Heredity is the transmission of traits from parents to offspring through genetic information. Every living organism, from bacteria to humans, passes along a biological blueprint that determines structure, function, and many observable characteristics. This blueprint is encoded in DNA, organized into units called genes, and packaged into structures called chromosomes. Humans carry about 19,400 protein-coding genes spread across 23 pairs of chromosomes.
How Heredity Works at the Molecular Level
Your DNA is a long molecule that stores instructions for building and running your body. Specific sections of that molecule are genes, each carrying the code for a particular protein or function. Chromosomes are the physical structures that carry genes, and they come in pairs: one from each parent. When you inherit a chromosome from your mother and the matching one from your father, you end up with two copies of each gene.
These two copies don’t always match. The different versions of a gene are called alleles. Which alleles you carry is your genotype, the invisible genetic code. What actually shows up in your body, your observable characteristics, is your phenotype. The distinction matters because phenotype isn’t determined by genes alone. Flamingos, for example, don’t carry a gene for pink feathers. Their pink color comes entirely from their diet. Without the right food, they’re white.
Dominant, Recessive, and In-Between
The simplest pattern of heredity follows rules first described by Gregor Mendel in the 1800s. He discovered that traits are determined by pairs of inherited factors (now called genes) and that some versions override others. A dominant allele produces its effect even when only one copy is present. A recessive allele only shows up when you carry two copies, one from each parent.
But inheritance isn’t always that clean. In incomplete dominance, two different alleles blend into an intermediate result. Snapdragons show this clearly: a plant with one red-flower allele and one white-flower allele produces pink flowers, splitting the difference. Codominance is different still. Instead of blending, both alleles express fully. If you have one allele for type A blood and one for type B, your blood cells display both A and B sugars on their surface, giving you type AB blood.
Polygenic Traits
Many of the traits people are most curious about, like height, skin color, and eye color, don’t follow simple one-gene rules. These are polygenic traits, meaning they’re shaped by two or more genes working together. Because so many genes contribute, these traits show a wide spectrum of outcomes rather than a few distinct categories. Your height, for instance, isn’t “tall” or “short” based on a single gene. It reflects the combined influence of hundreds of genetic variants, plus nutrition, health, and other environmental factors.
This same complexity applies to major health conditions. Cancer, heart disease, and diabetes all have genetic components, but those components are spread across multiple genes. That’s why having a family history of heart disease raises your risk without guaranteeing you’ll develop it. The genetic piece is real but not the whole picture.
Hereditary Health Conditions
Some diseases follow much more predictable inheritance patterns. These fall into a few categories based on where the relevant gene sits and whether one or two copies of the allele cause problems.
- Autosomal dominant: Only one copy of the altered gene is needed to cause the condition. If a parent has it, each child has a roughly 50% chance of inheriting it. Huntington’s disease and Marfan syndrome follow this pattern.
- Autosomal recessive: Two copies of the altered gene are required, one from each parent. Parents can be carriers without symptoms. Cystic fibrosis and sickle cell disease are examples.
- X-linked recessive: The gene sits on the X chromosome. Because males have only one X, a single altered copy causes the condition in boys, while girls with one altered copy are typically carriers. Hemophilia follows this pattern.
- Mitochondrial: Some genetic material lives outside the cell’s nucleus, in structures called mitochondria. These are inherited exclusively from the mother. Leber hereditary optic neuropathy, which causes vision loss, is one example.
Genes vs. Environment
Heredity sets the stage, but environment shapes the performance. Your genes don’t change throughout your life, but which genes are active can shift dramatically. This is the field of epigenetics. Your behaviors and surroundings can cause chemical changes to your DNA that turn genes on or off without altering the genetic code itself. One common mechanism involves small chemical tags attaching to DNA. When these tags accumulate on a gene, they silence it. When they’re removed, the gene becomes active again.
This means two people with identical DNA (like identical twins) can develop different traits or different disease risks over time, depending on diet, stress, chemical exposures, and other environmental factors. These epigenetic changes are reversible, unlike mutations, which permanently alter the DNA sequence. Your body is constantly adjusting which genes are expressed and at what level, responding to the world around you.
Heredity vs. Genetics
People often use these terms interchangeably, but they refer to different things. Heredity is the process itself: the transmission of traits from parents to offspring. Genetics is the scientific study of that process, focusing on the units of inheritance and the variation between individuals. Heredity is the phenomenon. Genetics is the discipline that investigates it. When someone says “it runs in the family,” they’re describing heredity. When a researcher maps the genes responsible, they’re doing genetics.