Genetic inheritance is the process by which biological information passes from parents to their children. It helps explain why children resemble their parents while also having their own combinations of traits.
At the center of inheritance are genes, segments of DNA that contain instructions used by cells. Most people have two copies of nearly every gene—one inherited from their mother and one from their father. The particular versions of those genes, along with how genes interact with one another and with the environment, influence traits such as blood type, eye color, height, and susceptibility to certain diseases.
Inheritance is not simply a matter of receiving a fixed package of traits. It is a process involving the transmission and reshuffling of genetic material, with occasional changes in DNA that can introduce new genetic variation.
Genes, DNA, and chromosomes
DNA, or deoxyribonucleic acid, is the molecule that stores hereditary information. A gene is a functional segment of DNA. Some genes provide instructions for making proteins, while others help regulate when and where genes are active.
DNA is organized into structures called chromosomes. In humans, most body cells contain 46 chromosomes arranged in 23 pairs. One chromosome in each pair generally comes from the mother and the other from the father.
The 22 pairs known as autosomes are similar in males and females. The remaining pair consists of the sex chromosomes. Typically, females have two X chromosomes, while males have one X and one Y chromosome. The X and Y chromosomes carry different sets of genes, which is why some patterns of inheritance differ between them.
The genes occupying corresponding positions on a pair of chromosomes are called alleles when referring to different versions of the same gene. For example, a gene may have several possible DNA sequences in the human population. An individual can inherit two identical copies or two different versions.
How parents pass genes to their children
The key to sexual reproduction is that parents do not pass their entire chromosome sets directly to their children. Instead, specialized cells called eggs and sperm are produced with one set of chromosomes rather than two.
This process, called meiosis, reduces the chromosome number by half. Each egg or sperm therefore normally contains 23 chromosomes. When an egg and sperm fuse during fertilization, their chromosomes combine to produce a new cell with 46 chromosomes.
This means a child receives approximately half of their nuclear DNA from each biological parent. But the inherited chromosomes are not exact copies of the chromosomes that either parent received from their own parents. Meiosis reshuffles genetic material before it is passed on.
That reshuffling is one reason full siblings can be genetically different from one another. They inherit DNA from the same parents, but each egg or sperm contains a different combination of the parents’ genetic material.
Why children resemble their parents but are not identical
During meiosis, matching chromosomes can exchange corresponding sections of DNA through a process called crossing over or recombination. Chromosomes are also distributed into reproductive cells in different combinations.
As a result, the genetic material passed from a parent to a child is a mixture of DNA inherited by that parent from previous generations.
This creates enormous genetic diversity. Even though close relatives share substantial amounts of DNA, the exact combination of variants inherited by each person is generally unique.
Identical twins are a special case. They develop when a single fertilized egg separates into two embryos. Because they originate from the same fertilized egg, they begin with essentially the same nuclear DNA, although genetic and environmental differences can emerge during development and throughout life.
Dominant and recessive inheritance
A common introduction to genetics uses the terms dominant and recessive. These concepts describe particular relationships between genetic variants, but they do not mean that a dominant allele is stronger, better, or more common.
In a simple dominant pattern, one copy of a particular variant can be sufficient to produce an associated trait. In a simple recessive pattern, the trait generally appears only when a person inherits the relevant variant from both parents.
For example, if a genetic condition is caused by a recessive variant, a person with one copy may be an unaffected carrier. A carrier can pass the variant to a child without having the condition themselves.
Real human genetics is often more complicated than these textbook examples. Many traits are influenced by multiple genes, and the effect of a genetic variant can depend on other variants, biological processes, and environmental factors.
Not all traits follow simple inheritance patterns
Some genetic traits follow relatively straightforward inheritance patterns, but many do not.
Codominance occurs when two different alleles are both expressed in a recognizable way. Human blood type provides a classic example: the A and B alleles of the ABO blood-group system can both be expressed in a person who inherits one of each.
Incomplete dominance occurs when the combined genetic effects produce a phenotype that is intermediate between two other forms, although the exact biological mechanism varies among traits.
Many characteristics are polygenic, meaning they are influenced by numerous genes. Height, for example, depends on the combined effects of many genetic variants as well as factors such as nutrition and health during development. Traits like these do not usually fit neatly into categories such as dominant or recessive.
Genes can also interact with one another. A variant in one gene may affect how another gene’s effects appear. This makes the relationship between DNA and observable traits more complex than a one-gene, one-trait model suggests.
What mutations contribute to inheritance?
A mutation is a change in DNA sequence. Mutations can arise from errors during DNA copying or from other sources of DNA damage.
Most mutations do not have a major effect on an individual’s observable characteristics. Some have harmful effects, some can be beneficial under particular circumstances, and many are effectively neutral.
For a DNA change to be passed from a parent to a child, it generally must occur in a cell that contributes to reproduction, such as an egg or sperm, or in the cell lineage that produces those cells. Changes that occur only in ordinary body cells are generally not inherited by offspring.
Inherited genetic variants are one source of the differences found among people. New mutations can introduce additional variation into populations, while recombination and the reshuffling of chromosomes create new combinations of variants.
How genetic inheritance relates to genetic conditions
A genetic condition can result from a change in a single gene, changes involving multiple genes, chromosome abnormalities, or interactions between genetic and environmental factors.
Some conditions have recognizable inheritance patterns. A condition associated with an autosomal dominant variant can occur when a person inherits one disease-associated copy of the gene. An autosomal recessive condition generally requires two disease-associated copies. Other conditions are associated with variants on the X chromosome or arise from changes in mitochondrial DNA.
A person’s genetic inheritance can therefore influence the probability of developing a condition without always determining what will happen. Some variants substantially increase risk without guaranteeing disease, while others have effects that depend on additional genetic or environmental factors.
What role does the environment play?
Genes provide biological information, but they do not operate in isolation. Characteristics arise through interactions among genes, development, and the environment.
Nutrition, physical activity, exposure to certain substances, infections, stress, and many other environmental factors can influence biological traits. The relative contribution of genes and environment differs substantially from one trait to another.
Even for a strongly heritable trait, heritability does not mean that a particular individual’s trait is determined by genes to a specific percentage. Heritability is a statistical measure describing how much variation in a trait within a particular population can be associated with genetic differences. It is not a measure of how “genetic” an individual person is.
What happens across generations?
Inheritance can be traced through families because genetic variants can be transmitted from parents to children and, in turn, to later generations.
A child receives one chromosome of each pair from each parent, but each parent carries genetic material from both of their own parents. Through recombination, the DNA passed to a child is a new mixture of ancestral genetic material.
This is why a genetic variant can sometimes appear to skip generations in a family. In a recessive inheritance pattern, for instance, two people who do not have a condition can each carry a recessive variant and pass both copies to a child.
At the population level, inheritance is one of the mechanisms through which genetic variation persists, changes in frequency, and contributes to evolution over generations. Natural selection, genetic drift, mutation, and migration can all alter the distribution of genetic variants in populations.
The basic idea in one chain
Genetic inheritance can be understood as a sequence of events:
DNA contains genes → genes exist in different variants → chromosomes carry those genes → meiosis reshuffles and separates chromosomes into eggs or sperm → fertilization combines genetic material from two parents → the resulting genome influences development and traits in interaction with the environment.
The important point is that inheritance transfers genetic information, not a predetermined blueprint for every aspect of a person’s life. Genes influence how cells function and how organisms develop, but traits emerge from a dynamic biological system in which genes, other genes, development, and environment continually interact.
