Non-Mendelian inheritance describes any pattern where traits do not follow the predictable ratios and rules established by Gregor Mendel’s foundational work. Mendel’s experiments established the Law of Segregation (an individual possesses two alleles for a trait and passes only one to the offspring) and the Law of Independent Assortment (alleles for different traits are passed on independently). These laws rely on simple dominance, where one allele completely masks another. While Mendel’s findings are the bedrock of genetics, most traits observed in nature are more complicated, often involving multiple genes, partial dominance, or genes located outside of the main chromosomes. This complexity means that inheritance frequently deviates from the simple ratios Mendel observed.
When Alleles Blend or Share Power
The simplest deviations occur when the interaction between the two alleles of a single gene is not one of complete dominance. In Incomplete Dominance, the heterozygous offspring displays a phenotype that is an intermediate blend of the two parental phenotypes. A classic example is the snapdragon flower, where a cross between a red-flowered plant and a white-flowered plant produces offspring with pink flowers. Neither the red nor the white allele is fully dominant, resulting in a blended color.
In contrast to blending, Codominance occurs when both alleles are fully and simultaneously expressed in the heterozygote. This means that the trait of each parent is visible at the same time. The human ABO blood group system provides a clear example of codominance, specifically involving the A and B alleles. An individual who inherits both the A allele and the B allele will have type AB blood, meaning their red blood cells express both A antigens and B antigens simultaneously.
Multiple Genes Working Together
Inheritance patterns become more complex when a single trait is influenced by two or more distinct genes. Polygenic Inheritance involves multiple genes contributing small, cumulative effects to a single characteristic, resulting in a continuous spectrum of phenotypes rather than clear-cut categories. Human height, skin color, and eye color are examples of polygenic traits, where many genes interact to determine the overall stature.
These traits are often called quantitative traits because they can be measured and vary gradually across a population, such as the range from light to dark skin tones. Another type of complex gene interaction is Epistasis, where the expression of one gene masks or modifies the effect of a completely different gene. A well-known illustration is coat color in Labrador retrievers. One gene determines the color pigment (black or brown), but a separate gene controls whether that pigment is deposited into the hair shaft. If the deposition gene is non-functional, the dog will have a yellow coat regardless of the color determined by the first gene, because the second gene has shut down the color expression pathway.
Inheritance Outside the Standard Chromosomes
Some inheritance patterns deviate because the genes are located on chromosomes that do not follow standard segregation. Sex-Linked Inheritance involves genes located on the sex chromosomes, primarily the X chromosome, leading to distinct patterns between males and females. Since males possess one X and one Y chromosome, they are hemizygous for X-linked traits, meaning they only have a single copy of those genes. A recessive gene on the X chromosome, such as the one causing red-green color blindness, will always be expressed in a male because there is no second X chromosome to mask it.
A different pattern, known as Extranuclear Inheritance, involves genetic material found outside the cell nucleus in organelles like mitochondria. Mitochondria contain their own small, circular DNA (mtDNA). In humans, mtDNA is inherited almost exclusively from the mother, as the sperm’s mitochondria are generally not transferred to the egg during fertilization. This purely maternal transmission bypasses Mendelian segregation entirely, meaning all offspring of a mother carrying a mitochondrial mutation will inherit the trait, regardless of the father’s genes.
The Role of Environment in Gene Expression
The final phenotype, or observable trait, is not solely determined by the inherited DNA sequence but also by the surrounding environment. Environmental factors significantly influence how genes are expressed, a concept often studied through epigenetics. Epigenetic modifications are chemical tags placed on DNA or its associated proteins that can turn genes “on” or “off” without altering the underlying genetic code.
Factors such as diet, temperature, and exposure to chemicals can induce these epigenetic changes, leading to phenotypic results that defy simple genetic predictions. For example, the coat color pattern of Siamese cats is temperature-sensitive; the gene for dark pigment is only expressed in cooler areas of the body, resulting in darker fur on the ears, paws, and tail. In humans, genetic potential for height can be influenced by nutrition during childhood, demonstrating how environment interacts with polygenic inheritance to shape the final trait.