For the first time in the mid‑19th century, scientists began to unravel the mechanisms that govern inheritance and evolution. Charles Darwin’s work on natural selection laid the foundation for modern biology, but it was Gregor Mendel’s pea‑plant experiments that formally defined the principles of heredity.
Mendel introduced the concepts of genes—specific DNA sequences that determine traits—and alleles, the alternative versions of a gene. His observations led to the laws of segregation and independent assortment, explaining how dominant and recessive alleles combine to produce observable phenotypes.
In prokaryotes, asexual reproduction by binary fission creates genetically identical offspring. In contrast, eukaryotes reproduce sexually through mitosis and meiosis, with each gamete carrying half the genetic material. Human gametes—sperm and egg—contribute one allele from each gene, producing diverse genotypes.
Typically, one allele dominates, masking the other in the phenotype. For example, the round‑seed allele (R) dominates the wrinkled allele (r) in peas. A plant with genotype Rr will display round seeds, but can pass the r allele to its progeny.
When a homozygous Rr plant self‑pollinates, the offspring genotypes are RR, Rr, rR, and rr. Only the rr individuals show wrinkled seeds, illustrating the need for two copies of a recessive allele to express its trait.
Genotypes that contain two identical alleles (RR or rr) are homozygous, while those with one of each (Rr) are heterozygous.
Not all genes follow the simple dominant‑recessive model. Two key alternatives are:
Animals with striking patterns, such as zebras and leopards, often exhibit codominance. In peas, a codominant Rr genotype would yield a mixture of smooth and wrinkled peas rather than a blended shape.
Blood types are determined by alleles A, B, and O. A and B are codominant, producing AB blood when both are present. O represents the absence of A or B antigens. Possible genotypes include AA, AO, BB, BO, AB (or BA), and OO.
For instance, a person with type O must inherit an O allele from each parent, though the parents may carry AO, BO, or OO genotypes. Consequently, neither parent can have type AB blood.
While both involve heterozygous phenotypes that differ from either homozygous state, incomplete dominance blends traits into an intermediate form, whereas codominance displays both traits simultaneously.
Polygenic traits, such as height or skin color, involve many genes and form a continuous spectrum, distinct from codominant inheritance.
