshadowing a different version. This establishes a binary, “either/or” outcome for a specific trait.
Defining the Genetic Language
Understanding simple dominance requires familiarity with the language used to describe heredity. A gene is a segment of DNA that provides instructions for a specific trait, such as flower or eye color. Alleles are the variant forms of a gene. Because humans inherit one set of chromosomes from each parent, we carry two alleles for every gene.
In this inheritance pattern, one allele is designated as dominant and the other is recessive. The dominant allele completely masks the expression of the recessive allele when both are present. The recessive trait is only outwardly expressed when an individual inherits two copies of the recessive allele.
The combination of alleles an individual possesses is called the genotype, often represented by letters (e.g., \(AA\), \(Aa\), or \(aa\)). The observable, physical manifestation of that genotype is the phenotype. For a trait exhibiting simple dominance, individuals with the \(AA\) (homozygous dominant) and \(Aa\) (heterozygous) genotypes display the same dominant phenotype. The recessive phenotype only appears in the \(aa\) (homozygous recessive) genotype.
Predicting Traits: Punnett Squares and Examples
The Punnett square is a visual tool used to predict the possible genotypes and phenotypes of offspring resulting from a genetic cross. This square grid maps the alleles contributed by each parent to determine the probability of outcomes in the next generation. For a trait governed by simple dominance, the results of a cross between two heterozygous parents (\(Aa \times Aa\)) reveal the pattern of inheritance.
In this common cross, the Punnett square shows that offspring have three possible genotypes: \(AA\), \(Aa\), and \(aa\), in a \(1:2:1\) ratio. Because the dominant allele completely masks the recessive one, the \(AA\) and \(Aa\) genotypes both result in the dominant phenotype. This yields the classic \(3:1\) phenotypic ratio, where three-quarters of the offspring display the dominant trait and one-quarter display the recessive trait.
A common example in humans is the ability to taste the chemical phenylthiocarbamide (PTC). The ability to taste PTC is a dominant trait, while the inability to taste it is recessive. If two heterozygous parents (\(Tt\)) who can taste PTC have a child, the child has a \(75\%\) chance of being a taster and a \(25\%\) chance of being a non-taster. Another example involves earlobe attachment, where detached earlobes are dominant over attached earlobes.
When Dominance Isn’t Simple
Simple dominance describes complete masking and stands in contrast to other common inheritance patterns. Incomplete dominance is one alternative, where the heterozygous genotype results in a phenotype that is a blend of the two parental traits. For instance, crossing a red-flowered plant with a white-flowered plant may produce offspring with pink flowers, rather than the red flowers expected under simple dominance.
Another distinct pattern is codominance, which occurs when both alleles are fully and simultaneously expressed in the heterozygous individual. Unlike simple dominance, codominance allows both traits to be visible at the same time. The human \(ABO\) blood group system provides an example: an individual with \(A\) and \(B\) alleles will have type \(AB\) blood, meaning both \(A\) and \(B\) markers are present on the red blood cells.
These alternative patterns highlight the strict definition of simple dominance, which requires the heterozygous organism to be phenotypically identical to the homozygous dominant organism. Simple dominance is defined by the complete expression of one allele over the other, a binary relationship. This complete masking effect distinguishes simple dominance from the blending or simultaneous expression seen in incomplete dominance and codominance.