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7. GLOSSORY Acrocentric: A type of chromosome with the centromere near one end. The human acrocentric chromosomes (13, 14, 15, 21 and 22) have satellited short arms that carry genes for ribosomal RNA. Allele: One of the alternative versions of a gene that may occupy a given locus. Alpha-fetoprotein (AFP): A fetal glycoprotein excreted into the amniotic fluid, which reaches abnormally high concentration in amniotic fluid (and maternal serum) when the fetus has certain abnormalities, especially an open neural tube defect. Amniocentesis: A procedure used in prenatal diagnosis to obtain amniotic fluid, which contains cells of fetal origin that can be cultured for analysis. Amniotic fluid is withdrawn from the amniotic sac by syringe after insertion of a hollow needle into the amnion through the abdominal wall and uterine wall. Aneuploidy: Any chromosome number that is not the exact multiple of the haploid number; or an individual with an aneuploidy chromosome number. The common forms of aneuploidy in humans are monosomy (absence of a single chromosome) and Trisomy (presence of an extra chromosome). Autosome: Any nuclear chromosome other than the sex chromosome; 22 pairs in the human karyotype. A disease caused by mutation in an autosomal gene or gene pair shows autosomal inheritance. Banding: One of several techniques that stain chromosomes and structural abnormalities. Birth defect: An abnormality present at birth, not necessarily genetic. Centromere: The primary constriction on the chromosome, a region at which the sister chromatids are held quired and at which the kinetochore is formed. Required for normal segregation in mitosis and meiosis. Chorionic villus sampling: A procedure used for prenatal diagnosis at 8 to 10 weeks’s gestation. Fetal tissue for analysis is withdrawn from the villous area of the chorion either transcervically or transabdominally under ultrasonographic guidance. Chromosomal disorder: A clinical condition caused by an abnormal chromosome constituition in which there is duplication,loss, or rearrangement of chromosomal material. Chromosomal satellite: A small mass of chromatin containing genes for ribosomal RNA, at the end of short arm of each chromatid of an acrocentric chromosome. Chromosome: One of the threadlike structures in the cell nucleus; consists of chromatin and carries the genetic information (DNA). Congenital: Present at birth, not necessarily genetic. Consanguineous: Related by descent from a common ancestor. Dicentric: As structurally abnormal chromosome with two centromers. Dysmorphism: Morphological development abnormalities, as seen in many syndromes of genetic or environmental origin. FISH: Fluorescence In Situ Hybridization. Mapping a gene by molecular hybridization of a cloned DNA sequence, labeled by fluorescence, to a chromosome spread or a cell nucleus on a slide. Fragile site: Non staining gap in the chromatin of a metaphase chromosome, such as a fragile site at Xq27 in fragile X syndrome. Genetic counseling: The provision of information and assistance to affected individuals or family members at risk of a disorder that may be genetic, concerning the consequences of the disorder; the probability of developing or transmitting it, and the ways in which it may be prevented or ameliorated. Genetic disorder: A defect wholly or partly caused by genes. Genetic screening: Testing on a population basis to identify individuals at risk of developing or of transmitting a specific disorder. Heterochromatin: Chromatin that stains darkly throughout the cell cycle, even in interphase. Generally thought to be late replicating and genetically inactive. Satellite DNA in regions such as centromeres, acrocentric short arms, 1qh, 9qh,16qh, and Yqh constitute constitutive heterochromatin, whereas the chromatin of the inactive X chromosome is referred to as facultative heterochromatin. Heteromorphism: A normal morphological or staining variant of a chromosome. Index case: An affected family member who is the first to draw attention to a pedigree of a genetic disorder. Inversion: A chromosomal rearrangement in which chromosome undergoes two breaks and is reconstituted with the segment between the breaks inverted. Inversions are of two types: Paracentric in which both breaks occur in one arm and the centromere is not included during the process, and Pericentric in which there is a break in each of the arm and centromere is involved in the process. In vitro fertilization: A reproductive technology in which sperm are allowed to fertilize an egg in tissue culture and the fertilized eggs are then introduced back into the uterus to allow implantation. Isochromosome: An abnormal chromosome in which one arm is duplicated (forming two arms of equal length, with the same loci in reverse sequence) Karyotype: The chromosome constitution of an individual. The term is also used for a photomicrograph of the chromosome of an individual systematically arranged and for the process of preparing such a photomicrograph. Maternal serum screening: Laboratory test that relies on measuring the levels of particular substances, such as alpha feto protein, human Chorionic gonadotropin, and unconjugated estriol, in a pregnant women’s blood to screen for fetuses affected with certain trisomies or neural tube defects. Microdeletion: A chromosomal deletion that is too small to be seen under microscope. Monosomy: A chromosome constitution in which one member of a chromosome pair is missing. Mosaic: An individual or tissue with at least two cell lines differing in genotype or karyotype, derived from a single zygote. Mosaic development: Embryological development in which different regions of the embryo develop independently from surrounding regions. Nuchal translucency: An ultrasonographic finding of an echo free space between the skin line and soft tissue overlying the cervical spine in the subcutaneous tissue of fetal neck. Associated with fetal aneuploidy. p: In cytogenetics, the short arm of chromosome. Pedigree: In medical genetics, a family history of a hereditary condition, or a diagram of a family history indicating the family members, their relationship to the proband, and their status with respect to a particular hereditary condition. Philadelphia chromosome (Ph): The structurally abnormal chromosome 22 that typically occurs in a proportion of the bone marrow cells in most patients with chronic myelogenous leukemia, the abnormality is a reciprocal translocation between the distal portion of 22q and distal portion of 9q Point mutation: A single nucleotide base pair change in DNA. Polymorphism: The occurrence together in a population of two or more alternative genotypes, each of a frequency greater than that which could be maintained by recurrent mutation alone. A locus is arbitrarily considered to be polymorphic if the rarer allele has a frequency of 0.01, so that the heterozygote frequency is at least 0.02. Any allele rarer than this is a rare variant. Proband: The affected family member through whome the family is ascertained. Also called the propositus or index case. Probe: In molecular genetics, a labeled DNA or RNA sequence used to detect the presence of a complementary sequence by molecular hybridization; or a reagent capable of recognizing a desired clone in a mixture of many DNA or RNA sequences. Also, the process of using such a molecule. q: In cytogenetics, the long arm of a chromosome. Rearrangement: Chromosome breakage followed by reconstitution in an abnormal combination. If unbalanced, the rearrangement can produce an abnormal phenotype. Recurrence risk: The probability that a genetic disorder present in one or more members of a family will recur in another member of the same or a subsequent generation. Retinoblastoma [Rb]: Rb is a rare, highly malignant cancer of the developing retinal cells. It can occur either sporadically or be familial, being inherited in an autosomal dominant manner. There is a constitutional interstitial deletion of 13q21. Ring chromosome: A structurally abnormal chromosome in which the telomere of each chromosome arm has been deleted and the broken arms have reunited in ring formation. Robertsonian translocation: A translocation between two acrocentric chromosomes by fusion at or near the centromere, with loss of the short arms. Segregation: In genetics, the disjunction of homologues chromosome at meiosis. Sex chromosomes: The X and Y chromosomes. Single gene disorder: A disorder due to one or a pair of mutant alleles at a single locus. Submetacentric: A type of chromosome with arms of different length. Synapsis: Close pairing of homologous chromosomes in prophase of the first meiotic division. Syndrome: A characteristic pattern of anomalies, presumed to be causally related. Telomere: The end of each chromosome arm. Human telomeres end with tandem copies of the sequence (TTAGGG)n, which is required for the proper replication of chromosome ends Translocation: The transfer of a segment of one chromosome. If two non homologous chromosomes exchange pieces, the translocation is reciprocal. Trisomy: The presence of an extra chromosome is reffered as Trisomy and is usually caused by faliur of separation of one of the pairs of homologous chromosomes during anaphase of maternal meiosis-I (non-disjunction). Ultrasonography: A technique in which high frequency sound waves are used to examine internal body structures; useful in prenatal diagnosis. X; autosome translocation: Reciprocal translocation between an X chromosome and an autosome.