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.
Genetics and Mendel’s Peas
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.
DNA, Genes, Alleles and Chromosomes
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: Father of Genetics
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.
Dominant Alleles and Mendelian Genetics
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.
Punnett Square and Inheritance
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 dominance and recessive traits chart indicates a 1:2:1 ratio with 50 percent of the offspring having heterozygous alleles like their parents.
Dominant Allele Disorders
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.
Dominant Alleles and Non-Mendelian Inheritance
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.
Dominant Allele Examples
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.
|Dominant Traits||Recessive Traits|
|Ability to Roll Your Tongue||Lacking Ability to Roll Your Tongue|
|Unattached Earlobes||Attached Earlobes|
|Huntington’s Disease||Cystic Fibrosis|
|Curly Hair||Straight Hair|
|A and B Blood Type||O Blood Type|
|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|
Incomplete Dominance vs. Mendelian Genetics
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 vs. Codominance
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.
- Genetics Generation: Mendelian Genetics
- University of Utah: Genetic Science Learning Center: What are Traits?
- The Human Genome: Poems on the Book of Life: Mendel’s Peas
- University of Washington: Genotypes and Phenotypes
- Cystic Fibrosis Foundation: CF Genetics: The Basics
- U.S. National Library of Medicine: Genetics Home Reference: Huntington Disease
About the Author
Dr. Mary Dowd studied biology in college where she worked as a lab assistant and tutored grateful students who didn't share her love of science. Her work history includes working as a naturalist in Minnesota and Wisconsin and presenting interactive science programs to groups of all ages. She enjoys writing online articles sharing information about science and education. Currently, Dr. Dowd is a dean of students at a mid-sized university.