What is Aneuploidy?

Generally, ploidy refers to the number, and here ploidy is the number of chromosomes present in the diploid set of homologous chromosomes that make up a genome of an organism.

Aneuploidy is one such type of ploidy, where the abnormal number of chromosomes are present in the individual where 45 or 47 chromosomes are present instead of 46.

Everything which are used up in pairs can be used only if their sets are available, considering from shoes, shocks, earrings, lenses, ear pods etc. Thus pairs are very important in the genetics where chromosomes play a key role in functioning of an organism in an appropriate manner.

Where the pair of chromosomes should be 2n, lesser or higher from this number causes many disorders and abnormality in an individual.

Aneuploidy is the condition where the person have a missing or an extra chromosome. Usually, any change in the number or structure of a chromosome is known as mutation and it is considered as one of the chromosomal disorders.

Chromosomal disorders are classified into two types based on the structural abnormality and numerical abnormality.

This numerical abnormality is further sub divided into euploidy and aneuploidy where euploidy is the condition where there is the presence of complete set of chromosomes (2n).

The other sub type is Aneuploidy where there is an addition or deletion in a 2n set of chromosomes.

Features of Aneuploidy

Aneuploidy is defined as the addition of an extra chromosome or removal or absence of a chromosome from any of its pair.

Each species contains a definite set of chromosomes where human contain 46 chromosomes which are 23 in pair to form a typical body cell and proper functioning of the parts.

If a cell contains one or two chromosomes extra or lesser than the normal pair then it is said to be aneuploidy.

This results in many disorders or abnormalities in a person, where in aneuploidy “an” refers to improper; and “ploidy” refers to number, and hence improper number of chromosomes causes this disorder.

Types of Aneuploidy

Aneuploidy is classified generally as hypoploidy and hyperploidy;

1. Hypoploidy

Hyploidy is generally defined as loss or lesser number of chromosomes compared with the normal 2n chromosomes.

The hyploidy is further sub divided as monosomy and nullisomy

Monosomy: Loss of one chromosome in a pair(2n-1)

Nullisomy: Loss of one pair (2n-2). The children having this condition has only lesser chance of leading a life.

2. Hyperploidy

This results in addition of one or more chromosomes. It is subdivided into types as

Trisomy: This condition results in extra chromosome on cells. It often results in Klinefelter’s syndrome with a trisomy in chromosome 21.

Tetrasomy: This condition results in addition of a pair of chromosomes.

Diploid: This condition results in addition of one chromosome in an monoploid organisms.

There are three most common types of aneuploidy that is occurring often; there are;

Monosomy

Here the organism contains one chromosome lesser (2n-1) in number than the normal, where humans contain 45 chromosomes instead of 46.

One of the examples for monosomy is TURNER’S SYDROME, where girls are mostly affected with this condition due to loss of one chromosome in the allosome set.

It leads to various developmental abnormalities and many severe symptoms.

The people with this condition are sterile in nature and short in stature and webbing of neck region is seen.

Trisomy

The individual contains addition of a n extra number of chromosomes. Which is 2n+1 in any one pair of the chromosome. Where they contain 47 chromosomes instead of 46.

One of the examples of trisomy is DOWN SYNDROME, which is due to the presence of an extra number in chromosome 21 which leads to many abnormalities.

It occurs mostly in 1 of 800 births. This factor mostly occurs if the child’s mothers age is above 35. This occurs due to the abnormal cell division during mitotic and meiotic phases.

The symptoms of this disorder include short stature, mental retardation, small head, poor muscle tone.

Causes of Aneuploidy

The chromosomal disorders mostly occur due to an occurrence of non-disjunction of chromosomes.

They occur when the two homologous sister chromosomes failed to separate during the stages of meiosis.

Meiosis I: In meiosis I the failure of non-disjunction of homologous chromosomes occurs during cell division.

Meiosis II: In Meiosis two, the failure of separation of sister chromatids occurs.

Mitosis: Non-disjunction of chromosomes also occurs in mitosis phases. In humans, the non-disjunction of chromosomes during mitosis does not pass through the next generation.

But in mitotic, non-disjunction causes other abnormalities. Cancer also occurs mostly due to the non-disjunction of chromosomes in the mitotic cells.

Considering an aneuploid condition, where the normal sperm combines with an aneuploid condition egg then it form an abnormal zygote as 2n+1 or 2n-1.

Where as in normal conditions normal sperm and a normal egg combines to form a normal zygote which is 2n.