Human Genetic Variation
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in situ hybridization: Base pairing of a sequence of DNA to metaphase chromosomes on a microscope slide.

inherited: Transmitted through genes from parents to offspring.

insertion: Type of chromosomal abnormality in which a DNA sequence is inserted into a gene, disrupting the normal structure and function of that gene.

library: Collection of cloned DNA, usually from a specific organism.

linkage: Association of genes and/or markers that lie near each other on a chromosome. Linked genes and markers tend to be inherited together.

locus: Place on a chromosome where a specific gene is located; a kind of address for the gene.

mapping: Process of deducing schematic representations of DNA. Three types of DNA maps can be constructed: physical maps, genetic maps, and cytogenetic maps; the key distinguishing feature among these three types is the landmarks on which they are based.

marker: Also known as a genetic marker, a segment of DNA with an identifiable physical location on a chromosome whose inheritance can be followed. A marker can be a gene, or it can be some section of DNA with no known function. Because DNA segments that lie near each other on a chromosome tend to be inherited together, markers are often used as indirect ways of tracking the inheritance pattern of genes that have not yet been identified but whose approximate locations are known.

Mendelian inheritance: Manner in which genes and traits are passed from parents to children. Examples of Mendelian inheritance include autosomal-dominant, autosomal-recessive, and sex-linked genes.

messenger RNA (mRNA): Template for protein synthesis. Each set of three bases, called a codon, specifies a certain amino acid in the sequence of amino acids that compose the protein. The sequence of a strand of mRNA is based on the sequence of a complementary strand of DNA.

metaphase: Phase of mitosis, or cell division, when the chromosomes align along the center of the cell. Because metaphase chromosomes are highly condensed, scientists use these chromosomes for gene mapping and identifying chromosomal aberrations.

microarray technology: New way of studying how large numbers of genes interact with each other and how a cell's regulatory networks control vast batteries of genes simultaneously. The method uses a robot to precisely apply tiny droplets containing functional DNA to glass slides. Researchers then attach fluorescent labels to DNA from the cell they are studying. The labeled probes are allowed to bind to complementary DNA strands on the slides. The slides are put into a scanning microscope that can measure the brightness of each fluorescent dot; brightness reveals how much of a specific DNA fragment is present, an indicator of how active it is.

mitochondrial DNA (mtDNA): Genetic material of the mitochondria, the organelles that generate energy for the cell.

multifactorial trait: Trait that is controlled by many genes and is also influenced by the environment.

mutation: Permanent structural alteration in DNA. In most cases, such DNA changes either have no effect or cause harm, but occasionally a mutation can improve an organism's chance of surviving and passing the beneficial change on to its descendants.

neutral mutation: Mutation that results in a changed amino acid sequence but does not alter the protein's function.

nucleotide: One of the structural components, or building blocks, of DNA and RNA. A nucleotide consists of a base (one of four chemicals: adenine, thymine, guanine, and cytosine) plus a molecule of sugar and one of phosphoric acid.

nucleus: Central cell structure that houses the chromosomes.

oligo: Oligonucleotide, short sequence of single-stranded DNA or RNA. Oligos are often used as probes for detecting complementary DNA or RNA because they bind readily to DNA and RNA complements.

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