| balanced chromosome | <genetics> A chromosome which is unable to pair with its homologue and participate in homologus recombination during meiosis because it contains several inversion mutations (that is, has segments which have become flip-flopped). (09 Oct 1997) |
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| B chromosome | <genetics> Small acentric chromosome, part of the normal genome of some races and species of plants. (18 Nov 1997) |
| bivalent chromosome | A pair of chromosome's temporarily united. (05 Mar 2000) |
| male chromosome complement | The large majority of males have a 46, xy chromosome complement (46 chromosomes including an x and a y chromosome). A minority of males have other chromosome constitutions such as 47,xxy (47 chromosomes including two x chromosomes and a y chromosome) and 47,xyy (47 chromosomes including an x and two y chromosomes). (12 Dec 1998) |
| marker chromosome | An abnormal chromosome that is distinctive in appearance but not fully identified. For example, the fragile x chromosome was once called the marker x. (12 Dec 1998) |
| p arm of a chromosome | The short arm of a chromosome (from the french petit meaning small). All human chromosomes have 2 arms: the p and q arms. (12 Dec 1998) |
| giant chromosome | <cell biology> Giant chromosomes produced by the successive replication of homologous pairs of chromosomes, joined together (synapsed) without chromosome separation or nuclear division. They thus consist of many up to 1000) identical chromosomes (strictly chromatids) running parallel and in strict register. The chromosomes remain visible during interphase and are found in some ciliates, ovule cells in angiosperms and in larval Dipteran tissue. The best known polytene chromosomes are those of the salivary gland of the larvae of Drosophila melanogaster which appear as a series of dense bands interspersed by light interbands, in a pattern characteristic for each chromosome. The bands, of which there are about 5,000 in Drosophila melanogaster, contain most of the DNA (ca 95%) of the chromosomes and each band roughly represents one gene. The banding pattern of polytene chromosomes provides a visible map to compare with the linkage map determined by genetic studies. Some segments of polytene chromosome show chromosome puffs, areas of high transcription. (18 Nov 1997) |
| Giemsa chromosome banding stain | <technique> A unique chromosome staining technique, used in human cytogenetics to identify individual chromosomes, which produces characteristic bands. It utilises acetic acid fixation, air drying, denaturing chromosomes mildly with proteolytic enzymes, salts, heat, detergents, or urea, and finally Giemsa stain; chromosome bands appear similar to those fluorochromed by Q-banding stain. Synonym: Giemsa chromosome banding stain. (05 Mar 2000) |
| metacentric chromosome | A chromosome with a centrally placed centromere that divides the chromosome into two arms of approximately equal length. (05 Mar 2000) |
| ring chromosome | A structurally abnormal chromosome in which the end of each chromosome arm has been lost and the broken arms have been reunited in ring formation. A ring chromosome is denoted by the symbol r. (12 Dec 1998) |
| Christchurch chromosome | An abnormal small acrocentric chromosome (no. 21 or 22) with complete or almost complete deletion of the short arm; found in cultured leukocytes in some cases of chronic lymphocytic leukaemia, also in some normal relatives of patients. (05 Mar 2000) |
| chromosome | <cell biology> The self-replicating genetic structures of cells containing the cellular DNA that bears in its nucleotide sequence the linear array of genes. The DNA of eukaryotes is subdivided into chromosomes, that consist of a number of chromosomes whose DNA is associated with various proteins. The chromosomes become more tightly packed at mitosis and become aligned on the metaphase plate. Each chromosome has a characteristic length and banding pattern. In prokaryotes, chromosomal DNA is circular, and the entire genome is carried on one chromosome. See: C banding, G banding. (10 Nov 1998) |
| chromosome 10 | 10q deletion occurs de novo and shows various malformations, high wide forehead with normocephaly, wide and bulbous tip of the nose, microretrognathia and severe mental retardation. This monosomy is rather rare and is reportedly associated with total colonic aganglionosis with small bowel involvement (TCSA), a variant of Hirschsprung disease. The clinical phenotype of 10p duplication, which is due to malsegregation of a familial translocation, includes severe postnatal growth retardation, profound mental retardation, several major and minor anomalies, dolichocephaly, harelip producing the appearance of a turtle's beak, cleft lip/palate in the absence of harelip, large low set ears, osteoarticular anomalies with hyperflexion of upper limbs and abduction-flexion of lower limbs, etc. Lethality seems considerable. Most cases of trisomy 10qter result from a parental translocation or inversion. More severe clinical manifestations are reported for trisomy 10q24, owing to heart and renal malformations and profound mental retardation. Trisomy 10q25 lacks major malformations, the mental retardation is moderate and the prognosis is favourable. The clinical features include high protruding forehead, round, broad and flat face, fine and arched eyebrows, downward slanting palpebral fissures, blepharophimosis, hypertelorism, hypoplastic and pinched nasal bridge, a small and often beaked nose, cleft palate, ligamentary hyperlaxity, and hypotonia. Inner organ malformations are rare but mental deficiency is severe. 10q monosomy is quite rare and the main features are severe mental retardation, microcephaly, low birth weight, prominent nose bridge, long face, and anomalies of external genitalia. The phenotype of ring chromosome 10 is not very characteristic and includes cardiac and renal anomalies, small stature and moderate mental retardation. Prenatal diagnosis of trisomy 10 is reported. Dysmorphic features include foetal nuchal edema, cleft lip/palate, small lower jaw, rocker-bottom foot, polydactyly, hitch-hiker thumb, syndactyly and inner organ malformations. Genes on chromosome 10 include those encoding glutamate oxaloacetate transaminase, orithine amino transferase and hexokinase 1. Chromosome 10 shows 2 fragile sites in the long arm, 10q23 and 10q25, which probably accounts for its increased involvement in chromosomal anomalies. (05 Mar 2000) |
| chromosome 11 | 11p13 monosomy usually occurs de novo and is called the WAGR syndrome. The most constant anomaly is bilateral Aniridia with other ocular anomalies. It is also associated with mental and growth retardation, ambiguous genitalia, nephroblastoma (Wilms tumour) or gonadoblastoma. Familial Aniridia is described with cryptic inversion involving breakpoints within band 11p13. 11p trisomy involving segment 11p12 to 11p14 shows no characteristic ocular anomaly nor signs of malignancy but rather a high convex forehead, frontal upsweep of hair, wide nose bridge, hypertelorism, short wide beaked nose, round chubby cheeks, cleft lip/palate, hypotonia and severe mental retardation. 11p15 duplication shows features of Beckwith-Wiedemann syndrome, macrosomia, dysmorphic facies, cleft palate and mild mental retardation. 11q2 trisomy nearly always results from a malsegregation of a parental translocation. The phenotype includes long prominent philtrum, retracted lower lip, microretrognathia frequently accompanied by malformations of the palate and by glossoptosis, suggestive of Pierre Robin syndrome, preauricular pits and flexion contracture of the limbs. Mental retardation and inner organ malformations are severe. A specific translocation (11;22) involving most frequently breakpoints 11q23 and 22q11 leads to a trisomy with a phenotype very similar to that of 11q2 trisomy. Some additional features probably due to the associated 22 trisomy are preauricular tags, anal atresia or stenosis. The prognosis is characterised by high frequency of early deaths. 11q-syndrome with deletion 11q24 shows congenital heart defects and coarse facial features. The main clinical features of a terminal deletion 11q23 include trigonocephaly, hypertelorism, micrognathia and heart defects. The critical chromosome segment appears to be within the 11q24.1 segment. Considerable growth and mental retardation are usual. The deletion occurs de novo in the majority of cases. Ring chromosome 11 is rare and the phenotype includes mental retardation, failure to thrive/small stature, microcephaly and cafe-au-lait spots. Paracentric inversion inv(11)(q13q25) is associated with polysplenia syndrome including bilateral left sidedness sequence accompanied by complex cardiac malformations and failure of normal asymmetry in morphogenesis. Important genes are localised on chromosome 11, include those for non-alpha globins, whose mutations are responsible for sickle cell anaemia and |
| chromosome 12 | Deletion of the proximal short arm of chromosome 12 is rare and occurs de novo. Microcephaly, narrow forehead, pointed nose and micrognathia are present. Mental and growth retardation are significant but inner organ malformations are generally not present. 12p trisomy nearly always results from a familial translocation. The phenotype includes turricephaly with flat apex, high bulging forehead, flat rectangular face, pronounced hypertelorism, a very short nose with a broad and poorly defined bridge, a short neck with cutaneous folds, ear abnormalities, hypotonia, severe growth and mental retardation and signs of precocious aging in adolescents. Tetrasomy 12p is consistent with Pallister-Killian syndrome. The critical region appears to be confined to 12p11.2. 12q2 trisomy is uncommon and results most frequently from malsegregation of a parental translocation. The patients show a relatively large head with frontal bossing, rectangular face with chubby cheeks and short limbs, especially in the proximal segment. Mental retardation is severe and growth retardation variable. Among others, genes for lactate dehydrogenase B, phenylalanine hydroxylase and haemolytic anaemia due to glyceraldehyde -3-phosphate dehydrogenase deficiency are assigned to chromosome 12. (05 Mar 2000) |