A system of chromosome classification and nomenclature.

Abstract

Chromosome morphology is usually studied on the basis of the position of the primary constriction (or centromere or kinetochore), which is a principal landmark in contracted metaphase chromosomes. Cytologists have adopted several methods to determine the centromere locations and described the chromosomes as telocentric (centromere at one end of the chromosome), acrocentric (centromere near one end of the chromosome), submetacentric (centromere nearer to one end of the chro mosome than the other) and metacentric (centromere at or near the middle of the chromosome). These four categories are not sharply distinct, but grade imper ceptibly into each other (Stebbins 1971). The chromosomes are also described as median, nearly median, nearly submedian, submedian, nearly subterminal, sub terminal, nearly terminal and terminal (Sharma and Sharma 1960, Sharma and Choudhury 1964, Adhikary 1963, 1974, Bose and Flory 1965. Thus there exists a great deal of confusion in describing a chromosome on the basis of centromere position. Levan et al. (1964) recognised the location of centromere in median point (M), median region (m), submedian region (sm), subterminal region (st), terminal region (t) and terminal point (T). However, this system did not give due consideration to a number of oftquoted nomenclatures of centromere locations like nearly submedian and nearly subterminal. The system of Levan et al. (1964) was based on a) dif ference (d) between long arm (1) and short arm (s), where the whole chromosome =10 units; b) arm ratio (r) as the ratio of short arm/long arm; and c) centromeric index (i) as 100 s/c in the different regions, where c is the total length of the chro mosome. Nevertheless, this system gave biased preference to the short arm of the chromosome in determining the arm ratio and the centromeric index. The criterion of difference (d) between long arm (1) and short arm (s) is not always helpful as evi dent from our study of karyotypes of Aloe barbadensis and Furcraea gigantea (Abraham and Nagendra Prasad 1979, 1980). Recently, Adhikary (1974) proposed a system incorporating all the possible and up-to-date usage of centromere locations with reference to intermediate regions between terminal and median points. He recognised 4 fixed points and 4 inter mediate regions. The fixed points were median (M), terminal (T), submedian (SM) and subterminal (ST). The intermediate regions were nearly median (nm), nearly terminal (nt), nearly submedian (nsm) and nearly subterminal (nst). This system was based on arm ratios, R1 (short arm/long arm) and R2 (long arm/short arm).

Karyotype analysis of seven cultivated varieties ofCapsicum annuumL.

“…For chromosome morphology and classification, the system proposed by Abraham and Nagendra Prasad (1983) was followed.…”

Studies on the Karyotype of the Black Rat, Rattus rattus, Collected at the University of Nigeria Zoological Garden

“…It comprises of one median chromosome (15), one nearly median chromosome (Y), two nearly submedian (-) chromosomes (16 and 17), and one terminal chromosome (18).…”

“…The centromeric index (i) was calculated. The classification of the chromosomes based on centromere position follows that proposed by [15]. Appropriate ideogram (karyotype) representing the entire chromosome length and morphology was constructed.…”

Comparative Karyotyping of Five Marine Species of Genus Cypraea (Mesogastropoda: Prosobranchiata).

“…Fundamental number (FN) was calculated on the basis that metacentrics, submetacentrics and subtelocentrics having a value of four and acrocentrics has a value of two . Nomenclature of chromosome type adopted by Levan et al (1964) and Abrahan and Nagendra (1983) were used in the present study. …”