What Is Aneuploidy And How Is It Different From Polyploidy?

Aneuploidy is a chromosomal anomaly in which the number of chromosomes is wrong. The inability of the chromosomes to correctly separate during meiosis may be the reason for this. A chromosomal anomaly known as polyploidy occurs when there is an additional set of chromosomes. Chromosomes failing to split properly during meiosis or a sperm fertilising an egg with more than one set of chromosomes can both result in this.

As the majority of you are already aware, our genes determine every physical characteristic of our bodies, including the colour of our eyes and how likely we are to contract certain diseases. Knowing so much, you probably already know that our chromosomes, which are found in each and every one of our cells, are tightly coiled and twisted up with those genes. These chromosomes serve as the template for all upcoming cells and house all of our genetic material.

Our chromosome count is extremely crucial, and having a wrong number can have severe and even fatal consequences. Aneuploidy originates from an error in the separation of chromosomes during meiosis, whereas polyploidy is caused by an error in the number of chromosomal “sets.” These, despite having similar names, are noticeably distinct. Prior to delving into the specifics of aneuploidy vs. polyploidy, it is important to take a deeper look at chromosomes.

Human Body and Chromosomes

Chromosomes are copied during cellular division and then distributed between the two daughter cells. The bulk of human cells have 46 chromosomes, which are divided into 23 pairs. A child inherits half from the mother and half from the father. The genetic material that results from this merger and subsequent recombination is what gives life its tremendous diversity.

Chromosomal counts are extremely crucial during meiosis, the biological reproduction of sex cells, because they have a big impact on the growing embryo. Meiosis, which is divided into meiosis I and meiosis II, is the process of cellular replication for sex cells. Four daughter cells, each with the appropriate amount of chromosomes, are produced when sex cells replicate properly. These daughter cells are the best possible candidates for fertilisation. This is regrettably not always the case.

Aneuploidy and Meiosis

Meiosis occurs in two stages, I and II, each of which has its own prophase, metaphase, anaphase, and telophase, as was already mentioned. Aneuploidy happens when the chromosomes fail to split during either anaphase I or anaphase II. Nondisjunction, a condition where this occurs, is typically brought on by a failing mitotic checkpoint, which was meant to protect cells against improper replication. The chromosomal pairs might not divide properly if these checkpoints aren’t working properly.

Two homologous chromosomes are intended to divide during anaphase I of meiosis I, resulting in two daughter cells, each of which carries a homologous chromosome (composed of two sister chromatids). These sister chromatids would then separate during anaphase II in a perfect meiosis, giving rise to four daughter cells.

One of the subsequent daughter cells will have both homologous chromosomes while the other won’t if nondisjunction occurs during anaphase I. While the daughter cell with zero homologous chromosomes will divide into two daughter cells with zero chromosomes, the daughter cell with two homologous chromosomes will divide into two daughter cells with two chromosomes in each, rather than one.

For instance, if these sex cells fertilised an egg, the resulting embryo would either have 1 too many or 1 too few chromosomes. To be more precise, the fertilised embryo would have 45 or 47 chromosomes overall, which can cause a number of genetic problems.

When meiosis I proceeds normally and the homologous chromosomes divide during anaphase I, a variation of this happens. As a result, there will be two daughter cells, each containing two sister chromatids.

Anaphase II during meiosis II, however, can also experience nondisjunction. In one of the daughter cells, the sister chromatids may not be able to separate, leaving two daughter cells with one chromosome, one cell with two chromosomes, and one cell with no chromosomes. Similar to the scenario mentioned above, these cells might be an example of aneuploidy.

The term “monosomy” refers to the absence of a chromosome, while “trisomy” refers to the presence of an extra chromosome.

Diagrams of a Nondisjunction (Image Credit: Tweety207/Wikimedia Commons)

A miscarriage, stillbirth, or a child with life-limiting abnormalities will arise from aneuploidy in multiple chromosomes of the cell; the likelihood of survival is low. Aneuploidy can occur in some circumstances, such as trisomy 21, but not all. Downs syndrome develops when there is an extra copy of the 21st chromosome.

A condition known as mosaicism, in which only some of the body’s cells exhibit the mutation, occurs when a chromosomal mutation does not affect all the cells of a growing embryo. A less severe form of the condition linked to such an instance of aneuploidy may result from this.

What is Polyploidy?

Due to the fact that we have two sets of chromosomes, humans are referred to as diploid organisms (23 from each parent). However, there are further “ploidy” varieties, such as haploid (1 set), triploid (3 sets), and tetraploid (4 sets).

An abnormally high number of chromosomes occurs from a mistake in cellular replication in the case of aneuploidy (usually 1 too many, or 1 too few). However, when we discuss polyploidy, a person may possess a full additional set of chromosomes, totaling 69 chromosomes. As mentioned earlier, this is known as triploidy.

There are several possible reasons why this might happen. An full extra set of chromosomes may be passed down if there is a problem with a cell’s nondisjunction during anaphase, especially early in the development of the foetus. Another possibility for polyploidy is polyspermy, which occurs when more than one sperm cell fertilises an egg and adds a second set of chromosomes from the very first cell division.

triplod (Photo Credit : Wikimedia Commons)

Although there have been a few extremely rare instances in which a person was born with complete triploidy and only survived for a few months, cases of total polyploidy almost always result in an early miscarriage. On a smaller scale, specific cell types of the body, such the heart or liver, are more likely to exhibit this kind of chromosomal alteration. It is also possible to be tetraploid, which would mean having 92 chromosomes, and this has an even lower survival chance (4n).

While it is widespread in other species, especially angiosperms, polyploidy is less common in humans than aneuploidy. In fact, this is frequently considered as one of the causes for the rapid diversification that angiosperms experienced during their short evolutionary history.

Final Thought

Even though aneuploidy and poyploidy may both come from the same kind of replication error during cell division and seem similar, they are extremely different from one another and have very different effects.

While many frequent forms of aneuploidy allow for survival, a chromosomal alteration on the order of total polyploidy makes life all but impossible.