Inheritance starts with two copies, not one script
You asked how genetic inheritance works because a relative shares a condition, a test named an allele, or a school chart made it look like a coin flip that decides a life. The coin flip is only a model, and only for some traits.
Think of two copies of a book. For autosomes you receive one copy from each parent. The editions may match or differ. Those editions are alleles. The body that results is not a photocopy of either book.
The real process is meiosis, fertilization, and then a lifetime of reading those copies in an environment. Inheritance is a pattern of transmission. It is not a sentence.
Mendel is a good tool for a small class of traits
Gregor Mendel tracked single-gene traits in peas and found ratios that still teach well: two alleles, dominant and recessive, a 3:1 appearance in some crosses. The model is clean because the traits were clean.
Some human conditions still fit that picture closely enough to be useful: sickle cell disease and CFTR-related cystic fibrosis are textbook autosomal recessive examples. Huntington disease is a textbook autosomal dominant example. Blood groups are taught that way with extra footnotes.
Most common traits do not fit. Height, type 2 diabetes risk, skin color, intelligence headlines, and almost everything sold as a gene for a personality are not Mendelian. Use Mendel where a single gene dominates the story. Put him down for the rest.
- Useful for some single-gene (Mendelian) conditions
- A poor map for height, most common disease risk, and most everyday traits
- Two alleles at a locus is the starting picture, not the whole genome
- A family pattern is a clue, not a diagnosis
Dominant and recessive as patterns, not ranks
Dominant, in this vocabulary, means one altered copy can be enough for the pattern to show. Recessive means both copies usually need to be altered before the pattern shows. Neither word means stronger, better, or more evolved.
A person can carry a recessive allele and have no trait. Two such people can have a child who inherits both copies and does show the trait. That is the inheritance pattern families notice after the fact.
Dominant does not mean inevitable in every person, and recessive does not mean hidden forever. The words describe a transmission pattern for a particular gene-trait pair. They do not grade your genome.
Incomplete penetrance and the gap between genotype and life
Penetrance is the chance that a person with a given genotype shows the related trait. Incomplete penetrance means some people with the genotype never show it. Variable expressivity means those who do show it do not show it the same way.
This is the honest hole in every inheritance chart. Two siblings can share an allele. One becomes ill. One does not. The chart was not lying. It was incomplete. Other genes, chance, age, and environment sit in that hole.
A variant is therefore a pattern, not a destiny. Clinical genetics spends its life in this gap. A webpage that skips it is selling certainty it does not have.
Polygenic traits are the common case
Polygenic means many genes, each usually with a small effect. Add environment and you have most of human variation that people actually notice: height, blood pressure, common lipid levels, many risks that run in families without a single named gene.
A polygenic score is an attempt to sum those small effects from a file. It is a statistical object for a population, with limits of ancestry, assay, and study design. It is not a phenotype and not a diagnosis.
When a company says they found the gene for a complex trait, ask how much of the variation it explains. If the answer is a few percent, you are not looking at Mendel's peas.
X-linked inheritance without the folklore
Genes on the X chromosome do not follow the two-copy autosomal pattern in people with a single X. An allele on that X is present in one copy in typical 46,XY cells, and in two copies in typical 46,XX cells, with one X partly silenced.
That is why some conditions are described as X-linked and appear more often, or more severely, in people with one X. Hemophilia A is the usual classroom example. The folklore that fathers pass nothing on X, or that daughters are only carriers, is too neat.
X-linked is still a pattern, not a destiny. Penetrance, mosaicism, and skewed X inactivation all leak. If a family is making decisions about an X-linked condition, that is clinic work.
Mitochondrial DNA follows a maternal line
Mitochondria carry their own small genome. A child almost always inherits that genome from the person who provided the egg. Sperm mitochondria are usually not passed on. That is a separate inheritance pattern from the 23 nuclear volumes.
Because each cell has many mitochondrial circles, a person can carry a mix of mitochondrial sequences (heteroplasmy). The mix can differ by tissue. That is one reason mitochondrial conditions are hard to read from a slogan.
Nuclear DNA still does most of the work of making a mitochondrion. Maternal inheritance of mtDNA does not mean the rest of metabolism is maternal. Keep the pamphlet and the library distinct.
Recombination shuffles the volumes before they are passed
Before eggs and sperm are finished, homologous chromosomes pair and swap segments. That shuffle is recombination. It is why a child can carry a chromosome 1 that is a mosaic of both grandparents on that side.
Without recombination, whole chromosomes would travel as uncut blocks. With it, the two copies of a book are rebound each generation. Linked genes still travel together more often than distant ones. That is the basis of genetic mapping.
New variants also appear. A child can carry an allele neither parent has in their body cells if it arose in egg or sperm. De novo is the name. Inheritance includes the new, not only the old.
- Autosomal: two copies, one from each parent
- Dominant or recessive: patterns for some single-gene traits
- X-linked: genes on X, unequal copy number by karyotype
- Mitochondrial: almost always the maternal mtDNA line
- Polygenic: many genes plus environment, the usual case
Patterns are not a diagnosis
A pedigree can suggest a pattern. A lab can name an allele. Neither is a diagnosis of a person until a clinician puts the pattern, the assay, and the body in the same room. This page is informational.
genome.sh can look up a public variant or gene without uploading a genome. That tells you what databases currently say about a name. It does not tell you which alleles you inherited, and it does not tell you what will happen.
If you came here from a family story, the next distinction is the one that prevents the story from eating the person: genotype is the letters. Phenotype is the life. They are related. They are not equal.
Questions
How does genetic inheritance work?
You receive one copy of each autosome from each parent, plus sex chromosomes, plus mitochondrial DNA almost always from the egg. Alleles may match or differ. Traits then depend on dominance, other genes, chance, and environment.
What is dominant vs recessive?
Dominant means one altered copy can be enough for a pattern to show. Recessive means both copies usually need to be altered. The words describe a transmission pattern, not a rank, and they do not make a trait inevitable.
Why don't I look like my parents?
You inherited a shuffled half from each of them, not a blend of their faces as a single file. Recombination rebinds chromosomes. Most visible traits are polygenic. Environment and chance do the rest.
What is incomplete penetrance?
Having a genotype associated with a trait and not showing the trait. Variable expressivity is showing it to different degrees. Both are reasons a family chart leaks.
What is polygenic inheritance?
Many genes, usually of small effect, plus environment. Height and most common disease risks are in this class. Mendel's ratios are the wrong tool.
How does X-linked inheritance work?
The gene sits on the X chromosome. People with one X have one copy of that gene. People with two X chromosomes have two, with one X partly inactivated. That geometry, not folklore about carriers, is the pattern.
Do you inherit mitochondrial DNA from your mother?
Almost always. mtDNA is transmitted along the maternal line, with rare exceptions. Nuclear DNA is still half from each parent. Mitochondrial conditions can also involve nuclear genes.
Can two healthy parents have a child with a genetic condition?
Yes. Recessive alleles can be silent in both parents. A new (de novo) variant can arise in egg or sperm. Chromosome counts can change during meiosis. Healthy parents are not a guarantee, and a condition in a child is not a verdict on the parents.