Terminology - SU 6

Chromosome mutation

Variations in the number and structure of chromosomes; often affects many genes and has large phenotypic effects.

Metacentric

Chromosome in which the two chromosome arms are approximately the same length.

Submetacentric

Chromosome in which the centromere is located toward one end, producing a short arm and a long arm.

Acrocentric

Chromosome in which the centromere is near one end, producing a long arm at one end and a knob, or satellite, at the other end.

Telocentric

Chromosome in which the centromere is at or very near one end.

Chromosome rearrangement

Chromosome mutations that change the structures of individual chromosomes.

Chromosome duplication

Mutation that doubles a segment of a chromosome.

Tandem duplication

Chromosome rearrangement in which a duplicated chromosome segment is adjacent to the original segment.

Displaced duplication

Chromosome rearrangement in which the duplicated segment is some distance from the original segment, either on the same chromosome or on a different one.

Reverse duplication

Duplication of a chromosome segment in which the sequence of the duplicated segment is inverted relative to the sequence of the original segment.

Segmental duplication

Duplicated chromosome segments larger than 1000 base pairs.

Unequal crossing over

Misalignment of the two DNA molecules during crossing over, resulting in one DNA molecule with an insertion and the other with a deletion.

Chromosome deletion

Loss of a chromosome segment.

Pseudodominance

Expression of a normally recessive allele due to a deletion on the homologous chromosome.

Haploinsufficient

Gene that must be present in two copies for normal function. If one copy of the gene is missing, a mutant phenotype is produced.

Chromosome inversion

Rearrangement in which a segment of a chromosome has been inverted 180 degrees.

Paracentric inversion

Chromosome inversion that does not include the centromere in the inverted region.

Pericentric inversion

Chromosome inversion that includes the centromere in the inverted region.

Position effect

Dependence of the expression of a gene on the gene’s location in the genome.

Dicentric chromatid

Chromatid that has two centromeres; produced when crossing over takes place within a paracentric inversion. The two centromeres of the dicentric chromatid are frequently pulled toward opposite poles in mitosis or meiosis, breaking the chromosome.

Acentric chromatid

Chromatid that lacks a centromere; produced when crossing over takes place within a paracentric inversion. The acentric chromatid does not attach to a spindle microtubule and does not segregate in meiosis or mitosis, so it is usually lost after one or more rounds of cell division.

Dicentric bridge

Structure produced when the two centromeres of a dicentric chromatid are pulled toward opposite poles, stretching the dicentric chromosome across the center of the nucleus. Eventually, the dicentric bridge breaks as the two centromeres are pulled farther apart.

Translocation

(1) Movement of genetic material between nonhomologous chromosomes or within the same chromosome. (2) Movement of a ribosome along mRNA in the course of translation.

Nonreciprocal translocation

Movement of a chromosome segment to a nonhomologous chromosome or chromosomal region without any (or with unequal) reciprocal exchange of segments.

Reciprocal translocation

Reciprocal exchange of segments between two nonhomologous chromosomes.

Robertsonian translocation

Translocation in which the long arms of two acrocentric chromosomes become joined to a common centromere, generating a metacentric chromosome with two long arms and another chromosome with two very short arms.

Alternate segregation

Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N₁ and N₂ and the chromosomes containing the translocated segments are T₁ and T₂ , then alternate segregation takes place when N₁ and N₂ move toward one pole and T₁ and T₂ move toward the opposite pole.

Adjacent-1 segregation

Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N₁ and N₂ and the chromosomes containing the translocated segments are T₁ and T₂ , then adjacent-1 segregation takes place when N₁ and T₂ move toward one pole and T₁ and N₂ move toward the opposite pole.

Adjacent-2 segregation

Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N₁ and N₂ and the chromosomes containing the translocated segments are T₁ and T₂ , then adjacent-2 segregation takes place when N₁ and T₁ move toward one pole and T₂ and N₂ move toward the opposite pole.

Fragile site

Constriction or gap that appears at a particular location on a chromosome when cells are cultured under special conditions.

Fragile-X syndrome

A form of X-linked intellectual disability that appears primarily in males; associated with a fragile site that results from an expanding trinucleotide repeat.

Structural variants

Collective term for chromosome rearrangements and copy-number variations.

Copy-number variations (CNVs)

Difference among individual organisms in the number of copies of any large DNA sequence (larger than 1000 base pairs).

Aneuloidy

Change in the number of individual chromosomes; most often an increase or a decrease of one or two chromosomes.

Polyploidy

Possession of more than two sets of chromosomes.

Nullisomy

Absence of both members of a homologous pair of chromosomes (2n − 2).

Monosomy

Absence of one of the chromosomes of a homologous pair.

Trisomy

Presence of an extra copy of a chromosome (2n + 1).

Tetrasomy

Presence of two extra copies of a chromosome (2n + 2).

Down syndrome (trisomy 21)

Human condition characterized by variable degrees of intellectual disability; characteristic facial features; slower growth and development; and an increased incidence of heart defects, leukemia, and other abnormalities. It is caused by the duplication of all or part of chromosome 21.

Primary Down syndrome

Human condition caused by the presence of three copies of chromosome 21.

Familial Down syndrome

Human condition caused by a Robertsonian translocation in which the long arm of chromosome 21 is translocated to another chromosome; tends to run in families.

Translocation carrier

Individual organism heterozygous for a chromosome translocation.

Edwards syndrome (trisomy 18)

Human condition characterized by severe intellectual disability, low-set ears, a short neck, deformed feet, clenched fingers, heart problems, and other disabilities; results from the presence of three copies of chromosome 18.

Patau syndrome (trisomy 13)

Human condition characterized by severe intellectual disability, small head, sloping forehead, small eyes, cleft lip and palate, extra fingers and toes, and other disabilities; results from the presence of three copies of chromosome 13.

Trisomy 8

Presence of three copies of chromosome 8; in humans, results in intellectual disability, contracted fingers and toes, low-set malformed ears, and a prominent forehead.

Uniparental disomy

Inheritance of both chromosomes of a homologous pair from a single parent.

Genetic mosaicism

Condition in which regions of tissue within a single individual have different chromosome constitutions.

Gynandromorph

Individual organism that is a genetic mosaic for the sex chromosomes, possessing tissues with different sex-chromosome constitutions.

Autoploidy

Condition in which all the sets of chromosomes of a polyploid individual are derived from a single species.

Allopolyploidy

Condition in which all chromosomes of a polyploid individual are from two or more species.

Unbalanced gametes

Gamete that has a variable number of chromosomes; some chromosomes may be missing and others may be present in more than one copy.

Amphidiploid

Type of allopolyploid in which two different diploid genomes are combined such that every chromosome has one and only one homologous partner and the genome is functionally diploid.