Chromosomal Disorders In Humans Classification

Genes are passed from parents to the child and are the building blocks of heredity. They hold DNA, which carries instructions for making proteins that do most of the work in the cells. Chromosomes contain thousands of genes that make proteins that direct a body’s development, growth, and chemical reactions. In the case there is some change or malfunction in this process, it can cause genetic disorders in humans and animals. 

Chromosomal Disorders in Humans 

We have 46 chromosomes in our bodies, with 23 pairs, which we get from our parents—half from the mother and the other half from the father. As studied in the earlier standards, females typically contain two X chromosomes, and males usually contain X and Y chromosomes in each body cell. These chromosomes store instructions for the body to grow and develop. 

Change in the number of chromosomes (that is contributed by parents) or the structure of the chromosomes results in chromosomal disorders and is considered a genetic disorder. Chromosomal disorders normally happen during the developmental stage of the foetus or from the time of conception. Chromosomal disorders can be classified mainly into two types, namely, numerical and structural. 

1. Numerical aberration

Numerical aberrations in chromosomes occur when more or less than the required number are present. That is less than 46 or more than 46 chromosomes. There are two sub-categories of numerical aberration, classified as:

Monosomies – There are supposed to be two copies of a chromosome. When there is only one copy instead of two, it is called monosomy, which is found in diploid cells. In monosomy, one chromosome of a homologous pair is missing, and the number of chromosomes becomes 45 instead of 46. The homologous pair fails to separate and has only one copy, but this is rare in humans. In most cases, the unborn child dies due to lacking a chromosome. 

Turner syndrome is one of the most common disorders caused by this abnormality. Turner syndrome is a genetic disorder common amongst girls. It causes several developmental problems, such as they are shorter than ordinary girls and do not experience normal puberty; they also have health problems like kidney and heart issues. 

Trisomies – As humans have 23 pairs and 46 chromosomes in total, we either get less than that or more due to changes or mutation. Unlike in the case of monosomy, where there is a copy missing, in trisomy, there is an extra copy of the chromosome, and there are 47 instead of 46 chromosomes. 

Disorders like Down syndrome, Edward syndrome, and Patau syndrome are the most common abnormalities that come in the category of trisomy. There are three types of Down syndrome, and the most common among them is Standard Trisomy 21; wherein the mother’s egg or the father’s sperm has an extra chromosome. People with down syndrome experience delays in development and learning. 

2. Structural Aberrations 

Structural aberration is the change in the structure of the chromosome. This can occur any time, like during egg cell formation in females or sperm cell formation in males, or even during the development of the foetus. Some structural disorders can cause health problems, while others do not cause significant health issues. There are four sub-categories of structural aberrations and are classified as follows:

Deletion – It is described as a mutation where there is a deficiency. Wherein, during DNA replication, a part of a chromosome or the DNA sequence is left out or is missing. From a single base to an entire piece of the chromosome, any number of nucleotides can be missing or deleted due to the aberration. Large deletions can be fatal as there are variations due to the loss of genes. 

Duplication – This means that a part of a chromosomal segment is present in two copies or is duplicated in the chromosome’s original location, leading to extra genetic material. However, the overall number of chromosomes is normal. A chromosome contains several genes because a duplication or presence of an extra gene can cause malfunctions in those genes. Therefore, the extra gene results in improper development of the baby due to “extra instructions”. 

Inversion – The rearrangement of chromosomes in a segment chromosome reversed end-to-end is called an inversion in structural aberration. This disorder or malfunction occurs when a chromosome suffers from breakage, and rearrangement occurs within it. There are two types of inversions – pericentric and paracentric. There is a breakage point in each arm of the chromosome in centromere in the case of pericentric. 

In contrast, the arm’s breakage occurs in the paracentric case but does not include the centromere. Inversions do not cause abnormalities as long as the rearrangement is balanced with no extra or missing DNA. However, there can be increased production of abnormal chromatids in the case of heterozygous individuals. 

Translocation – As the name suggests, translocation is a location change. In terms of genetics, it often refers to the part of the chromosome transferred to another chromosome. As chromosomes carry genes, the units of heredity, translocation can cause problems within chromosomes. In this kind of abnormality, a chromosome breaks, a portion of which attaches to another chromosome. Translocation of a chromosome increases the chance of causing diseases like leukaemia, breast cancer, and down syndrome. 

Conclusion

Chromosomal aberrations are genetic disorders that can occur before conception and can happen to the sperm or the egg or even during the developmental stage of the foetus. Prevention of this kind of disorder is not known since it happens at a cellular level. Medical experts believe that a few factors like women who give birth after 35 years of age, mother and father both with a history of heavy drinking and smoking, or exposure to toxic chemicals are likely to contribute to such disorders of the offspring.