AN75.1-5 | Principles of Genetics, Chromosomal Aberrations & Clinical Genetics — Glossary

An abnormal chromosome number that is not an exact multiple of the haploid number, such as trisomy (2n+1) or monosomy (2n-1)

Failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate properly during cell division, producing aneuploid gametes

The presence of three copies of a particular chromosome instead of the normal two (2n+1 = 47 in humans)

The presence of only one copy of a particular chromosome instead of the normal two (2n-1 = 45 in humans); only monosomy X (Turner) is viable

Having complete extra sets of chromosomes: triploidy (69), tetraploidy (92); usually lethal in humans

Fusion of two acrocentric chromosomes (13, 14, 15, 21, 22) at their centromeres, producing a balanced carrier with 45 chromosomes

Exchange of segments between two non-homologous chromosomes, balanced in the carrier but producing unbalanced gametes during meiosis

Loss of a segment of a chromosome, causing haploinsufficiency for the genes in that region

A structural rearrangement where a chromosomal segment is reversed 180 degrees; paracentric (not involving centromere) or pericentric (involving centromere)

An abnormal chromosome with two identical arms (two short arms or two long arms), formed by transverse division of the centromere

The presence of two or more genetically distinct cell lines derived from a single zygote, arising from post-zygotic mitotic error

The presence of cells from two genetically distinct zygotes in one individual, arising from fusion of two embryos or cell exchange

Trisomy 21 (95% free trisomy, 4% Robertsonian translocation, 1% mosaic), the commonest chromosomal cause of intellectual disability

Trisomy 18, characterised by clenched fists with overlapping fingers, rocker-bottom feet, and cardiac defects; 90% mortality in the first year

Trisomy 13, characterised by holoprosencephaly, midline facial defects, polydactyly, and cardiac defects; 90% mortality in the first year

45,X karyotype in females, characterised by short stature, webbed neck, streak gonads, and coarctation of aorta

47,XXY karyotype in males, characterised by tall stature, small firm testes, gynaecomastia, and infertility

The complete set of chromosomes in an individual, arranged and photographed during metaphase according to size and centromere position

Fluorescence In Situ Hybridisation — a technique using fluorescent DNA probes to detect specific chromosomal regions for diagnosing deletions, duplications, and translocations

Single Nucleotide Polymorphism — a variation at a single base pair position occurring in more than 1% of the population

A genetic variant present in at least 1% of the population; most are benign and contribute to normal human diversity

A permanent alteration in the DNA sequence that may be pathogenic (disease-causing) or benign, occurring at a frequency below 1% in the population

A chromosome banding technique using Giemsa stain after trypsin treatment, producing characteristic light and dark band patterns for chromosome identification

A four-chromosome structure formed during meiosis I in a translocation carrier, consisting of the two normal and two translocated chromosomes

A disorder caused by loss of paternal genes at 15q11-13, characterised by hypotonia in infancy, hyperphagia with obesity, intellectual disability, and hypogonadism