I hope most of you already heard about chromosomes . so What do you think chromosomes are ?? 

Walter Flemming discovered chromosomes in the nuclei of salamander cells. In 1879, he observed the behavior of these thread like structures in the nuclei during cell division.

Chromosome is the storage form of DNA within the cell. It is a single DNA molecule, associated with specific proteins. The DNA code of genes are collectively called nucleotides, long strings of nucleotides form genes, and groups of genes are packed tightly into threadlike structures called chromosomes.

How many chromosomes do Humans have ?

Humans have 23 pairs of chromosomes. 22 pairs are similar in both male and female. The 23rd pair is the sex chromosomes that determine the sex. It differs between male and female. Females have two X chromosomes whereas males have one X and one Y chromosome.

If you look into the above image. This shows the number of chromosome pairs that human normally has. twenty two pairs are called autosomes and one pair is sex chromosomes as shown above.

Everything will be fine if Human has correct number of chromosomes. but, its not the case always. sometime’s chromosome numbers in human being vary and that causes various disorders.

What happens if the number of chromosomes Vary?

Variation in the number of chromosomes may be the addition or even loss of one or more chromosomes, but not a complete set. The addition of a single chromosome to a diploid (‘di’ means two and ‘ploid’ means set of chromosomes) genome is called trisomy, and loss of a chromosome is known as monosomy.

In humans, most of these variations occur during meiosis (cell division while creating eggs or sperms). This is the major cause of birth defects in human beings. About one out of 20 babies born to women over 45 years of age are prone to have this problem.

There is a very chance that they may have three copies of chromosome 13, 18 or 21 instead of only two. This leads to syndromes.

Image curtsy : U S National Laboratory of Medicine

For example; three copies of chromosome 13 leads to Patau syndrome, three copies of chromosome 18 leads to Edwards syndrome and three copies of chromosome 21 leads to Down syndrome. All these three syndromes are the examples of trisomy.

As we consider cases that include the gain or loss of sex-determining chromosomes in humans, we have Klinefelter syndrome and Turner syndrome.

Klinefelter syndrome is due to an additional X-chromosome and Turner syndrome is due to loss of an X-chromosome instead of regular 46 chromosomes. Turner syndrome is an example of monosomy.

These are the most frequently observed cases on chromosome number variation. But, Molecular Biologists say, there are many random errors due to chromosome number variation that originates during the production of gametes. This phenomenon is known as non disjunction.

Rarely, some cells endup with complete extra sets of chromosomes, for a total of 69 chromosomes, they are called as Triploid. Similarly, cells with two additional sets of chromosomes are called as Tetraploid (92 chromosomes). In such conditions, where every cell of a body consists of an extra set of chromosomes, is not likely compatible with life.

I will try to explain each syndromes in detail in my upcoming posts. Stay Tuned !! Lot more to come .