Heredity is the biological process by which parents pass genetic information to their offspring, shaping everything from eye color and blood type to disease risk and body structure. At its core, heredity works through DNA, a molecule that stores instructions in a chemical code and copies itself each time a cell divides. But the full picture is richer than that single sentence suggests. Some traits follow clean, predictable inheritance patterns, while others emerge from the combined influence of hundreds or thousands of genes interacting with the environment. And some heritable information travels outside of DNA altogether.
How DNA Carries and Copies the Blueprint
Your body contains roughly 20,000 protein-coding genes, each a stretch of DNA that provides instructions for building a specific molecule. When cells divide, the entire genome must be duplicated so that each new cell gets a complete set of instructions. This copying process is remarkably accurate. Cells rely on specialized molecular machinery to replicate their DNA with extraordinary precision, avoiding harmful errors that could otherwise lead to disease.1PubMed Central. DNA replication fidelity and cancer
One reason for that accuracy is a built-in error-correction system called proofreading. The enzyme responsible for copying DNA can detect when it has inserted the wrong building block. When that happens, copying stalls, and the enzyme switches to a different mode that snips out the mistake before resuming. The balance between extending the new DNA strand and removing incorrect additions is what keeps the error rate vanishingly low.2Molecular Cell. What Is Heredity? The Biology of Inherited Traits – Section: Proofreading Errors still slip through occasionally, and those mutations are the raw material for evolution. But the proofreading system ensures that the vast majority of DNA is faithfully transmitted from one generation to the next.
Mendel and the Simplest Inheritance Patterns
The modern understanding of heredity traces back to Gregor Mendel’s experiments with pea plants in the 1860s. By tracking seven characteristics across generations of peas, Mendel worked out two foundational principles. The first, segregation, says that each parent carries two copies of each gene and passes only one to any given offspring. The second, independent assortment, says that different genes are typically passed along independently of one another.3Plant Physiology. Mendel: From genes to genome
Mendel’s framework works beautifully for traits controlled by a single gene with two clearly distinct versions. Conditions like sickle cell disease and cystic fibrosis follow this pattern: you either inherit the disease-causing variant or you don’t, and the outcome is relatively predictable from the parents’ genetic makeup. These are sometimes called Mendelian traits, and they are the examples most people encounter in school biology. The trouble is, they are also the exception rather than the rule.
Why Most Traits Do Not Follow Simple Rules
Height, skin color, blood pressure, intelligence, susceptibility to heart disease: the traits people care about most tend to be influenced not by one gene but by many. This concept, called polygenic inheritance, was formalized in 1918 when the statistician R.A. Fisher showed that if many genes each contribute a small effect to a trait, the result in a population is a smooth, bell-shaped distribution rather than the sharp either/or categories Mendel observed in peas.4PubMed Central. Polygenic inheritance, GWAS, polygenic risk scores, and the search for functional variants As the number of contributing genes grows very large, each gene’s individual contribution becomes correspondingly tiny.5Cell. An Omnigenic Model of Complex Traits
This is why two tall parents sometimes have a shorter-than-expected child, or why siblings can look strikingly different from each other despite sharing the same parents. Each child inherits a different random assortment of many small-effect genetic variants, and those different combinations produce a range of outcomes. The genes involved are each inherited in the same Mendelian way, passed along one copy at a time, but because there are so many of them, the aggregate result looks nothing like the crisp one-gene patterns from a textbook.6Biological Reviews. POLYGENIC INHERITANCE and NATURAL SELECTION
Polygenic traits are also harder to study because the contribution of any single gene is too small to notice without very large datasets. That difficulty has been overcome in recent decades through genome-wide association studies, which scan the DNA of hundreds of thousands of people to find the small genetic variants associated with a given trait. But even these massive studies typically explain only a fraction of the variation they set out to capture, a shortfall sometimes called “missing heritability.”
Incomplete Dominance and Blending Effects
Mendel’s pea experiments featured traits where one version of a gene was clearly dominant and the other recessive: a plant was tall or short, a seed was smooth or wrinkled. But many genes don’t work that way. In incomplete dominance, having one copy of each variant produces an intermediate outcome rather than a clean winner. Red and white snapdragon flowers produce pink offspring, for instance, because neither color variant fully overrides the other.
Research in plants confirms that incomplete dominance is widespread and matters for complex traits. A large-scale study of gene expression in maize hybrids found that roughly a third of analyzed genes showed expression levels that deviated from a simple average of the two parent strains, with most of those deviations representing partial or incomplete dominance rather than one variant fully dominating the other.7PubMed Central. The incomplete dominance of gene expression controlled by Trans-eQTL hotspots contributes to heterosis in maize Similar patterns appear in fish: studies of triploid cyprinids (a group that includes carp) have documented incomplete dominance in gene expression inherited across generations.8DNA Research. Combined effects of dosage compensation and incomplete dominance on gene expression in triploid cyprinids The broader lesson is that “dominant” and “recessive” are not the only modes, and for many genes the reality falls somewhere between the two.
Mitochondrial DNA Follows Its Own Rules
Not all of your DNA lives in the cell nucleus. Mitochondria, the tiny structures that generate energy inside cells, carry their own small genome. And unlike nuclear DNA, which is a roughly equal mix from both parents, mitochondrial DNA comes almost exclusively from your mother. Sperm do carry mitochondria into the egg at fertilization, but the paternal mitochondrial DNA is actively destroyed and never passed to the offspring.9PubMed. Maternal inheritance of mitochondrial DNA by diverse mechanisms to eliminate paternal mitochondrial DNA
This was established decades ago through studies of human families. In every family where researchers could detect a clear difference in mitochondrial DNA between the two parents, the children always carried the mother’s version.10PubMed Central. Maternal inheritance of human mitochondrial DNA Because mitochondrial DNA is passed along a strictly maternal line, it is useful for tracing ancestry through mothers. It is also relevant medically: mitochondrial diseases, which affect energy production in cells, are inherited from the mother regardless of the father’s genetic makeup.
The practical consequence is that heredity is not symmetrical between parents. Your father and mother each contribute half of your nuclear DNA, but your mitochondrial genome is entirely your mother’s contribution. If you’re tracing family traits, most follow nuclear inheritance patterns, but anything linked to mitochondrial function follows the maternal line exclusively.
Epigenetic Inheritance Goes Beyond the DNA Sequence
For most of the twentieth century, heredity was treated as synonymous with DNA sequence: you inherit your parents’ genes, full stop. That view has been complicated by the growing field of epigenetics, which studies heritable changes in how genes are used without any change to the DNA letters themselves. Chemical tags on DNA (such as methyl groups) and modifications to the proteins that package DNA can silence or activate genes, and some of these tags can be transmitted from parent to offspring.
The best-understood mechanism is DNA methylation, in which methyl groups attach to specific spots on the DNA strand and turn genes off. But newer research points to small non-coding RNA molecules as another vehicle for passing epigenetic information across generations. Studies show that mature sperm are packed with small RNAs, and these molecules are shaped by the father’s environmental exposures and potentially delivered to the embryo at fertilization, where they can influence early development.11PubMed Central. Paternal Contributions to Offspring Health: Role of Sperm Small RNAs in Intergenerational Transmission of Epigenetic Information
Animal experiments have shown that environmental exposures can trigger epigenetic changes that persist across multiple generations. In one study, exposure to DDT (a pesticide) induced changes across all three major epigenetic systems: DNA methylation, non-coding RNA profiles, and histone modifications (changes to the proteins that DNA wraps around). These alterations were transmitted to offspring who had never been exposed to DDT themselves.12PubMed Central. Alterations in sperm DNA methylation, non-coding RNA and histone retention associate with DDT-induced epigenetic transgenerational inheritance of disease The concept of ancestral intergenerational epigenetic inheritance suggests that events occurring long before conception, affecting parents or even grandparents, can be transmitted to later generations through gametes and the uterine environment.13PubMed Central. Epigenetic Inheritance: Concepts, Mechanisms and Perspectives
One specific form of epigenetic inheritance is genomic imprinting, where a gene’s activity depends on which parent it came from. For most genes, both the maternal and paternal copies are active. But for imprinted genes, only the copy from one specific parent is turned on. This means that for those particular genes, it matters which parent transmitted the variant, not just whether you have it.14PubMed Central. Genomic imprinting and parent-of-origin effects on complex traits Disorders like Angelman syndrome and Prader-Willi syndrome arise from problems with imprinted regions of chromosome 15, and which syndrome develops depends entirely on whether the deletion came from the mother or the father.
How Genes and Environment Work Together
A common misconception about heredity is that genes determine outcomes in a fixed way, that you inherit “the gene for” a trait and that settles the matter. In reality, most traits emerge from the interplay between genetic potential and environmental influences. The concept of a reaction norm captures this: a single set of genes can produce different outcomes across different environments.15PubMed Central. The reaction norm in gene x environment interaction Your height depends on your genes and on whether you got adequate nutrition during childhood. Your risk of type 2 diabetes depends partly on inherited variants and partly on diet, exercise, and body weight.
This is why scientists talk about “heritability” rather than genetic destiny. Heritability is a population-level statistic that describes how much of the variation in a trait, within a specific group of people in a specific environment, can be attributed to genetic differences. Twin studies are one of the classic tools for estimating heritability. They compare identical twins, who share all of their DNA, with fraternal twins, who share about half. If identical twins are more similar for a trait than fraternal twins, that suggests genes play a role.16International Journal of Epidemiology. How to estimate heritability: a guide for genetic epidemiologists
A critical point that many people miss is that high heritability does not mean a trait is unchangeable. Height is one of the most heritable human traits, with estimates above 80% in well-nourished populations, yet average height has increased dramatically over the past century in countries where nutrition improved. The genetic variants did not change; the environment did. Heritability tells you about the sources of variation among people in a particular setting, not about the biological limits of what is possible.
Polygenic Risk Scores and What They Can Tell You
One of the most active areas in modern genetics is the development of polygenic risk scores. These are calculated by adding up the tiny effects of many genetic variants across a person’s genome to produce a single number representing their genetic predisposition to a given trait or disease.17PubMed Central. Tutorial: a guide to performing polygenic risk score analyses The idea is simple in principle: since most diseases are influenced by hundreds or thousands of genetic variants, aggregating all of those small effects should give a useful estimate of someone’s overall risk.
In practice, polygenic risk scores show real promise but also real limitations. They currently explain only a small fraction of total trait variation for most conditions, though they are improving as the datasets behind them grow. For some diseases, the scores are already useful for stratification. In glaucoma, for instance, people in the top tenth of polygenic risk scores were diagnosed on average seven years earlier and were fifteen times more likely to develop advanced disease compared with those in the bottom tenth.18PubMed Central. Genome-wide association studies, Polygenic Risk Scores and Mendelian randomisation: an overview of common genetic epidemiology methods for ophthalmic clinicians Research has also shown that improved statistical methods for constructing these scores can dramatically increase their predictive power, with improvements of anywhere from 3% to over 300% depending on the disease.19PubMed Central. Leveraging effect size distributions to improve polygenic risk scores derived from summary statistics of genome-wide association studies
A significant caveat is that most polygenic risk scores were developed using data from people of European ancestry, and they tend to perform worse in other populations. This is an active equity concern in the field, since the tool is less useful for the very communities that are already underserved by medicine. As more diverse genetic datasets are collected, scores are expected to become more broadly applicable, but that gap remains real right now.
Inheritance Without DNA Changes
Heredity usually brings to mind the passing of genes, but some biologically important information is transmitted between generations through routes that have nothing to do with DNA sequence. One example is horizontal gene transfer, in which genetic material jumps between organisms that are not parent and offspring. This process is routine among bacteria (it is how antibiotic resistance spreads so rapidly), and it also occurs between bacteria and animals. Horizontally acquired genes that end up in an animal’s germline can then be passed to offspring through normal inheritance, blurring the line between “your genes” and “borrowed genes.”20PubMed Central. Horizontal gene transfer between bacteria and animals
Another form of non-genetic inheritance is the vertical transmission of gut microbes from mother to infant. The maternal microbiome serves as a primary source of bacteria for the newborn during and after birth.21PubMed Central. Studying Vertical Microbiome Transmission from Mothers to Infants by Strain-Level Metagenomic Profiling In one study, about 72% of the gut bacteria in newborns at three to seven days old came from their mothers. When mothers received antibiotics around the time of delivery, that figure dropped to roughly 25%, with the remainder colonized by environmental bacteria instead.22PubMed Central. Vertical Transmission of Gut Microbiome and Antimicrobial Resistance Genes in Infants Exposed to Antibiotics at Birth This microbial inheritance matters because the composition of a baby’s gut bacteria influences immune development, digestion, and potentially even long-term disease risk.
Perhaps the most exotic form of non-DNA inheritance involves prions, proteins that can adopt a self-perpetuating shape and pass that shape to other copies of the same protein. In yeast and other fungi, prion-like proteins serve as genuine heritable elements, altering the cell’s behavior through self-templating conformational changes that are transmitted from mother cell to daughter cell through cell division.23PubMed. Prions as protein-based genetic elements Some of these protein-based elements can even be transmitted through meiosis, the specialized cell division that produces sperm and eggs, meaning they behave like genetic elements despite containing no nucleic acid at all.24PubMed Central. Protein-Based Inheritance: Epigenetics beyond the Chromosome
The Germline Barrier and Why Acquired Traits Usually Are Not Inherited
In the late nineteenth century, the biologist August Weismann proposed what is now called the germ plasm theory: the idea that the cells that produce eggs and sperm (the germline) are set aside early in development, walled off from the rest of the body. Under this framework, changes that happen to your muscles, skin, or brain during your lifetime cannot reach the germline and therefore cannot be inherited. A weightlifter’s children are not born with larger muscles.25PubMed. Testing Weismann’s germ plasm theory in Arabidopsis
Weismann’s barrier holds up well in most animals, where germline cells are indeed segregated early in embryonic development. But it is less rigid in plants, which can produce reproductive cells from tissues that have been growing and dividing for years, accumulating mutations along the way. Recent work in Arabidopsis (a common laboratory plant) has explored how this plays out, finding that plants can have dual origins for their germline cells rather than the strict early-segregation model Weismann envisioned.26Current Biology. Dual origins of germline cells in Arabidopsis thaliana Epigenetic inheritance, as described earlier, also challenges the strictest version of Weismann’s theory: environmental exposures can alter chemical tags on DNA in sperm and eggs without changing the DNA sequence itself, creating a loophole that allows some environmentally induced changes to sneak across the germline barrier.
Genome Editing and the Future of Heredity
The development of CRISPR-Cas9 gene editing has raised the prospect of deliberately altering the human germline, making genetic changes that would be passed to future generations. Early experiments have been conducted in non-viable human embryos to test the feasibility. When researchers injected CRISPR-Cas9 into early-stage embryos targeting the hemoglobin β gene (linked to blood disorders), only about 4 out of 71 embryos contained the desired change. Attempts at a different gene locus achieved editing in about 7-14% of embryos, depending on the approach.27Cell. What Is Heredity? The Biology of Inherited Traits – Section: DNA double-strand breaks allow genome editing in human embryos
Those numbers highlight how far the technology remains from routine clinical use. Even when editing works, off-target changes (unintended edits elsewhere in the genome) remain a concern. And the ethical questions are enormous: editing an embryo’s germline means every cell in the resulting person, and potentially their children and grandchildren, would carry the change. Most scientific bodies currently recommend against heritable human genome editing for clinical purposes, though laboratory research continues. The line between understanding heredity and rewriting it is thinner than it has ever been, and where society draws that line is as much a question of ethics and governance as it is of biology.