Principles of Genetics and Breeding

1.3.3 Chromosome classification by centrometic position

1.3.3 Chromosome classification by centromeric position

Chromosomes are classified as follows, according to the position of the centromere.

a) Metacentric : Centromere is located approximately at the centre of the chromosome. Their arms are approximately equal and they appear U or V shape during movement.

b) Sub- metacentric: When the centromere lies a little distance away from the middle of the chromosomes may be called sub-metacentric.

c) Sub- telocentric: Centromere is located at sub-terminal position of the chromosome. One arm is much longer than the other. They appear J or L-shaped during movement.

c) Acrocentric or telocentric: Centromere is located at the end (terminal) of the chromosome.This is the most common karyotypes among fish species, and also it is found in a variety of distantly related fish taxa.

Table 2.1. Chromosome classification by centromeric position

Type	Centromeric position	Symbol	Centromeric index (range)
Metacentric	Nearly median	m	46-49
Submetacentric	Submedian	sm	26-49
Acrocentric	Subterminal	st	15-30
Telocentric	Terminal	t	<15

Each chromosome contain two identical parallel structures or chromatids. Each chromatid in turn consists of one or more thin filaments called chromonemata containing characteristic, condensed stainable regions called chromosomes. Chromonema contain alternating thick and thin regions. Centromere or kinetochore divides the chromosomes into two parts. The position of centromere varies from chromosome to chromosome. Centromere forms the site of implantation of the micro tubules that constitute chromosomal spindle fibers. The chromosomal ends are known as telomeres. Each extremity of the chromosome has a polarity and therefore it prevents other chromosomal segments to be fused with one another.

Genes, elementary units of heredity, are located throughout the length of the chromosome. In fishes and other vertebrate animals, each chromosome is known to contain hundreds and perhaps even thousands of genes.The basis of the chromonema is represented by DNA or deoxyribonucleic acid.

Advances in techniques, e.g. the use of colchicines, hypotonic treatment and air-drying cell suspensions, have allowed more detailed analyses of chromosome morphology. Measurement of chromosomes which give an as opposed to alpha-level karyology in which only the number of chromosomes is reported.These new techniques have permitted interspecific comparison of karyotypes and thus the identification of chromosomal rearrangement, which are an important or significant factor in evolution. accurate location of the position of the centromere have contributed to an important gain of knowledge,entitled beta-level karyology

Chromosomes are best seen during the cell division. The portions of the chromosomes on their side of the centromere are called as the chromosome arms. The total number of chromosome arms in a karyotype is called as fundamental arm number(FN).

1. Centromeric index is defined as the length of the shorter of the two chromosome arms multiplied by 100 and divided by the length of the whole chromosome and expressed as percentage.

$$Centrometric\ index = {The\ length\ of\ the\ shorter\ arm\ of\ a\ chromosome \over The\ total\ length\ of\ that\ chromosome}\ X\ 100 $$

This gives an idea of how much of percentage the short arm is of the whole chromosome. Based on the centromeric index, the chromosomes are classified as metacentric, sub-metacentric and telocentric.

2. Armratio is defined as the length of the longer arm of the chromosome divided by the length of the shorter arm.

$$Arm\ ratio = {Length\ of\ the\ long\ arm \over Length\ of\ the\ short\ arm}$$

Arm ratio is always greater than or equal to one and is an alternative method to centromeric index for describing the position of the centromere and the relative lengths of two arms.

3. Relative length is defined as the length of the whole chromosome multiplied by 100 and divided by the total length of all the chromosomes in the haploid set including one being measured and expressed as a percentage.

$$The\ relative\ length = {Length\ of\ a\ chromosome \over Total\ length\ of\ all\ the\ chromosomes\ in\ the\ haploid\ set}\ X\ 100 $$

The relative length gives us an indication of the proportion of the whole genome of the organism that is represented by the particular chromosome. However; this is a linear measurement and not a measure of mass and volume.

The chromosomes of a species can be arranged in order of size (by counting from the microscopic photograph). The complete set of chromosomes arranged according to their size is called as karyotypes . Karyotype of different species differ in size, shape, and number of chromosomes. It is, thus a phenotypic character similar to other morphological traits like body size.Therefore, it is believed that the karyotypes in different species have evolved through natural selection. Within a species, the males and females may have minor dissimilarity in the karyotypes, generally in a pair of chromosomes are very much similar are called as autosomes .

Morphologically, a chromosome in mitotic metaphase has only two distinguishing features: its length and a transverse constriction that makes the position of the centromere. From the chromosome length and centromere position three factors can be calculated: the centromere index , the arm ratio and the relative length of the chromosome. The first two factors, centromere index and the arm ratio, tells us all about the chromosome itself. The relative length tells us about the size of the chromosome in relation to the other chromosomes in the set.