In this article we will discuss about the chromosomal aberrations and variation of chromosome number in human.
Any anomaly in the normal structure of chromosome or any variation in the normal karyotype of human causes an abnormal development of human or resulting in zygotic loss, still births or infant death. The degree of the abnormalities in the human structure will ultimately depend upon the types of anomalies in the human normal karyotype.
Structural chromosomal anomaly:
This type of chromosomal anomalies are mainly as a consequence of the breakage of chromosomes. If any segment of any chromosome is lost due to breakage then it is known
as deletion, but if it occurs at one end of any chromosome, it is known as deficiency. Similarly a segment may become detached and then reattached — only the other way round — this is known as inversion.
A segment may be included twice over and known as duplication. Similarly, chromosomes of two different pairs may exchange segments which is known as reciprocal translocation or interchange.
Likewise, a special type of reciprocal translocation may occur in between two acrocentric chromosomes (centromere is situated terminally i.e. at the very near end of the chromosome) which can join in such a way that the two long arms are essentially preserved and this type is known as centric fusion or Robertsonian translocation.
The basic difference between the reciprocal and Robertsonian translocation is that in case of former the total number of chromosomes remains unchanged, whereas, in case of latter, involving the possible loss of functionally irrelevant chromosome material and shows the chromosome complement is one short. The following Tables (9.1 and 9.2) shows some of the chromosomally abnormal human beings and their symptomatic features.