Genetics is the study of heredity, and the field owes much to Gregor Mendel. His experiments with pea plants in the 1860s established that traits were inherited in a predictable fashion, laying the groundwork for understanding how genes operate.
While Mendel’s laws are a starting point for classical genetics, they represent an idealized system. The inheritance of many traits is more complex than the patterns he described. These exceptions, which do not follow Mendelian rules, are known as non-Mendelian genetics, and they reveal a more nuanced picture of how genetic information creates life’s diversity.
The Mendelian Foundation
To understand the exceptions, one must first understand the rules Mendel proposed. His work is built on two primary laws. The first is the Law of Segregation, which posits that for any given trait, an individual has two alleles, with one inherited from each parent. During the formation of reproductive cells (gametes), these two alleles separate so that each gamete receives only one.
This concept explains how traits can seem to skip a generation. For instance, in Mendel’s experiments, when a purple-flowered pea plant was crossed with a white-flowered one, all the offspring had purple flowers. When these offspring were crossed, the white-flower trait reappeared in about a quarter of the next generation. This demonstrated that the allele for white flowers was present but unexpressed in the first generation.
The second principle is the Law of Independent Assortment. This law states that the alleles for different traits are sorted into gametes independently of one another. For example, the allele a plant inherits for flower color does not influence the allele it inherits for seed shape. This independent sorting is a result of how chromosomes align during meiosis.
Beyond Simple Dominance
Mendel’s work centered on complete dominance, where one allele completely masks the effect of a recessive allele. Many genetic interactions are not this straightforward, as the relationship between alleles can result in phenotypes that differ from what simple dominance would predict.
One such pattern is incomplete dominance, where the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. A classic example is seen in snapdragons; a cross between a red-flowered plant and a white-flowered plant produces offspring with pink flowers. This creates a third, distinct phenotype that is a visual blend of the parental traits.
Another variation is codominance, where both alleles are fully and separately expressed in the heterozygous state. In certain chickens, an individual with alleles for both black and white feathers will have plumage with distinct black and white speckles. A similar example in humans is the ABO blood group system, where the A and B alleles are codominant. An individual with one A allele and one B allele will have type AB blood.
Traits with Multiple Genetic Inputs
The complexity of inheritance increases when more than two allele options exist for a single gene or when multiple genes contribute to a single trait. These situations expand the potential for phenotypic variation well beyond the simple scenarios Mendel studied.
Multiple alleles refers to a scenario where three or more alternative forms of a gene exist within a population. Although a population may harbor many alleles for a gene, any given individual can only possess two of them. The human ABO blood group system is a prime example, involving three alleles: IA, IB, and i. These combine to produce the four main blood types (A, B, AB, and O).
In contrast, polygenic inheritance occurs when a single characteristic is controlled by the cumulative effects of two or more different genes. This results in a continuous spectrum of phenotypes. Traits like human height, skin color, and eye color are governed by polygenic inheritance. For instance, human height is influenced by hundreds of genes, each contributing a small amount to the final outcome.
Location-Dependent Inheritance
The physical location of a gene on a chromosome can alter its pattern of inheritance, leading to outcomes that diverge from Mendelian predictions. Genes on sex chromosomes behave differently than those on non-sex chromosomes (autosomes). Additionally, some genetic material is not housed in the cell’s nucleus and follows a unique inheritance route.
Sex-linked inheritance involves traits determined by genes on the sex chromosomes, the X and Y chromosomes in humans. Because males (XY) have only one X chromosome, they are more likely to express recessive traits located on that chromosome. A well-known example is red-green color blindness, an X-linked recessive trait. A female must inherit two copies of the recessive allele to be colorblind, whereas a male only needs to inherit one from his mother to be affected. This is why red-green color blindness is more common in males.
A different form of location-dependent inheritance is mitochondrial inheritance. Mitochondria, the energy-producing organelles in our cells, contain their own small, circular DNA (mtDNA). This DNA is inherited exclusively from the mother because the egg cell provides the cytoplasm and all its organelles to the developing zygote. Sperm contain mitochondria, but they are destroyed after fertilization. As a result, mtDNA is passed down from mother to all of her offspring.