Genome Foundations

The genome is the entire set of genetic instructions found in a cell. In humans, the genome consists of 23 pairs of chromosomes, found in the nucleus, as well as a small chromosome found in the cells' mitochondria. Each set of 23 chromosomes contains approximately 3.1 billion bases of DNA sequence.4


An allele is one of two or more versions of a gene. An individual inherits two alleles for each gene, one from each parent. If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous. Though the term allele was originally used to describe variation among genes, it now also refers to variation among non-coding DNA sequences.4


A chromosome is an organized package of DNA found in the nucleus of the cell. Different organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes--22 pairs of numbered chromosomes, called autosomes, and one pair of sex chromosomes, X and Y. Each parent contributes one chromosome to each pair so that offspring get half of their chromosomes from their mother and half from their father.4   
Chromosome Illustration

Deoxyribonucleic Acid (DNA)

DNA is the chemical name for the molecule that carries genetic instructions in all living things. The DNA molecule consists of two strands that wind around one another to form a shape known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases--adenine (A), cytosine (C), guanine (G), and thymine (T). The two strands are held together by bonds between the bases; adenine bonds with thymine, and cytosine bonds with guanine. The sequence of the bases along the backbones serves as instructions for assembling protein and RNA molecules.


The study of heritable changes that do not affect the DNA sequence but influence gene expression.3


An exon is the portion of a gene that codes for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more DNA sequences called introns. The parts of the gene sequence that are expressed in the protein are called exons, because they are expressed, while the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between--or interfere with--the exons.4
Exon Illustration


The basic unit of heredity that occupies a specific location on a chromosome. Each consists of nucleotides arranged in a linear manner. Most genes code for a specific protein or segment of protein leading to a particular characteristic or function.3
Gene Illustration


The study of genes and heredity. Heredity is the passing of genetic information and traits (such as eye color and an increased chance of getting a certain disease) from parents to offspring.2


A genotype is an individual's collection of genes. The term also can refer to the two alleles inherited for a particular gene. The genotype is expressed when the information encoded in the genes' DNA is used to make protein and RNA molecules. The expression of the genotype contributes to the individual's observable traits, called the phenotype.4
Genotype Illustration

Germ Line                 

A germ line is the sex cells (eggs and sperm) that are used by sexually reproducing organisms to pass on genes from generation to generation. Egg and sperm cells are called germ cells, in contrast to the other cells of the body that are called somatic cells.4  
Germ Line Illustration


Genomics refers to the study of the entire genome, essentially all the genes that can be found in an organism.4    

Example: If you wanted to study the genomics of an organism or a person you could sequence all of their genes and all of their DNA and look for changes and make comparisons with other individual's genomes.4


Heterozygous refers to having inherited different forms of a particular gene from each parent. A heterozygous genotype stands in contrast to a homozygous genotype, where an individual inherits identical forms of a particular gene from each parent.4 
Heterozygous Illustration


Homozygous is a genetic condition where an individual inherits the same alleles for a particular gene from both parents.4 
Homozygous Illustration


An intron is a portion of a gene that does not code for amino acids. In the cells of plants and animals, most gene sequences are broken up by one or more introns. The parts of the gene sequence that are expressed in the protein are called exons, because they are expressed, while the parts of the gene sequence that are not expressed in the protein are called introns, because they come in between the exons.4 
Intron Illustration

Loss of Heterozygosity (LOH)

If there is one normal and one abnormal allele at a particular locus, as might be seen in an inherited autosomal dominant cancer susceptibility syndrome, loss of the normal allele produces a locus with no normal function. When the loss of heterozygosity involves the normal allele, it creates a cell that is more likely to show malignant growth if the altered gene is a tumor suppressor gene.3           


A microbiome is all of the genetic material found within an individual microbe such as a bacterium, fungal cell, or virus. It also may refer to the collection of genetic material found in a community of microbes that live together.4  
Microbiome Illustration


Microsatellite sequences are repetitive DNA sequences usually several base pairs in length. They are used as genetic markers to follow the inheritance of genes in families.4 

Wild-type gene

A term used to describe a gene when it is found in its natural, non-mutated (unchanged) form.2

Subcategory: Clinical Genomics

Cascade Genetic Testing

The process of extending genetic testing to individuals at risk for inheriting a pathogenic variant previously identified in a biologic relative. This process is repeated as more pathogenic variant carriers are identified within the family. This process is sometimes referred to as cascade screening, although cascade genetic testing is the preferred term.3


Penetrance refers to the likelihood that a clinical condition will occur when a particular genotype is present. For adult-onset diseases, penetrance is usually described by the individual carrier's age, sex, and organ site.2

Example: The penetrance for breast cancer in female carriers of BRCA1 pathogenic variants is often quoted by age 50 years and by age 70 years.2


Findings that are distinctive or characteristic of a particular disease or condition and make the diagnosis.2

Precision Medicine

A form of medicine that uses information about a person’s own genes or proteins to prevent, diagnose, or treat disease. In cancer, precision medicine uses specific information about a person’s tumor to help make a diagnosis, plan treatment, find out how well treatment is working, or make a prognosis. Also called personalized medicine.2

Example: Examples of precision medicine include using targeted therapies to treat specific types of cancer cells, such as HER2-positive breast cancer cells, or using tumor marker testing to help diagnose cancer.2


The study of the relationship between genetic variations and how our body responds to medications.6


A phenotypic trait or disease that resembles the trait expressed by a particular genotype, but in an individual who is not a carrier of that genotype.3

Example: A woman with early onset breast cancer who does not have a variant in any gene known to be associated with breast cancer.


The observable characteristics in an individual resulting from the expression of genes; the clinical presentation of an individual with a particular genotype.3

Example: An individual on colonoscopy found to have significant polyposis, which is a physical finding associated with variants in gene such as MUTYH and APC.