Genetic inheritance is the fundamental process by which characteristics are passed from parents to offspring. Understanding these patterns of heredity is central to comprehending the diversity and continuity of life forms on Earth.
The Foundations of Mendelian Inheritance
The principles of Mendelian inheritance, often referred to as “simple dominance,” provide a baseline for understanding how traits are passed down. Gregor Mendel, through his pioneering experiments with pea plants in the 19th century, established these foundational laws. He observed that certain traits appeared to mask others, leading to the concept of dominant and recessive alleles.
Alleles are different versions of a gene, and an organism inherits two alleles for each gene, one from each parent. A dominant allele expresses its trait even when only one copy is present, effectively masking the effect of a recessive allele. A recessive allele, conversely, only expresses its trait when two copies are inherited, meaning the individual is homozygous for that allele.
The genetic makeup of an organism, the combination of alleles it possesses for a trait, is known as its genotype. The observable characteristics or physical expression of these alleles is termed the phenotype. For instance, in pea plants, if ‘T’ represents the dominant allele for tallness and ‘t’ for dwarfness, a plant with genotype ‘TT’ or ‘Tt’ will be tall. Only a ‘tt’ genotype results in a dwarf phenotype.
When Mendel crossed two true-breeding parent plants, one tall (TT) and one dwarf (tt), all offspring in the first filial (F1) generation were tall (Tt). Upon self-pollinating these F1 tall plants, the second filial (F2) generation consistently showed a phenotypic ratio of approximately three tall plants to one dwarf plant. This classic 3:1 ratio in the F2 generation is a hallmark of Mendelian monohybrid crosses, demonstrating the segregation of alleles during gamete formation and their random recombination.
When Genes Blend: Incomplete Dominance
Incomplete dominance represents a deviation from Mendelian patterns, where the heterozygous phenotype appears as an intermediate blend of the two homozygous phenotypes. In this inheritance pattern, neither allele fully masks the other, resulting in a new, distinct phenotype in the heterozygote.
A classic example is snapdragon flower color. When a true-breeding red (RR) is crossed with a true-breeding white (WW), the resulting F1 generation produces pink offspring (RW). The red and white alleles do not exhibit complete dominance, and the red pigment appears diluted in the heterozygote.
If these pink F1 snapdragons are then self-pollinated, the F2 generation will display a phenotypic ratio of 1 red: 2 pink: 1 white. This 1:2:1 phenotypic ratio is characteristic of incomplete dominance, differing from the 3:1 ratio seen in Mendelian inheritance. It underscores that the heterozygous genotype produces a unique intermediate phenotype, rather than resembling one of the homozygous parents.
When Both Genes Show: Codominance
Codominance is another distinct inheritance pattern where both alleles are fully and simultaneously expressed in the heterozygous individual. Unlike incomplete dominance, there is no blending of traits; instead, both phenotypes are distinctly visible. The term “co-” signifies that both alleles are working together, or equally, to contribute to the phenotype.
A prime example of codominance in humans is the ABO blood group system. The A and B alleles are codominant, while the O allele is recessive. An individual inheriting both an A allele and a B allele will have AB blood type, meaning both A and B antigens are present on the surface of their red blood cells. Both alleles are expressed equally, not blended into a new type.
Another illustration of codominance is observed in roan cattle. When a red-coated cattle is crossed with a white-coated cattle, their offspring can have a roan coat. This roan appearance is not a blend of red and white to make pink, but rather a mixture of individual red hairs and individual white hairs, both distinctly visible. This simultaneous expression of both parental traits in the heterozygote is a hallmark of codominance.
Comparing Inheritance Patterns
The three inheritance patterns—Mendelian dominance, incomplete dominance, and codominance—differ fundamentally in how alleles interact in heterozygous individuals.
In Mendelian dominance, one allele completely masks the presence of the other. For example, a tall pea plant with one tall and one dwarf allele will be indistinguishable from a plant with two tall alleles.
In incomplete dominance, the interaction results in an intermediate phenotype in heterozygotes. Neither allele is fully dominant, leading to a blended appearance. Here, the heterozygous phenotype is distinct from either homozygous parent.
Codominance, however, involves the full and simultaneous expression of both alleles in the heterozygote. Instead of blending, both traits are distinctly visible. The AB blood type, where both A and B antigens are present, and roan cattle, with both red and white hairs, exemplify this pattern.
The phenotypic ratios also vary: Mendelian monohybrid crosses yield a 3:1 phenotypic ratio in the F2 generation, while incomplete dominance and codominance typically result in a 1:2:1 phenotypic ratio for the F2 generation, reflecting the unique heterozygous phenotype.