Appendix

Chromosomal aberration (chromosomal abnormality): A generalized name for all types of chromosomal mutations: deletions, translocations, inversions, and duplications. It can sometimes also denote the genomic mutations (aneuploidy, trisomy, etc.).

Allele: One of two or more alternative forms of a gene, each characterized by a unique nucleotide sequence.

Allele-specific oligonucleotide: A synthetic oligonucleotide probe complementary to a specific DNA sequence. Provides the possibility to identify alleles that differ on one or more bases.

Allelic disorders: Phenotypically different disorders caused by different mutations in the same locus (gene).

Allelic heterogeneity: The phenomenon of the existence within one nosological form of different variants of the disease, caused by different mutant alleles of one gene.

Alpha-fetoprotein (α-fetoprotein, AFP): An embryonic protein found in the fetus’s, newborn’s, pregnant woman’s blood, and amniotic fluid.

Amniocentesis: Puncture of amniotic fluid sac to obtain a sample of the amniotic fluid containing fetal cells.

Amplification: An increase in the number of copies of a specific region of DNA.

Aneuploidy: An altered set of chromosomes in which one or more chromosomes from the usual set are either lost or are represented by additional copies.

Anticipation: An earlier age of onset and a more severe course of the disease in each subsequent generation.

Anticodon: Unit of three tRNA bases complementary to a mRNA codon.

Antimutagenesis: The process of mutation prevention, i.e. return of the damaged chromosome or gene to its original state.

Association: 1) in genetic epidemiology, a phenomenon in which the particular allele frequency is significantly higher or lower in the group of patients compared to its frequency in the population to which these patients belong; 2) in dysmorphology, a combination of abnormalities of unknown etiology and pathogenesis, detected simultaneously more often than expected by chance.

Assortative mating: Non-random mating in which pair bonds are established according to a specific genotype.

Assorted marriages: Marriages in which the choice of a marriage partner based on one or more characteristics is not accidental.

Autosomal dominant inheritance: A type of inheritance in which one mutant allele located in the autosome can cause disease.

Autosomal recessive inheritance: A type of inheritance of a trait or disease in which a mutant allele located in the autosome must be inherited from both parents.

Autosome: Any of the nonsex chromosomes. Humans have 22 pairs of auto­somes.

Bacterial artificial chromosomes (BACs): Vectors that replicate in bacteria, with a capacity to carry 100–130 kilobases of cloned human DNA; used for high-resolution gene mapping and DNA sequencing.

Balanced polymorphism: A polymorphism maintained in a population by the heterozygote advantage, allowing even a dangerous allele in the homozygous state to persist in the population with a relatively high frequency.

Barr body: A sex chromatin, represents an inactive X-chromosome.

Bioinformatics: computational analysis and storage of biological and experimental data. Used in genomic and proteomic research.

Bivalent: A linked pair of homologous chromosomes observed in the metaphase of the first meiotic division.

Carrier: An individual with one copy of the gene that causes recessive disorder and one copy of the normal allele.

Case-control study: An epidemiological method that compares patients with a disease (case group) and the appropriate control group (no disease) relatively to the frequency of various putative risk factors.

Centimorgan (cM): A unit of genetic distance between loci; 1 cM corresponds to a recombination frequency in 1% between loci (equivalent, on average, to 1 million base pairs).

Centromere: The primary constriction of the chromosome, located at the junction of sister chromatids and the formation of kinetochores. Required for their proper separation during mitosis and meiosis.

Centrosomes: Two microtubule-organizing centers; can be observed at the poles of a dividing cell at the end of the prophase.

Chimera: An individual with cells derived from two genetically different zygotes.

Chorionic biopsy: A prenatal diagnostic procedure performed at 7–11 weeks of pregnancy in order to obtain a sample of cells.

Chromosomal aberration (chromosomal abnormality): A generalized name for all types of chromosomal mutations: deletions, translocations, inversions, and duplications. It can sometimes also denote genomic mutations (aneuploidy, trisomy, etc.).

Chromosomal disorder: A disorder caused by constitutional chromosomal or genomic abnormalities.

Chromosomal mutation (aberration): Changes in chromosome structure.

Chromosome set: The total number of chromosomes in the nucleus of a gamete, zygote, or somatic cell.

Cis position: The position between two sequences on the same chromosome, literally “on the same side”. The opposite of the transposition.

Clone: A genetically identical offspring of one cell.

Cloning vector: A small plasmid, phage, or DNA-containing animal virus that can be inserted with heterogeneous DNA.

Codominant alleles: Different alleles, each of them is expressed in a heterozygote (for example, blood group according to the AB0 system).

Codon: An elementary unit of the genetic code, consisting of three adjacent bases, that encodes either inclusion of specific amino acid residue in the polypeptide chain, or a transcription stop-start signal.