This glossary collects the genetics and inheritance terms used across Clinical Genetics Primer, defined in plain language. It is meant as a quick reference while you read about specific conditions, not as a diagnostic tool. If a term here raises questions about your own health or a family member's, a genetic counselor or clinician is the right person to talk to.
Autosomal dominant inheritance
This is a pattern of inheritance in which a condition is located on one of the numbered, non-sex chromosomes and only one changed copy of the gene, inherited from one parent, is enough to cause the condition. A person with an autosomal dominant condition has roughly a one in two chance of passing the changed gene to each child.
Autosomal recessive inheritance
In this pattern, the gene involved sits on one of the numbered, non-sex chromosomes, and a person needs two changed copies, one from each parent, to be affected. A child of two unaffected carriers has about a one in four chance of inheriting both changed copies and being affected.
Carrier
A carrier has inherited one changed copy of a gene associated with a condition that is inherited in an autosomal recessive or sex-linked pattern, along with one typical copy, and does not show symptoms. A carrier can still pass the changed copy on to a child.
De novo variant
A de novo variant is a genetic change that is seen for the first time in a person and is not present in either parent. These changes can arise in an egg or sperm cell, or very early after fertilization, and they explain many cases of a genetic condition appearing in a family with no prior history.
Ectoderm
The outermost of the three primary germ layers in an early embryo. It gives rise to the skin, hair, nails, sweat glands, and tooth enamel, the structures affected in ectodermal dysplasia.
Ectodermal dysplasia
Ectodermal dysplasia is a general term for a group of related conditions involving abnormal development of structures that arise from the embryonic ectoderm, including skin, hair, nails, teeth, and sweat glands. There are many distinct forms, each with its own genetic cause.
EDA gene
The EDA gene provides instructions for a protein that helps the ectoderm and the layer beneath it communicate properly during embryonic development. Changes in this gene are the most common cause of hypohidrotic ectodermal dysplasia and are usually inherited in an X-linked pattern.
FMR1 gene and CGG repeat
The FMR1 gene normally contains a short, repeated DNA sequence known as a CGG repeat, usually numbering under about 45 copies. When this repeat expands beyond roughly 200 copies, the gene is silenced and stops producing its protein, which causes fragile X syndrome; an intermediate expansion is called a premutation and can still be passed on or expand further in later generations.
Gene
A gene is considered the basic unit of inheritance. Genes are passed from parents to offspring and contain the instructions a cell uses to build proteins, which in turn shape physical and biological traits.
Genetic counselor
A genetic counselor is a specialized healthcare professional who helps individuals and families understand how a genetic condition could affect them, interprets genetic test results, and helps people think through their options. A genetic counselor also provides support as a family adjusts to a diagnosis or a known risk.
Genetic testing
Genetic testing is a laboratory test that examines a person's DNA for changes linked to a disease or trait. It can be used to confirm or rule out a suspected genetic condition, estimate the chance of passing a condition to a child, or test a pregnancy before birth.
Genomic imprinting
Genomic imprinting is a process in which only one copy of certain genes, either the one from the mother or the one from the father, is normally active, while the other copy is chemically silenced. This silencing does not change the underlying DNA sequence, but it means that whether a gene copy came from the mother or the father can matter for health, as seen in conditions such as Angelman syndrome.
Genotype
A genotype describes the specific version of a gene or DNA sequence a person has at a given location in their genome. Genotype is distinct from phenotype, which refers to the observable trait that results from it.
Germline mosaicism
Germline mosaicism occurs when a genetic change is present in some, but not all, of a parent's egg or sperm cells, while the rest of the parent's body is unaffected. Because of this, a parent with no symptoms can still have more than one child affected by the same condition, since the change can be passed down through any of the affected germ cells.
Inheritance pattern
An inheritance pattern describes the way a genetic condition is typically passed from parents to children, such as autosomal dominant, autosomal recessive, or X-linked. Knowing a condition's inheritance pattern helps a genetic counselor estimate the chance that other family members are affected or could pass the condition on.
MECP2 gene
The MECP2 gene is located on the X chromosome and provides instructions for a protein that is important for normal brain function. Changes in this gene underlie almost all cases of classic Rett syndrome, and most of these changes arise new in the affected child rather than being inherited from a parent.
Gene variant
A difference in DNA sequence compared with a reference sequence. "Variant" is the modern, neutral term; it includes changes that cause disease, changes with no known effect, and changes still being studied.
Variant
See gene variant above.
Chromosome
A structure made of DNA tightly coiled around proteins, found in the nucleus of a cell. Humans typically have 46 chromosomes arranged in 23 pairs, including one pair of sex chromosomes.
X chromosome
One of the two sex chromosomes. Most people with two X chromosomes are female, and most people with one X and one Y chromosome are male. Genes on the X chromosome follow X-linked inheritance patterns.
Mutation (gene variant)
A mutation, also called a gene variant, is a change in the DNA sequence of a gene. Variants can be inherited from a parent or can arise during a person's lifetime, and not every variant causes a health problem; some have no effect at all.
Newborn screening
Newborn screening is a set of laboratory tests performed on babies shortly after birth, usually from a blood sample, to look for a defined set of genetic and metabolic conditions. The goal is to catch certain treatable conditions early, before symptoms appear, since the exact list of conditions screened varies by region.
Pedigree
A pedigree is a chart that maps the relationships among members of a family across generations and notes who has a particular trait or condition. Genetic counselors use pedigrees to help work out how a condition is likely being inherited in a specific family.
Phenotype
A phenotype is an individual's observable trait, such as height, eye color, or a specific clinical feature of a condition. A person's phenotype results from the combination of their genotype and environmental factors.
Prenatal testing
Prenatal testing is used during pregnancy to look for genetic or chromosomal changes in a fetus before birth. Some prenatal tests, such as a blood test that analyzes fetal DNA circulating in the pregnant person's blood, are noninvasive, while others involve sampling fluid or tissue directly.
Prevalence
Prevalence refers to how common a condition is within a population at a given time, often expressed as a number of affected people out of a larger group. For many of the rare genetic conditions covered on this site, prevalence estimates are imprecise and can vary between sources, so we describe rarity in general terms rather than citing a single exact figure unless a reliable source states one.
Proband
A proband is the person affected by a genetic condition, or concerned they may be at risk, who is typically the first family member to bring the condition to the attention of healthcare professionals. A proband's diagnosis often starts the process of evaluating other family members.
Synapse
A synapse is the point of contact between two neurons where one nerve cell passes a chemical or electrical signal to another. Many neurogenetic conditions, including some discussed on this site, are understood in part through how they disrupt normal synapse formation or function during brain development.
TSC1 and TSC2 genes
The TSC1 and TSC2 genes provide instructions for two proteins, hamartin and tuberin, that work together to help regulate cell growth. A variant in either gene can cause tuberous sclerosis complex; a person is typically born with one altered copy, and a separate change affecting the other copy in certain cells later in life allows tumors to form.
UBE3A gene
The UBE3A gene is normally inherited from both parents, but in most of the brain only the copy from the mother is active due to genomic imprinting. When the maternal copy is lost or altered, the gene's protein is missing from brain cells, which causes Angelman syndrome.
Uniparental disomy
Uniparental disomy occurs when a person inherits two copies of a chromosome, or part of a chromosome, from one parent and no copy from the other parent. For most genes this has no effect, but for genes that undergo genomic imprinting, uniparental disomy can leave a person without any active copy of an important gene, which can cause a genetic disorder.
Variant of uncertain significance (VUS)
A variant of uncertain significance is a genetic change identified through testing where it is not yet clear whether the change is actually connected to a health condition. More information, such as additional family testing or further research, is often needed before a variant like this can be reclassified as either benign or disease-related.
X-linked inheritance
X-linked inheritance refers to a pattern in which the gene involved is located on the X chromosome. Because males have only one X chromosome, they are often more noticeably affected by X-linked conditions than females, who have a second X chromosome that can offset the effect of a changed copy.
Many of these terms recur across the site's condition pages. If a definition here does not fully answer your question about a specific diagnosis, the condition's own page, or a conversation with a genetic counselor, is the better next step.