**Abstract**

Chromosome identification depends on the morphological features of the chromosome and therefore karyotype and its banding pattern analyses are the most suitable technique to identify each and every chromosome of a chromosome complement. Moreover, aberrations caused by breaks play an important role in the evolution of a chromosome set and chromosome complement by decreasing or increasing the chromosome number. Therefore, both the aspects are discussed in detail in the present chapter. At present, the chapter will highlight the karyotype and its components, karyotype trends, evolution and its role in speciation, banding pattern and techniques, chromosome differentiation and linearization, banding applications and their uses, detection and analysis of chromosomal aberrations, chromosome and chromatid types of aberrations and mechanism of the formation of chromosome aberrations and breaks for karyotype evolutionary trends.

**Keywords:** chromatin, karyotypes, karyoype trend, karyotype evolution, chromosome banding techniques and pattern, chromosome aberrations and detection

## **1. Introduction**

Chromatin is a mixture of DNA, RNA and proteins could be easily visible during the interphase and prophase of the cell division cycle. Chromatin from interphase (loose mixture of DNA, RNA Protein) to thick mitotic structure (tightly packed or compressed mixture of DNA, RNA Protein) packed through Nucleosome model of DNA packing. Chromatin is divided into euchromatin and heterochromatin. Euchromatin is a part of chromatin which takes less stain, loosely packed, genetically active, involved in active transcription, dispersed appearance with more DNA content than RNA. On the other hand, heterochromatin is slightly opposite to the euchromatin with dark stained region, tightly packed, genetically inactive, not involved in the active transcription, thick appearance with more RNA content than DNA. Heterochromatin could be of two type's constitutive and facultative heterochromatin. Constitutive heterochromatins are permanently conserved or condensed and in stable form i.e. not changed from heterochromatin to euchromatin and vice-versa. It consists of multiple repeats of DNA sequences with quite less density of genes in this region which are transcriptionally inactive. Most probably, thick and condensed state of the constitutive heterochromatin, replicates late in S-phase with reduced frequency of genetic recombination.

Facultative heterochromatins are not permanently conserved or condensed and in unstable form i.e. easily changed from euchromatin to heterochromatin and vice-versa [1–3].

Heterochromatic regions could be easily recognized on chromosome structure in the form of chromomeres, chromocentres and knobs. Chromomeres are regular features of all prophase chromosomes but their number, size, distribution and arrangements are specific for a particular species at a particular stage of development. Chromocentres are the regions with varying size near the centromere in the proximal regions of chromosome arms. Some of the Chromocentres could be resolved into large number of strings of chromomeres which are much larger in size as compared to the chromomeres found in the distal region of the chromosmes arms during the mid-prophase. The relative distribution of the chromocentres on the chromosome structure, sometimes considered to be of significant evolutionary value. Knobs are considered to be a spherical bodies or regions with spherical in shape and sometimes diameter of these spherical bodies is equal in width to chromosome arm, but the size may vary i.e. less or more than the diameter of chromosome arm. For example, a very distinct such type of chromosome knob could be observed in maize (*Zea mays*) at pachytene stage of meiosis I. It could be considered as a valuable chromosome marker for distinguishing chromosome of related species and races [4–6].

#### **2. Concept of karyotype and its components**

Karyotype may be defined as the study of chromosome morphology of a chromosome complement in the form of size, shape, position of primary constriction or centromere, secondary constriction, satellite, definite individuality of the somatic chromosomes and any other additional features. Karyotype highlights closely or distantly related species based on the similarity or dissimilarity of the karyotypes. For example, a group of species resemble each other in the number, size and form of their chromosomes. There may be 12 different types of karyotype categories depending on increasing asymmetry in chromosome complement [7]. The degree of asymmetry of chromosome complement depends on the four arm ratios (1 to 4) and the size of the smallest and largest chromosome and three different proportions of the metacentric chromosomes (ABC) of a given chromosome complement. Arm ratio 1 being the most symmetrical and 4 is the most asymmetrical. There are various quantitative karyotypic ratios to observe the karyotype variations and precise description of the karyotype such as relative length, centromeric index, total form percent, disprsin index, disparity index, coefficient of variation, volume of chromosomes, value of relative chromatin and so on. Asymmetric karyotype may be defined as the huge difference between the largest and smallest chromosome as well as less number of metacentric chromosomes in a chromosome complement. Similarly, symmetric karyotype may be defined as the small difference between the largest and smallest chromosome as well as more number of metacentric chromosomes in a chromosome complement [8, 9].

The principle ways in which karyotypes differ from each other are (i) basic chromosome number, (ii) form and relative size (V➔J or L➔I) of different chromosomes of the same set, (iii) number and size of satellites (related to those positions of the chromosome which form nucleoli) and secondary constrictions (NOR region of chromoosmes), (iv) absolute size of the chromosomes, (v) distribution of material with different staining properties i.e. euchromatin and heterochromatin [10, 11].

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**Figure 1.**

*Karyotype trend in* Luzula *species by fission and fusion of the chromosomes.*

*Chromosome Banding and Mechanism of Chromosome Aberrations*

Karyotypic trend may be defined as the evolutionary changes in the chromosome complement by increasing or decreasing its base chromosome number which showed a definite direction of movement or pattern of its movement either from polyploidy to diploid or vice versa. For example, *Luzula* species (Juncaceae), also called wood rush, a monocot with holocentric chromosomes showed huge variation in genome and pattern or direction of chromosome movement from diploid to polyploidy or vice versa through symploidy and agmatoploidy phenomenon. The phenomenon could be related to the ascending or descending dysploidy which is also known as pseudoaneuploidy where chromosomes rearrange themselves within or between the chromosomes to decrease or increase the chromosome number in the chromosome complement of a particular species. Simploidy is the phenomenon of fusion of chromosomes together to reduce the chromosome number while the agmatoploidy breaks the chromosomes (fission) to increase the chromosome number for a particular species (**Figure 1**). The trend of *Luzula* species are as follows, L. purpureo-splendens (2n = 2x = 6; chromosome length 6.66 μm), L.elegans (2n = 2x = 6; chromosome length 4.62 μm), *L. alpinopilosa* (2n = 2x = 12 ± 1; chromosome length 2.55 μm), L.nivea (2n = 2x = 12; chromosome length 1.70 μm), L. sylvetica (2n = 2x = 12; chromosome length 1.48 μm), *L. multiflora* (2n = 6x = 36; chromosome length 1.32 μm), and *L. sudetica* (2n = 8x = 48; chromosome length 0.52 μm). The trend could be explained to understand that species with chromosome number 12 has merged their chromosomes together through a process of symploidy to occur speciation of a new diploid with 2n = 6. This could be possible because the size of the chromosomes increasing in L elegans and L. purpurosplendans. Similarly, there is a possibility of agmatoploidy phenomenon has been occurred and the size the chromosomes decreased in *L. sudetica*. Moreover, it clearly

*DOI: http://dx.doi.org/10.5772/intechopen.96242*

**2.1 Karyotype trend**
