Chromosomes
The word chromosomes is coined from Greek khrōma 'colour' + sōma 'body' and is defined as a thread-like structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of gene are called chromosomes.
Chromosomes were discovered in the late 19th century by a German Biologist Walther Flaming in 1882.when he was examining the rapidly dividing cells of salamander larvae. Since their discovery chromosomes have been found in the cells of all eukaryotes. Their number however varies from species to species. Penicillium, a fungus has only one pair of chromosomes, while some ferns have more than 500 pairs. A mosquito has 6, frog 26, mouse 40, honeybee 32, corn 20, sugarcane 80 and human cells have 46 chromosomes consisting of 23 pairs.
Explanation in this work is based on human chromosomes for our convenience.
STRUCTURE OF CHROMOSOMES
The most ideal stage to study the structure of chromosomes is the metaphase of mitosis. A metaphase chromosome consists of two identical components called chromatids, which are held together at a specific region called primary constriction. It is usually found in the centre and hence it is commonly described as centromere. It shows a plate-like proteinaceous structure called kinetochore where the microtubules of the spindle become attached during cell division. The portions of a chromatid found on either side of a centromere, are called arms which may be equal (isobrachial) or unequal (heterobrachial), based on the position of centromere.
Sometimes, a chromosome may have an additional constriction apart from the centromere, called secondary constriction. Portion of the chromatid situated beyond the secondary constriction, is called as a satellite body. Such a chromosome with a satellite body is known as a sat-chromosome. Secondary constrictions most often take part in the formation of nucleolus. Hence, such secondary constrictions are called as nucleolar organizers. Structure of Chromosomes
The terminal end of a chromosome is called as telomere. It is functionally different from the rest of the chromosome. A chromatid may breakup into pieces and the pieces may rejoin, but no segment becomes connected to the telomere. Thus, the telomere shows a sort of polarity.
The chromatid, when observed under the electron microscope appears to be composed of a very fine filamentous structure called chromonema. It represents a molecule of double stranded DNA extending from one end of the chromosome to the other. The chromonema occurs in a highly coiled state. The chromonema is in turn composed of a chain of tiny, bead-like structures called nucleosomes. A nucleosome or nubody consists of a core particle surrounded by a DNA strand. The core particle is formed by a group of proteins called histones. In a nucleosome there is an octomer of 8 histone molecules, two each of H2A, H2B, H3 and H4. In between the nucleosomes is the DNA portion not associated with histones and is called Linker DNA. A linker DNA with the nucleosome is described as a chromatosome.
Chemically, the chromosomes are described as nucleo-proteins, since they contain DNA and proteins. The chromosomal proteins have nothing to do with the genetic potency of the organism. These proteins, called histones, regulate the gene action.
TYPES OF CHROMOSOMES
The chromosome types are divided into two categories, autosomes and sex chromosome. These types are explained below:

Autosomes
Autosomes are structures that contain the hereditary information. They do not contain information related to reproduction and sex determination. They are identical in both sexes, i.e., male and female species of humans. There are 46 (2n) chromosomes in humans. Of these 46 chromosomes, there are 44 pairs of autosomes and contain information related to the phenotypic characters.

Allosomes/ Heterosomes
The allosomes are sex chromosomes that are different from autosomes in form, behavior and size. There is pair of allosomes in humans. The X chromosomes are present in the ovum and either the X or Y chromosome can be present in the sperm. These chromosomes help in determination of sex of the progeny. If the offspring receives X chromosome from the mother as well as father, it results in a female child (XX). If the offspring receives one X and one Y chromosome from the parents, it results in a male child (XY). In simple words, it is the donation of X or Y chromosome by the father that helps in determination of the sex of the child.

Apart from these two categories, chromosomes can further be divided according to the location of the centromere and number of centromeres.

Chromosome Types: Based on Centromere Position
Chromosomes are divided into four types based on the centromere position. These four types are as follows:

Metacentric Chromosome
The metacentric chromosome has its centromere centrally located between the two arms. This gives the chromosome a typical 'V' shape that is seen during the anaphase. The arms of this chromosome are roughly equal in length. In certain cells, fusion of two acrocentric chromosomes leads to formation of metacentric chromosome.

Submetacentric Chromosome
The arms of the submetacentric chromosome are said to be unequal in length. This is because the kinetochore is present in the sub median position. This gives rise to the 'L' shape of the submetacentric chromosome.

Telocentric Chromosome
Also known as the monarchial type of chromosomes, they have a centromere that is located towards the end of the chromosome. Thus, telocentric chromosomes have a 'rod' shaped appearance. In some cases, the telomeres extend from both the chromosome ends. The Telocentric chromosome is not present in humans.

Subtelocentric Chromosome
Chromosomes that have a centromere that is located closer to the end than the center are called subtelocentric chromosomes.

Acrocentric Chromosome
The location of the centromere in the acrocentric chromosome is subterminal. This causes the short arm of the chromosome to become really short making it very difficult to observe.

Holocentric Chromosome
In holocentric chromosomes, the centromere runs through the entire length of the chromosome. These chromosomes are very common in cells belonging to organisms in the animal and plant kingdom. Chromosome Types: Based on Centromere Number
The number of centromeres present on the chromosome help in determining the type of chromosome. These different chromosome types based on the number of centromeres are as follows:

Acentric Chromosome
These are chromosomes that totally lack centromere. These chromosomes are observed due to effects of chromosome-breaking process like irradiation.

Monocentric Chromosome
Monocentric chromosomes are those that contain a single centromere. This type of chromosome is present in most of the organisms. The Monocentric chromosomes can be called acrocentric, if the centromere is located at the end of the chromosome. If the centromere is located towards the center it is called metacentric chromosome. And, telocentric chromosomes are those in which the centromere is present at the chromosome's end.

Dicentric Chromosome
Dicentric chromosomes are those that have two centromeres that are present on its arms. These chromosomes are formed after two chromosomal segments with a centromere each, are fused end to end. This causes them to lose their acentric fragments, leading to formation of dicentric chromosome.

Polycentric Chromosome
Polycentric chromosomes are those that contain more than two centromeres. These chromosomes are very common in plants, for example, the Adder's-tongue fern has 1262 chromosomes.

The types of chromosomes in humans are divided into two categories as seen above. They include the autosomes and sex chromosomes. Thus, in all, humans contain 22 pairs of autosomes and one pair of sex chromosome. This leads to presence of 46 chromosomes present per cell. Chromosomes are very important for each and every organism as the future of their offspring depends on the normal division and separation of chromosomes.
Abnormalities in chromosomes
Chromosome abnormality
A chromosome abnormality is a change in the number or the arrangement of the chromosomes. There are many different types of possible changes. In some individuals, a piece of a chromosome is missing. This is called a deletion. In other individuals, a piece of a chromosome is duplicated. This is called a duplication. Sometimes, a piece of one chromosome is broken off and attached to another chromosome. This is called a translocation. A person may have an entire extra copy of a chromosome or a missing chromosome. These are called trisomy and monosomy, respectively. There are other types of chromosome abnormalities as well, such as ring chromosomes, insertions, inversions, and isochromosomes (which occur when one half of the chromosome is missing and the other half is duplicated).
In most cases, chromosome abnormalities are diagnosed by a karyotype. A geneticist with specialized training looks for any changes in the number or arrangement of the chromosomes.

Three major single chromosome mutations are; deletion (1), duplication (2) and inversion (3).

The two major two chromosome mutations are; insertion (1) and Translocation (2).
A chromosome anomaly, abnormality, aberration, or mutation is a missing, extra, or irregular portion of chromosomal DNA. It can be from an atypical number of chromosomes or a structural abnormality in one or more chromosomes. A karyotype refers to a full set of chromosomes from an individual that can be compared to a "normal" karyotype for the species via genetic testing. A chromosome anomaly may be detected or confirmed in this manner. Chromosome anomalies usually occur when there is an error in cell division following meiosis or mitosis. There are many types of chromosome anomalies. They can be organized into two basic groups, numerical and structural anomalies.
Numerical disorders
This is called aneuploidy (an abnormal number of chromosomes), and occurs when an individual either is missing a chromosome from a pair (monosomy) or has more than two chromosomes of a pair (trisomy, tetrasomy, etc.).In humans, an example of a condition caused by a numerical anomaly is Down Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three copies of chromosome 21, rather than two). Trisomy has been determined to be a function of maternal age.
An example of monosomy is Turner Syndrome, where the individual is born with only one sex chromosome, an X.
Structural abnormalities
When the chromosome's structure is altered, this can take several forms: * Deletions: A portion of the chromosome is missing or deleted. Known disorders in humans include Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q deletion disorder. * Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Known human disorders include Charcot-Marie-Tooth disease type 1A, which may be caused by duplication of the gene encoding peripheral myelin protein 22 (PMP22) on chromosome 17. * Translocations: A portion of one chromosome is transferred to another chromosome. There are two main types of translocations: * Reciprocal translocation: Segments from two different chromosomes have been exchanged. * Robertsonian translocation: An entire chromosome has attached to another at the centromere - in humans these only occur with chromosomes 13, 14, 15, 21, and 22. * Inversions: A portion of the chromosome has broken off, turned upside down, and reattached, therefore the genetic material is inverted. * Insertions: A portion of one chromosome has been deleted from its normal place and inserted into another chromosome. * Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material. * Isochromosome: Formed by the mirror image copy of a chromosome segment including the centromere.
Chromosome instability syndromes are a group of disorders characterized by chromosomal instability and breakage. They often lead to an increased tendency to develop certain types of malignancies.
Inheritance
Most chromosome abnormalities occur as an accident in the egg or sperm, and therefore the anomaly is present in every cell of the body. Some anomalies, however, can happen after conception, resulting in Mosaicism (where some cells have the anomaly and some do not). Chromosome anomalies can be inherited from a parent or be "de novo". This is why chromosome studies are often performed on parents when a child is found to have an anomaly. If the parents do not possess the abnormality it was not initially inherited; however it may be transmitted to subsequent generations.