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In this article we will discuss about the classification of human chromosomes on the basis of size and position of the centromere.
All the human chromosomes in normal numbers (46) can be arranged in groups based on size and positions of the centromere.
The resulting classification of the chromosomes is given in the following table:
Methods of Human Chromosome Classification and Nomenclature:
All known eukaryotes possess more than two chromosomes per haploid set, so it is necessary to define a system of descriptive nomenclature which allows individual chromosomes to be distinguished from others in the same nucleus. The nomenclature which is used is summarised below and specifies the type and position of the centromere and the relative lengths of the chromosome arms.
Most chromosomes can be defined in terms of three parameters—the d-value, the r-value and the centromeric index (i), which delimits regions of the chromosome in which the centromere can be placed irrespective of chromosome size.
The d-value is the difference in length between the long and short arms of the chromosome, i.e., d = q – p, where, q and pare the lengths of the long and short arms, respectively, and the total length of the chromosome has been divided into 10 arbitrary units.
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The r -value is the ratio of the lengths of the two chromosome arms, i.e., r = q/p. The centromeric index is the distance from the centromere to the tip of the short arm, expressed as a percentage of the total chromosome length, i.e., i = 100 p/(p+q).
Human chromosomes are placed into 7 categories based on these parameters and their size are as shown:
Table 8.1 shows some of the important genetic loci for which the mode of inheritance was reasonably well established.
