Institute of Human Genetics

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Cytogenetics

Introduction

Cytogenetic is a study of genetic material at the cellular level. Within the nucleus of each cell, the genetic material (DNA), tightly coiled and conjugated with various proteins, forms a number of rod-like structure called chromosomes.

There are 23 pairs of chromosomes in a human cell. 22 pairs are same in males and females and are known as autosomes. The 23rd pair of chromosome is known as sex chromosome. In males the sex chromosomes are XY while in females the sex chromosomes are XX.

There is also an emerging branch of molecular Cytogenetics that can be described as the microscopic analysis of chromosomes and chromatin in metaphase and interphase using labeled DNA Probes.

It is an application of the revolution produced by recombinant DNA technology over the past 25 years and observed that single- stranded DNA sequences can anneal in appropriate conditions to the denatured DNA of chromosomes in standard microscopic preparation of mitotic cells.

Certain micro deletions associated with various genetic disorders like Prader villi, De-George, Angelman syndrome and so on needs this technology called as FISH (Flrorescent-in-situ-Hybridization).


”Metaphase of Human Chromosomes”

This is the study of structure, function and evaluation of chromosomes, the vehicles of inheritance that reside in the cell nucleus. There are about 1,000 genes together in a kind of genetic soup in the nucleus of each human chromosome totaling around 31,000 genes in human genome.

Genes control everything and they all interact. Before cells divide, genes come together into 23 couples; i.e. chromosome pairs. The DNA sequences comprising genes always go to the same location of a particular chromosome, which for convenience are numbered from biggest (1 – most genetic material) to the smallest (22) plus the sex chromosomes (XX female, XY male).

Eggs from female and sperm from male are specific cells that contain a single set of 23 chromosomes. Conception completes the chromosome set and cell replication and differentiation begins: chromosome pairs separate and move to opposite sides of the nucleus, the nucleus and cell divides, individual chromosome replicate into a pair and the process repeats.

If a pair of chromosomes does not split clearly then, after division one cell will have too many chromosomes and the other will have too few. Karyotyping (chromosome study) is aimed at detecting the family of disorders that come from having too many or too few chromosomes per cell.





The above placed picture/clipping of chromosome 7 has a distinctive bands, the same are present in all chromosomes. This can be visualized with the sophisticated and advanced techniques, which enable us to identify the small segment of genetic material that is deleted from its normal location or rearranged onto an alternate neighborhood where it may not function properly. This may be the cause of many disorders and disease process

Thus human cytogenetic study is particularly concerned with how this process may go wrong and how structural alterations arise. The study of these alteration in number or structure of chromosomes is helpful in the diagnosis of couple with repeated fetal loss, dysmorphic child, delayed development, epileptic child, certain kind of cancers, mental retardation etc.


Karyotype analysis of couples with Recurrent Pregnancy loss (To Know More Click Here)

This is the most common application of genetic study where women experience repeated fetal loss (RPL) without any apparent reason. It has been observed that 3 to 7% of families with RPL have chromosomal abnormalities of Robertsonian or Reciprocal translocation in chromosomes of either parent. When such chromosomes takes part in embryogenesis, resulting defective embryo do not grow or result in a child with abnormal conditions.

Another common condition where Karyotype analysis is needed is the child with mental retardation, dysmorphism, delayed development, epilepsy etc. Most common cause of MR is Down Syndrome & Fragile–X syndrome.