Dominant Trait Definition
A dominant trait is an inherited characteristic that appears in an offspring if it is contributed from a parent through a dominant allele. Traits, also known as phenotypes, may include features such as eye color, hair color, immunity, or susceptibility to certain diseases and facial features such as dimples and freckles.
In sexually reproducing species, each individual has two pairs of chromosomes; humans have 23 pairs of chromosomes, and so 46 chromosomes in total. The chromosomes contain thousands of genes that code for the proteins that express and control all of the biochemical and physical features of an organism; this set of genes is an organism’s genotype.
Within each chromosome, there are two copies of each gene. Each chromosome carries the same gene in the same position (called a locus) so that they are paired. However, each locus may have two different versions of each gene: one received from the mother and one from the father.
Each of the alternative versions of a gene is called an allele. Alleles come in two different forms: recessive (denoted as a small letter, e.g., a) and dominant (denoted as a capital letter, e.g., A).
If an individual carries the same two alleles for a gene, they are homozygous for that gene (aa or AA); this is the case whether the alleles are recessive or dominant. If the two alleles are different, the individual is heterozygous for the gene (Aa).
Assuming Mendelian Genetics, which is a simplified explanatory tool:
- A recessive trait will only be expressed if the offspring has two copies of the recessive allele that codes for the trait (recessive homozygous, aa).
- A dominant trait will always be expressed in the offspring if the dominant allele is present, even if there is only one copy of it (heterozygous or dominant homozygous, Aa or AA).
Gregor Mendel was a 19th-century Austrian monk who first formulated the idea of inherited traits after conducting simple hybridization experiments with pea plants.
At the time, it was mostly believed that reproduction resulted in offspring with traits that were a blend of the parents’ traits. However, Mendel noticed that when he crossed purple-flowered pea plants with white-flowered pea plants, the offspring had purple flowers.
He then bred these first generation offspring with themselves (this is possible in many plant species). In the second generation of pea plants, he noticed that 75% of the offspring were purple and 25% were white.
Mendel suggested that there was a ‘trait’ that was being passed from the first parent population of white flowers, into the third generation; we now know those traits to be genes.
In the case of the peas, the purple flower coloration is controlled by a dominant gene (designated here as P), while the white coloration is controlled by a recessive gene (p). The parent generation contained homozygous purple (PP) and homozygous white (pp) genes.
When these were bred to create the first generation, the offspring were Pp, having each taken a dominant allele from one parent and a recessive allele from the other. So, although both alleles were passed down, the white color alleles were masked by the dominant purple color alleles.
Offspring of the second generation could therefore be PP, Pp, or pp, with PP and Pp individuals displaying purple color and pp individuals displaying white color. Using a punnet square, it is possible to see how the 3:1 color ratio was achieved in the second generation:
Mendel furthered his research by studying other characteristics of peas, such as pod color (yellow or green), pea shape (round or wrinkled), flower position (axial or terminal), and height of the plants (tall or short); for each, the same outcome was seen as for flower color.
Using these basic principles of inheritance, it is possible to predict the percentage of different dominant traits expressed in the offspring, in a wide range of reproductive events. However, most traits within a phenotype of a complex organism are controlled by a number of different genes, so the reality is not always as simple as in Mendel’s experiments.
Examples of Dominant Traits
Human Dominant Traits
There are many characteristics of the human phenotype, which are controlled by dominant alleles:
- Dark hair is dominant over blonde or red hair.
- Curly hair is dominant over straight hair.
- Baldness is a dominant trait.
- Having a widow’s peak (a V-shaped hairline) is dominant over having a straight hairline.
- Freckles, cleft chin and dimples are all examples of a dominant trait.
- Having almond-shaped eyes is a dominant trait whereas having round eyes is a feature controlled by recessive alleles.
- The trait of detached earlobes, as opposed to attached earlobes, is dominant.
- Right-handedness is dominant over left-handedness.
- The ability to roll the tongue is dominant over the inability to do so.
- Astigmatism is dominant over normal vision.
- The presence of webbed fingers is a dominant trait.
- The development of 6 fingers instead of 5 is controlled by dominant alleles.
- Brown eyes are dominant over blue eyes (however, eye color is controlled by more than one gene and is thus a polygenetic trait and cannot be explained by Mendelian genetics. People with green and hazel eyes have a mix of alleles for brown and blue eyes).
Other traits, which are not physically visible, are also controlled by dominant alleles, for example:
- Immunity to poison ivy is controlled by a dominant allele.
- High blood pressure is a dominant trait.
- A & B blood types are dominant over the O blood type.
- Susceptibility to migraines is a dominant trait.
- Tone deafness is dominant over normal hearing.
It is important to note that dominant alleles are not better than recessive alleles; dominant traits can cause serious health problems for individuals (such as high blood pressure). Dominant traits are also not necessarily more common than recessive traits; although, if they have an effect on the health of individuals within a population, they may become more or less common in the gene pool over time, due to natural selection.
Dominant Traits for Selective Breeding
Just like in Mendel’s experiments, humans have been using genetics for selective breeding in animals, as well as fruit and vegetables, for thousands of years.
Dominant traits that are favorable, such as white wool in sheep, smooth coats in horses, and short legs in dachshunds, can be increased in a population by breeding individuals who have the dominant alleles. By consistently breeding individuals with the desired dominant trait, the dominant allele becomes more common in the population.
However, selective breeding does have downsides. When efforts are made to breed from a small founding population that is homozygous for the desired dominant trait, variation within the gene pool is low.
Recessive genes that may cause health problems to increase in frequency within the population, and are expressed when they end up homozygous. This is known as inbreeding and can cause issues such as an increased rate of cancers, heart disease, and vision or hearing disorders.