It was not until the middle of the 19th century that anyone was conducting what would turn out to be conclusive and irrefutable work in the mechanisms underlying human genetics, both in the short term (inheritance, or the passing of traits from parents to offspring) and over the long term (evolution, or shifts in the allele frequencies of given populations over hundreds, thousands or even millions of generations).
In mid-1800s England, a biologist named Charles Darwin was busy preparing to publish his major findings in the areas of natural selection and descent with modification, concepts that are now at the top of every life scientist's terminology list but were at the time anywhere between unknown and controversial.
Mendel: The Start of Understanding Genetics
At the same time, a young Austrian monk with a science-rich formal educational background, some serious gardening experience, and a supernatural level of patience named Gregor Mendel combined these assets to produce a number of important hypotheses and theories that advanced the life sciences by a tremendous leap virtually overnight, among them the law of segregation and the law of independent assortment.
Mendel is best known for introducing the idea of genes, or molecular instructions contained in DNA (deoxyribonucleic acid) pertaining to a given physical trait, and alleles, which are different versions of the same gene (typically, each gene has two alleles).
Through his now-famous experiments with pea plants, he produced the concepts of dominant and recessive alleles and the notions of phenotype and genotype.
The Basics of Heritable Traits
Prokaryotes, which are single-celled organisms such as bacteria, reproduce asexually, by making exact copies of themselves using a process called binary fission. The result of prokaryotic reproduction is two daughter cells that are genetically identical to the parent cell and to each other. That is, the offspring of prokaryotes, in the absence of genetic mutations, are simply copies of each other.
Eukaryotes, in contrast, are organisms that reproduce sexually in the cell-division process of mitosis and meiosis, and include plants, animals and fungi. Each daughter call gets half of its genetic material from one parent and half from the other, with each parent contributing a randomly selected allele from each of its genes to the genetic mix of the offspring via gametes, or sex cells, produced in meiosis.
(In humans, the male produces gametes called sperm cells and the female creates egg cells.)
Mendelian Inheritance: Dominant and Recessive Traits
Usually, one allele is dominant over the other, and completely masks its presence at the level of expressed, or visible, traits.
For example, in pea plants, round seeds are dominant over wrinkled seeds because if even one copy the allele coding for the round trait (represented by a capital letter, in this case R) is present in the plant's DNA, the allele coding for the wrinkled trait has no effect, though it can be passed on to the next generation of plants.
An organism's genotype for a given gene is simply the combination of alleles it has a that gene, e.g., RR (the result of both parental gametes containing "R") or rR (the result of one gamete contributing "r" and the other an "R"). The organism's phenotype is the physical manifestation of that genotype (e.g., round or wrinkled).
If a plant with the genotype Rr is crossed with itself (plants can self-pollinate, a handy ability to have when locomotion is not an option), the four possible genotypes of the resulting offspring are RR, rR, Rr and rr. Because two copies of a recessive allele must be present for the recessive trait to be expressed, only "rr" offspring have wrinkled seeds.
When an organism's genotype for a trait consists of two of the same alleles (e.g., RR or rr), the organism is said to by homozygous for that trait ("homo-" meaning "same"). When one of each allele is present, the organism is heterozygous for that trait ("hetero-" meaning "other").
In both plants and animals, not all genes obey the aforementioned dominant-recessive scheme, resulting in various forms of non-Mendelian inheritance. The two forms of major genetic significance are incomplete dominance and codominance.
In incomplete dominance, heterozygous offspring display phenotypes intermediate between the homozygous dominant and homozygous recessive forms.
For example, in the four o'clock flower, red (R) is dominant over white (r), but Rr or rR offspring are not red flowers, as they would be in a Mendelian scheme. Instead, they are pink flowers, just as if the parental flower colors had been blended like paints on a palette.
In codominance, each allele exerts equal influence over the resulting phenotype. However, rather than a uniform blending of the traits, each trait is fully expressed, but in different parts of the organism. While this may seem confusing, examples of codominance are sufficient to illustrate the phenomenon, as you'll see momentarily.
- Because in codominance the concept of "recessive" is not in play, no lowercase letters are used in the description of genotype. Instead, genotypes might be AB or GH or whatever letters are appropriate to signify the traits under consideration.
Codominance: Examples in Nature
You have no doubt noticed various animals that have stripes or spots on their fur or skin, such as zebras and leopards. This is an archetypal example of codominance.
If pea plants adhered to a codominant scheme, any given plant with the genotype Rr would have a mixture of smooth peas and wrinkled peas, but no intermediate, i.e., roundish-but-wrinkly, peas.
The latter scenario would be indicative of incomplete dominance, and all of the peas would have the same shape; purely round and purely wrinkled peas would not be evident anywhere on the plant.
Human blood types serve as a great example of codominance. As you may know, human blood types can be classified as A, B, AB or O.
These result from each parent contributing either an "A" red blood cell surface protein, a "B" protein or no protein, which is designated "O." Thus the possible genotypes in the human population are AA, BB, AB (this could also be written "BA" since the functional result is the same and which parent contributes which allele is irrelevant), AO, BO or OO. (It is important to recognize that while the A and B proteins are codominant, O is not an allele but really the absence of one, so it is not labeled in the same way.)
Blood Types: An Example
You can work out the various genotype-phenotype combinations here for yourself, a fun exercise when you know your blood type and are curious about either your parents' possible genotypes or those of any children you may have.
For example, if you have the blood type O, both of your parents must have donated a "blank" to your genome (the sum of all of your genes). This does not mean, however, that either of your parents necessarily has O as a blood type, because either or both could have the genotype AO, OO or BO.
Thus the only certainty here is that neither of your parents could have type AB blood.
More on Incomplete Dominance vs. Codominance
While incomplete dominance and codominance are clearly similar forms of inheritance, it is important to keep in mind the difference between the blending of traits in the former and the production of an additional phenotype in the latter.
In addition, some incompletely dominant traits have contributions from multiple genes, such as human height and skin color. This is somewhat intuitive because these traits are not a simple blend of parental traits and exist along a continuum instead.
This is known as polygenic ("many genes") inheritance, a scheme that bears no relationship to codominance.
- Biology LibreTexts: Incomplete Dominance and Codominance
- OpenText BC: Extensions of the Laws of Inheritance
- NCBI Bookshelf: Medical Genetics Summaries: ABO Blood Groups
- Scitable by Nature Education: Genetic Dominance: Genotype-Phenotype Relationships
- Scitable by Nature Education: Gregor Mendel and the Principles of Inheritance
- Untamed Science: Non-Mendelian Genetics