Dominant Trait Definition
A dominant trait is an inherited characteristic that appears in an offspring if it is contributed by a dominant allele from a parent. Phenotypes are characteristics such as eye color, hair color, immunity to certain diseases, and facial characteristics such as dimples and freckles.
A sexually reproducing species has two pairs of chromosomes; humans have 23 pairs, making a total of 46 chromosomes. In an organism, the chromosomes contain thousands of genes that code for the proteins that determine its biochemical and physical characteristics. This set of genes is called its genotype.
Two copies of each gene are found on each chromosome. Each chromosome has the same gene at the same position (called a locus), so they are paired. Each locus, however, may have two versions of each gene: one received from the mother and one from the father. The alternative versions of a gene are called alleles. There are two types of alleles: recessive (denoted as a small letter, e.g., a) and dominant (denoted as a capital letter, e.g., A).
An individual is homozygous for a gene if they carry both alleles (aa or AA); this is true whether the alleles are recessive or dominant. An individual is heterozygous for the gene (Aa) if the two alleles differ.
Assuming Mendelian Genetics, which is a simplified explanatory tool:
- If the offspring have two copies of the recessive allele coding for the recessive trait (recessive homozygous, AA), the recessive trait will be expressed.
- Even when there is only one copy of a dominant allele (heterozygous or dominant homozygous, Aa or AA), the dominant trait will always be expressed in the offspring.
After conducting simple hybridization experiments with pea plants, Gregor Mendel first formulated the idea of inherited traits.
At the time, most people believed that reproduction resulted in offspring with traits inherited from the parents. Nevertheless, Mendel noticed that when he crossed purple flowered pea plants with white flowered pea plants, the offspring had purple flowers. After that, he bred these offspring with themselves (this is possible in many plant species).
He noticed that 75% of the offspring of his pea plants were purple and 25% were white in the second generation. A ‘trait’ passed from the first parent population of white flowers into the third generation is now known to be a gene.
The purple flower coloration of peas is controlled by a dominant gene (designated here as P), while the white coloration is controlled by a recessive gene (p). Genes homozygous for purple (PP) and white (PP) were found in the parent generation.
These were bred to create the first generation, and the offspring were Pp, having each taken a dominant allele from one parent and a recessive allele from the other. Although both alleles were passed down, the dominant purple color allele masked the white color allele. The second generation’s offspring may be PP, Pp, or pp, with PP and Pp individuals displaying purple color and pp individuals displaying white color.
Further research by Mendel examined other characteristics of peas, such as pod color (yellow or green), pea shape (round or wrinkled), flower placement (axial or terminal), and plant height (tall or short); the same result was observed for each.
Using these basic principles of inheritance, it is possible to predict the percentage of different dominant traits expressed in offspring across a wide range of reproductive events. Mendel’s experiments were simple, but most phenotypes in a complex organism are controlled by a number of genes, so the reality is not always so straightforward.
Examples of Dominant Traits
Human Dominant Traits
The human phenotype is characterized by many traits controlled by dominant alleles:
- Blonde and red hair are less dominant than dark hair.
- Hair with curly ends dominates hair with straight ends.
- It is a dominant trait to be bald.
- It is more dominant to have a widow’s peak (a V-shaped hairline) than to have a straight hairline.
- Dimples, freckles, and cleft chins are examples of dominant traits.
- Having almond-shaped eyes is a dominant trait, whereas having round eyes is a recessive trait.
- In contrast to attached earlobes, detached earlobes are dominant.
- It is more common for people to be right-handed than left-handed.
- Rolling the tongue is more dominant than not rolling it.
- Normal vision is dominated by astigmatism.
- It is a dominant trait to have webbed fingers.
- It is determined by dominant alleles whether six fingers develop instead of five.
- (However, eye color is a polygenetic trait and cannot be explained by Mendelian genetics. People with green and hazel eyes have alleles for both brown and blue eyes).
A dominant allele can also control traits that are not physically visible, such as:
- Poison ivy immunity is controlled by a dominant allele.
- There is a dominant trait associated with high blood pressure.
- The A and B blood types dominate over the O blood type.
- There is a dominant trait associated with migraine susceptibility.
- There is a greater prevalence of tone deafness than normal hearing.
Individuals with dominant traits can suffer serious health problems (such as high blood pressure) because dominant traits are not better than recessive traits. There is also no guarantee that dominant traits are more common than recessive traits; however, if they have an effect on the health of individuals within a population, they may become more or less common over time due to natural selection.
Dominant Traits for Selective Breeding
Many people have used genetics for selective breeding in animals, as well as fruits and vegetables, since Mendel’s experiments.
By breeding individuals with dominant alleles, desirable traits can be increased in a population, such as white wool in sheep, smooth coats in horses, and short legs in dachshunds. When a dominant allele is consistently bred into the population, it becomes more common.
Selective breeding does, however, have some downsides. There is little variation within the gene pool when a small founding population is homozygous for the desired dominant trait. Health problems caused by recessive genes become more common in the population as they become homozygous. An increased risk of cancer, heart disease, and vision and hearing problems can result from inbreeding.
Related Biology Terms
- Recessive Traits – Recessive traits are phenotype characteristics, which are displayed when a gene has two recessive alleles at a locus within a chromosome.
- Genes – Made up of DNA, genes are the basic unit of heredity, which code for the expression and control of proteins.
- Genotype – The set of genes in each individual that is responsible for the expression of particular traits.
- Phenotype – The physical characteristics or traits that are expressed as a consequence of an organism’s genotype.
A Dominant Trait is a genetic trait that is expressed when an individual has at least one copy of the dominant allele. It masks the expression of the recessive allele.
Dominant Traits are inherited in a dominant-recessive pattern. If an individual inherits at least one dominant allele, they will express the dominant trait. If they inherit two recessive alleles, they will express the recessive trait.
A Dominant Trait is expressed when an individual has at least one copy of the dominant allele, while a Recessive Trait is only expressed when an individual has two copies of the recessive allele.
Yes, a Dominant Trait can skip a generation if the dominant allele is present in a heterozygous state in the parent generation. In this case, the Dominant Trait may not be expressed in the offspring of that generation, but can reappear in subsequent generations.
Examples of Dominant Traits in humans include dimples, widows peak, unattached earlobes, and Huntington’s disease.