The number of copies of a segment of DNA differs from individual to individual CNVs can make the difference between a healthy genotype and an unhealthy one. Example: Abnormal huntingtin genes, having more than 36 CAG codons producing the amino acid glutamine, produce Huntington disease. People with fewer repeats remain healthy.

Genome

The complete set of chromosomes for a species. Identifies the characteristics that define a species. Example: The entire human genome.

Genotype

An individual’s set of chromosomes. Identifies the genetic basis for individual characteristics. Example: Having one allele for Type A blood and a second allele for Type O blood.

Phenotype

The observable characteristics of an individual. Identifies the end result of the interactions between genes and environment. Example: Having hazel eyes or being 6 feet tall.

Chromosome

Twenty-three pairs of chromosomes make up the entire human genome. Example: The X and Y sex chromosomes.Twenty-three pairs of chromosomes make up the entire human genome. Example: The X and Y sex chromosomes.

Gene

A functional hereditary unit occupying a fixed location on a chromosome. Genes are responsible for the production of particular proteins. Example: Genes for blood type or for the production of dopamine receptors.

Allele

Alternative versions of a particular geneTrait variations between individuals in a species. Example: The A, B, and O alleles for blood type.

Meiosis

Cell division that reduces the number of chromosomes to half in reproductive cells Meiosis creates a vast number of possible genetic combinations. Example: A single human can produce eggs or sperm with (more than 8 million) combinations of his or her chromosomes.

Mutation

A change in a gene or chromosome that can be passed to offspring. Results of mutations can be advantageous, neutral, or disadvantageous. Example: A mutation causes the disease cystic fibrosis, which causes premature death.

SNPs (“snips”)

Variations that occur in a gene when a single base is changed from one allele to the next. SNPs can be a major source of variation between individuals of a species. Example: APOE alleles ε2 , ε3 , and ε4 that affect a person’s chances of developing Alzheimer disease.

Copy-Number Variations (CNVs)

The number of copies of a segment of DNA differs from individual to individual

CNVs can make the difference between a healthy genotype and an unhealthy one. Example: Abnormal huntingtin genes, having more than 36 CAG codons producing the amino acid glutamine, produce Huntington disease. People with fewer repeats remain healthy.