Dominant Allele: What Is It? & Why Does It Happen? (With Traits Chart)

If you're the only one in your biological family with blue eyes, you may question how that happened.

The likely answer has to do with Mendelian inheritance, not switched babies at birth or deep, dark family secrets. Brown-eyed parents with a recessive allele (gene variation) for blue eyes have a one in four chance of giving birth to a blue-eyed child.

Dominant alleles, like the gene variant for brown eyes, make proteins and enzymes that result in brown eyes, for instance.

Modern genetics dates back to the 1860s when Gregor Mendel, an Austrian monk with an interest in science and math, experimented with peas in his garden over the course of eight years. Mendel's keen observations led to the principles of Mendelian inheritance.

Through systematic crossings of purebred pea plants, Mendel discovered how dominant vs. recessive traits work. Years later, non-Mendelian genetics and complex heredity emerged as scientists encountered the many exceptions to Mendelian inheritance and simplistic heredity.

The nucleus of the cell contains deoxyribonucleic acid (DNA) – the "blueprint" of a living organism. Genes are snippets of DNA in the chromosomes that influence inheritable traits such as natural athletic ability. Different forms of genes are called alleles. Many possible types of alleles exist within a species.

A child receives one allele for eye color from the mother and one from the father. When a child receives two alleles for brown eyes, the gene is homozygous dominant for that trait. If a child receives two different alleles for eye color, the gene for eye color is heterozygous.

Gregor Mendel is commonly called the father of genetics for his pioneering work in identifying the difference between dominant and recessive traits. By cross-pollinating pea plants year after year, Mendel figured out the genotype vs. phenotype distinction.

He also noted that certain traits skip a generation due to a hidden copy of a gene that's double recessive.

Mendelian genetics is a simplistic model that worked well with common pea plants. Mendel studied the color and position of blossoms, stem length, seed shape and color and pod shape and color of pea plants from one generation to the next.

Once Mendel identified the dominant genetic traits, he was able to see what happens in homozygous vs. heterozygous crossings.

The Punnett square illustrates Mendelian genetics. A person with two alleles for brown eyes is homozygous dominant. Someone with two alleles for blue eyes has a homozygous recessive allelic pair. Heterozygous individuals have one allele for brown and one allele for blue eyes, for instance.

The Punnett square predicts allelic pairs of offspring. For instance, the predicted genotype of children born to two parents with heterozygous alleles is often shown in a chart.

The dominance and recessive traits chart indicates a 1:2:1 ratio with 50 percent of the offspring having heterozygous alleles like their parents.

Non-reproductive cells in the human body contain two copies of every gene: one from the mother and one from the father. Normal copies of a gene are called the wild-type. Autosomal dominant disorders like Huntington disease occur when a person inherits even one copy of a single gene that's defective.

A person can also be an asymptomatic carrier of diseases like cystic fibrosis that only occur when both parents pass on mutations of the CFTR gene.

Non-Mendelian inheritance models involve multiple types of dominance not seen in garden peas. Codominance refers to two traits appearing in a heterozygote offspring, rather than one trait dominating the other in the phenotype. Red blood cells illustrate codominance.

For instance, blood type AB results from equal dominance of type A and type B dominant alleles. Incomplete dominance happens when heterozygote offspring have an intermediate phenotype such a red flower and a white flower producing pink flowers.

Mendel's principles include the fundamental theory of inheritance and the principle of segregation. His work focused on the difference between dominant and recessive traits in genotype and inherited phenotype.

Mendel found that dominant traits – like purple flowers – are seen more often than recessive traits when purebred, homozygote peas cross.

Recessive traits do not reappear until the F₁ (first-generation) hybrids mature and self-pollinate. Gregor Mendel also noted that dominant traits outnumber recessive traits by a 3:1 ration in F₂ (second generation). In terms of Mendel's plants, he did not see examples of codominance or blending.

\(csv-table
Dominant Traits Recessive Traits
Ability to Roll Your Tongue Lacking Ability to Roll Your Tongue
Unattached Earlobes Attached Earlobes
Dimples No Dimples
Huntington's Disease Cystic Fibrosis
Curly Hair Straight Hair
A and B Blood Type O Blood Type
Dwarfism Normal Growth
Baldness in Males No Baldness in Males
Hazel and/or Green Eyes Blue and/or Grey Eyes
Widow's Peak Hairline Straight Hairline
Cleft Chin Normal/Smooth Chin
High Blood Pressure Normal Blood Pressure\)

Polygenic inheritance refers to traits determined by more than one gene. The many alleles that contribute to traits like human height are not in one set place.

Different alleles can be closely linked on chromosomes, unlinked on chromosomes or even be on different chromosomes and still influence expression of certain trait. Environment may also play a role in gene expression.

Incomplete dominance and codominance are both part of non-Mendelian inheritance, but they're not the same thing. Incomplete dominance is a blending of traits vs. an additional phenotype because both alleles are expressed in codominance.

In humans, eye color, skin color and many other traits are influenced by many allele variants that give rise to multiple shades from light to dark.