Question

The human X chromosome is about the same size as human chromosomes 4 and 5. Trisomy of the X chromosome occurs about once in every thousand human female births, and yet trisomy 4 and trisomy 5 are almost never seen among living humans. Why do these differences in the frequency of trisomic individuals occur, in spite of similar chromosome size?

Answer 1

The trisomy of X chromosome have less severe consequences as compared to trisomy of autosomal chromosomes and the affected individual is normally fertile with few occasional problems. This is due to the phenomenon of X-chromosome inactivation in which all but one X chromosome becomes heterochromatized and becomes transcriptionally inactive. The inactivated X chromosome is known as barr body and an individual with trisomy X will have two barr bodies in each of its cell. This X-chromosome inactivation normalizes the gene dosage between affected and normal individuals (as only X chromsome is active in each cell) and the affected individual is born with no or very few abnormalities. Therefore, trisomy X is not much severe and the affected female can reproduce normally, and have a normal life expectancy. Therefore, Trisomy of X chromosome has a high frequency among human population.

On the other hand, no such chromosomal inactivation is seen in autosomes and individuals with autosomal trisomy have altered gene dosage due to presence of three copies of an autosome. Thus individuals with autosomal trisomy are born with many birth defects and usually have a short life span. Trisomy of certain autosomes such as chromosome 4 or 5 can be embryonically lethal hence they are no observed among living humans.