Non-Mendelian inheritance describes patterns of how traits are passed down that do not follow the simpler dominant and recessive rules Gregor Mendel observed in pea plants. These variations reveal a greater complexity in how genetic information is expressed and inherited across generations. Understanding these diverse mechanisms helps explain the vast array of characteristics seen in living organisms, from flower colors to human health conditions.
When Traits Blend or Both Appear
Some genetic interactions at a single gene locus do not result in a clear dominant or recessive outcome. Incomplete dominance occurs when neither allele completely masks the other, resulting in a blended or intermediate physical characteristic in the offspring. For example, when a red snapdragon flower (carrying two alleles for red pigment) is crossed with a white snapdragon (carrying two alleles for no pigment), the resulting offspring display pink flowers. The pink color arises because the single red allele in the heterozygote produces half the pigment of a homozygous red plant.
Codominance presents another scenario where both alleles are fully and distinctly expressed in the heterozygote, without blending. The ABO blood group system in humans is a well-known example, specifically for individuals with AB blood type. Both the A and B alleles are simultaneously expressed on the surface of red blood cells, producing both A and B antigens. Roan cattle provide another illustration; a cross between a red cow and a white bull produces offspring with both red and white hairs interspersed throughout their coat.
The ABO blood group system also demonstrates the concept of multiple alleles, where more than two possible alleles exist for a single gene within a population. While an individual organism carries only two alleles for any given gene, a population may have several variations. For instance, the ABO blood type gene has three main alleles: I^A, I^B, and i. These combine to produce the four common blood types: A, B, AB, and O.
Multiple Genes Working Together
Traits can arise from the combined influence of several genes, rather than just one. Polygenic inheritance involves multiple genes contributing to a single trait, often leading to a continuous spectrum of characteristics. Human height is a classic example, influenced by hundreds of genes, each contributing a small additive effect. Skin and eye color are also polygenic traits, displaying a wide range of shades due to the interplay of numerous genes. This cumulative effect means there isn’t a simple “tall” or “short” category, but a gradual variation across the population.
Another complex interaction between different genes is epistasis, where the expression of one gene modifies or masks the expression of another gene at a different location on a chromosome. Labrador retriever coat color provides a clear example of epistasis. One gene determines pigment type (black or brown), while a separate gene dictates pigment deposition into the fur. A dog may inherit alleles for black pigment, but if it also inherits two recessive alleles for pigment deposition, its coat will be yellow, regardless of the pigment production gene. The gene for pigment deposition overrides the expression of the pigment color gene.
Inheritance from Specific Parents or Chromosomes
Some inheritance patterns are uniquely tied to specific chromosomes or non-nuclear DNA. Sex-linked inheritance refers to traits determined by genes located on the sex chromosomes, typically the X or Y chromosome in humans. Since females have two X chromosomes (XX) and males have one X and one Y chromosome (XY), the inheritance patterns often differ between the sexes. Red-green color blindness, for instance, is an X-linked recessive trait. A female must inherit two copies of the recessive allele (one on each X chromosome) to be colorblind, whereas a male only needs one copy of the recessive allele on his single X chromosome to express the trait. Hemophilia, a bleeding disorder, also follows an X-linked recessive pattern, making it more prevalent in males.
Mitochondrial inheritance involves traits determined by genes found within the mitochondria, which are cellular organelles with their own small circular DNA. Unlike nuclear DNA, mitochondrial DNA is almost exclusively inherited from the mother. If a mother has a mitochondrial disorder, all of her children are at risk of inheriting the condition, although the severity can vary. Fathers do not pass mitochondrial traits to their offspring.