Chromosomal alterations and human disorders

                On the last post I talked about the relevance of chromosomal alterations for evolution. Therefore, I am going to explain a little more about the effects of these events on the phenotype of humans. As I said before, modifications on chromosome structure, duplication or deletion of entire or parts of chromosomes, can alter the genome, changing the genetic sequence or/and the normal way of gene expression. Chromosomal abnormalities in humans usually lead to patterns known as syndromes, combinations of signs and symptoms (Haydon, 2008).

            As I have mentioned, there are numerical and structural chromosomal alterations. The numerical ones are known as aneuploidy in syndromes. They occur as a result from meiosis error that originates gametes without any chromosome of a chromosome pair and gametes with the both 2 chromosomes. When these gametes fertilize they provide individuals with monosomy (1 chromosome) or trisomy (3 chromosomes) for that pair (Haydon, 2008).

            The most recognized autosomal aneuploidy is the Down syndrome, or trisomy 21 (Some symptoms at the figure above). Despite individuals with this syndrome have developmental delay and heart defect, they usually live well into adult life (Haydon, 2008). Klinefelter syndrome (47, XXY) is the most common sexual trisomy, whereby men have tall stature, small testes, scant body hair and infertility (Haydon, 2008). The only monosomy compatible to life is sexual: the Turner syndrome (45, X). The symptoms are short stature and infertility. Despite, women 45, X usually have a normal life, I believe this is due to the natural activation of only one X in women, which I have explained in previous posts.

            Structural chromosomal alterations are result of breakage and subsequent reunion of chromosome regions. We can have deletions of regions, losing material, or duplication by gaining a copy of a segment at the original location on the chromosome (Theisen and Shaffer, 2010). Also, it is possible to have rearrangements such as inversions of segments after two-break events; translocations by exchange of segments between chromosomes; insertions by translocation and insertion of segments into a new region of the same or other chromosome. These events are pathogenic when they disrupt important genes (Theisen and Shaffer, 2010). An example of deletion is found at the cri du chat syndrome, in chromosome 5. Symptoms include microcephaly, low-set ears, round face, high-pitched cat-like cry, mental retardation and some health problems(Theisen and Shaffer, 2010).

I would like to talk about more syndromes but there are so many for a single post.

Thanks for reading!

REFERENCE LIST:

Theisen, A & Shaffer L G (2010). Disorders caused by chromosome abnormalities. The Application of Clinical Genetics. 2010: 3; 159–174

Haydon, J (2008). Chromosome disorders. In: Genetics in Practice: A Clinical Approach for Healthcare Practitioners (ed. Wiley). pp. 85-100. Hoboken, NJ, USA.

Image source (19/04/2014): http://www.rayur.com/chromosomal-disorder-down-syndrome-trisomy-21.html