Download 7. glossory - Shodhganga

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 Amniotic fluid is withdrawn from the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 Fetal tissue for analysis is withdrawn from the villous area of the chorion either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.