Children inherit their genetic information from their biological parents. That inheritance helps determine many characteristics, from blood type and natural hair texture to aspects of height, eye color, and susceptibility to certain diseases. But inheritance is not a simple process of copying a parent’s traits. Each child receives a unique combination of genetic material, and environmental factors also influence how many traits develop.
The basic unit of inheritance is the gene, a segment of DNA that contains instructions used by cells. Genes are arranged along structures called chromosomes, which are found in the nucleus of most cells.
In humans, most body cells contain 46 chromosomes arranged in 23 pairs. One chromosome in each pair usually comes from the biological mother and the other from the biological father. The exception is egg and sperm cells, which contain 23 chromosomes each. When an egg and sperm combine at fertilization, the resulting embryo receives a complete set of 46 chromosomes.
DNA carries the instructions for inherited traits
DNA, short for deoxyribonucleic acid, is the molecule that stores genetic information. Its sequence of chemical building blocks provides instructions for making proteins and regulating biological processes.
A gene can have different versions, called alleles. For example, a particular gene may have alleles that influence different forms of a biological characteristic. A person typically inherits one allele from each biological parent for genes located on paired chromosomes.
The combination of alleles a person carries is part of their genotype. The observable characteristics that result from genetic information, together with developmental and environmental influences, are called the phenotype.
This distinction matters because possessing a particular genetic variant does not always mean that a trait will appear in a straightforward way. Some traits are strongly influenced by a single gene, while many others depend on numerous genes and environmental conditions.
Why children are genetically different from their parents and siblings
If children simply received identical copies of their parents’ chromosomes, siblings would be genetically much more alike than they actually are. The production of eggs and sperm introduces extensive genetic variation.
During meiosis, the specialized cell division that produces eggs and sperm, chromosome pairs separate so that each reproductive cell receives only one chromosome from each pair. Before the chromosomes separate, corresponding chromosomes can exchange sections of DNA in a process called crossing over or genetic recombination.
As a result, the chromosomes in an egg or sperm are mixtures of genetic material inherited by that parent from their own parents. Which chromosomes and DNA segments end up together varies from one reproductive cell to another.
Fertilization adds another layer of randomness. Any one of a person’s eggs or sperm may participate in conception, so the combination inherited by one child is different from the combination inherited by another.
This is why siblings can resemble one another strongly while still having different combinations of genetic variants.
Dominant and recessive inheritance
Some genetic traits follow relatively simple inheritance patterns. One familiar pattern involves dominant and recessive alleles.
A dominant allele can influence a trait when a person has just one copy of it. A recessive allele generally affects the phenotype only when a person has two copies, one inherited from each parent.
Suppose a gene has two alleles, represented simply as A and a. A person could inherit AA, Aa, or aa. If A is dominant and a is recessive, AA and Aa may produce the same observable phenotype, while aa produces a different one.
A parent carrying one dominant and one recessive allele can pass either allele to a child. If both parents carry a recessive allele, their child may inherit two recessive copies.
This pattern is important for understanding certain inherited conditions, but it does not describe most human traits. Traits such as height, skin pigmentation, and many aspects of facial appearance are influenced by multiple genes and do not fit a simple dominant-versus-recessive model.
Not all alleles are simply dominant or recessive
Genetic inheritance can involve several other patterns.
With codominance, two different alleles can both contribute to the phenotype. The ABO blood group system provides a classic example: the A and B alleles are both expressed when a person inherits one of each, producing type AB blood.
With incomplete dominance, neither allele completely determines the phenotype when paired with another allele, resulting in an intermediate or otherwise distinct phenotype.
Some genes also have more than two common alleles in a population, although an individual still generally carries only two alleles for an autosomal gene.
These patterns illustrate why predicting a child’s characteristics from the parents’ appearances alone can be misleading.
Many traits are controlled by many genes
For a large number of human characteristics, there is no single gene that determines the outcome. Instead, many genetic variants contribute small or varying effects.
Polygenic traits are traits influenced by multiple genes. Height is one example. A person’s eventual height reflects the combined effects of many genetic variants as well as factors such as nutrition and health during development.
Other characteristics can involve both multiple genes and substantial environmental influence. Body weight, for example, is affected by numerous biological factors as well as diet, physical activity, sleep, medications, socioeconomic circumstances, and other aspects of a person’s environment.
This means that inheritance often affects probabilities and biological tendencies rather than producing a predetermined outcome.
Some genes affect how other genes are expressed
Genes do not operate independently. Variants in one gene can influence biological pathways that affect the activity or effects of other genes.
Cells also regulate which genes are active and when. Gene expression refers to the process by which information in a gene is used by a cell, often to produce a functional RNA molecule or protein.
Environmental conditions and cellular signals can alter gene expression without changing the underlying DNA sequence. Chemical modifications to DNA and associated proteins are part of a broader system called epigenetic regulation.
Some epigenetic patterns can be influenced by conditions in the parent’s body and, in particular circumstances, certain molecular marks can persist through reproduction. However, this does not mean that ordinary experiences or acquired characteristics are routinely encoded into DNA and passed directly to children. The vast majority of inherited genetic information is transmitted through DNA in egg and sperm cells.
How sex chromosomes are inherited
One of the 23 chromosome pairs is the sex chromosome pair. In the typical human chromosome system, females have two X chromosomes, while males have one X and one Y chromosome.
Egg cells normally carry an X chromosome. Sperm cells carry either an X or a Y chromosome. If an X-bearing sperm fertilizes the egg, the resulting chromosome combination is typically XX; if a Y-bearing sperm fertilizes it, the combination is typically XY.
The X and Y chromosomes carry different sets of genes, so some inherited conditions have distinctive patterns of transmission. For example, an X-linked recessive condition can occur more readily in people with only one X chromosome because there is no second X chromosome carrying another copy of the gene that might compensate for the variant.
Not every aspect of biological sex is determined solely by the XX or XY chromosome pattern, and variations in chromosomes, genes, hormones, and development can produce other biological patterns.
Mitochondrial DNA follows a different inheritance pattern
Most of a cell’s DNA is located in the nucleus, but mitochondria, the structures that help cells produce energy, contain their own small amount of DNA.
Mitochondrial DNA is generally inherited through the egg, so children typically receive their mitochondrial DNA from their biological mother rather than their biological father. This is different from nuclear DNA, which is inherited from both parents.
Mitochondrial inheritance is relevant to certain genetic conditions caused by variants in mitochondrial DNA.
New genetic variants can arise
A child does not necessarily inherit every genetic variant unchanged from a parent. DNA can acquire changes, known as mutations or genetic variants, as cells divide.
Some variants arise in reproductive cells and can therefore be passed to a child. Others occur after fertilization during the individual’s development and may be present only in certain cells or tissues.
A new variant is not automatically harmful. Genetic changes can have harmful, beneficial, or neutral effects, and the effect depends on the gene, the specific change, and the biological context.
Genetic variation is also a normal part of human reproduction. It contributes to the differences among individuals within a population.
What does a child actually inherit?
A child inherits far more than visible physical traits. Genetic information contributes to the development and function of virtually every part of the body.
Inherited genetic differences can influence:
- Blood type
- Certain aspects of physical appearance
- How the body processes particular substances
- Some aspects of growth and development
- Susceptibility to particular diseases
- Responses to some medications
- Certain biological characteristics and physiological processes
But genes generally work together with development and the environment. Having a genetic predisposition to a condition does not necessarily mean that a person will develop it, just as lacking a known genetic risk does not guarantee that a person will remain free of a disease.
Some inherited conditions are caused primarily by variants in a single gene. Others result from combinations of genetic variants and environmental factors. Still others are influenced by chromosome abnormalities or changes in mitochondrial DNA.
Why a child can resemble one parent more than the other
A child’s resemblance to a parent depends on which genetic variants the child inherits and how those variants interact.
For a particular gene, a child receives one allele from each biological parent. But the visible effect of those alleles depends on the inheritance pattern. For traits influenced by many genes, the child receives a different mixture of variants at many locations throughout the genome.
Chance therefore plays a major role in which characteristics become more noticeable. A child may inherit combinations that make certain features resemble one parent strongly, while other features resemble the other parent or fall somewhere between them.
A child’s appearance also cannot reveal their entire genetic inheritance. Many inherited differences have no obvious outward effect.
Genetic inheritance is not the same as inheriting a trait directly
Parents pass DNA variants, not finished characteristics.
For example, a parent does not pass a specific height to a child. The child inherits genetic variants that can influence growth, and those variants interact with nutrition, hormones, health, and other developmental factors.
The same principle applies to many complex characteristics. Genetics provides biological information and influences how the body develops and functions, but development occurs within an environment.
Understanding inheritance therefore requires separating three related ideas: the DNA sequence a person inherits, how cells use that genetic information, and the characteristics that ultimately develop. The connection among them is powerful but rarely as simple as one gene producing one visible trait.
At its core, inheritance works through the transmission and reshuffling of DNA. Each biological parent contributes genetic material through reproductive cells, those contributions combine at fertilization, and the resulting genome provides the biological foundation on which a unique individual develops.
