Genetic disorders that can be revealed by kariotype analysis.

The interesting fact is that it is estimated that chromosomal aberrations occur in some 7.5% of all zygotes. Most of them is lethal though, as e.g. 60% of miscarriages occurring spontaneously in first three months of pregnancy are caused by chromosomal aberrations. Only 0.5% of newborns show such genetic disorders.

        The most common general symptoms of aneuploidy are general development retardation, many developmental defects. Generally in trisomy the symptoms are not so severe as in monosomy. It seems that it is better to have additional chromosomal material than have less of it. That is best shown by the fact that monosomies of any autosomal chromosomes are lethal and also the monosomy of X chromosome (which will be described later) is usually lethal in early foetal development.

There are few genetic diseases that are results of aneuploidy:

1. Down’s Syndrome – it is the most common of trisomies (1/650 newborns) and perhaps also the most well-known. It is caused by trisomy in the 21st pair of chromosomes∗. Among the symptoms is always mental retardation to different extent. Other phenotypic features are various – they occur to different extent, be present, or not at all. Some of the most common phenotypic features include: big distance between I and II toes, open mouth, short neck, short and broad palms, big tongue, heart defects.    
2. Edwards’ Syndrome – occurs in 1/3500 newborns and is caused by trisomy in the 18th pair of chromosomes. Some more common symptoms include: severe mental retardation, low birth weight, deformations of skull, feet and arms, eye defects,  anomalies of skeleton and internal organs. 95% of foetuses with Edwards’ Syndrome are miscarriaged, the rest, which is born alive, dies usually in infancy.    
3. Patau’s Syndrome – occurs in 1/5000 newborns and is caused by trisomy in the 13th pair of chromosomes. The common symptoms are: rozszczep wargi i podniebienia, defects of eyes, ears, heart and kidneys, limb anomalies. High infant mortality is also the case.  
4. Klinefelter’s Syndrome – occurs in 1/1000 newborn boys. Kariotypes may come in a few forms – 47, XXY is the most common, but also 48, XXXY and 49,XXXXY, occur. Among the symptoms are: secondary testis disappearance, no spermatogenesis, low testosterone level, secondary sex characteristics not fully developed, gynaecomasty, mental retardation.
5. Turner’s Syndrome is an example of a disorder caused by monosomy of sex chromosome which means the kariotype is 45,XO. The frequency of appearance is 1/2500 newborn girls. In Poland, there are about 9000 women with the disease and 100 are born with it annually. The characteristic features of girls suffering from the disorder are: too much skin on the neck, lymphatic oedema of limbs, being short height, reproductive organs not fully developed, numerous defects of internal organs. A girl treated with growth hormone, taking it everyday for several years, can eventually reach the height of a small normal woman as an adult.

It is worth noting that the risk of giving birth to a child showing aneuploidy rises with the age of the mother. Therefore, if the mother is over 35, or genetic disorders occurred in close family of either the father or the mother, the prospective parents are advised to seek help from genetic counselling.  

Bibliography:

1. Goździcka-Józefiak,A., Bobowicz, M.A.,& Kędzia, H., Genetyka molekularna i biochemia wybranych chorób u ludzi, Wydawnictwo Naukowe UAM, Poznań 2001
2. Badania molekularne i cytogenetyczne w medycynie – elementy genetyki klinicznej, pod red. J. Bal, wyd. Springer PWN, Warszawa 1998    
3. Winter P.C., Hickey G.I., & Fletcher, H.L.,  Instant notes in Genetics, BIOS Scientific Publishers Ltd., 1998.