Cytogenetics and Cancer Research

Down Syndrome is a popular topic in my Cytogenetics and Cancer Research blog now. I decide to write another post about the Down Syndrome to grab the attention from my readers. As I mentioned in my last post, Down Syndrome is caused by the non-disjunction of the chromosome during the meiosis. However, little is known about the causes for non-disjunction to occur as well as Down Syndrome in term of Cytogenetics. There are many hypotheses have been put forward to explain about the non-disjunction.
 

1. Enhanced Selective Miscarriage of Trisomy 21 Conceptions by Older Mothers

Analysis of the data shows that Trisomy 21 occurs in about 1 per 200 2000 of the births to mothers aged 18-24. The rate gradually increases in frequency to 1 per 300 births to mothers between 25 and 35. The rate continues to increase to 1 per 100 births to mothers at age 45 or older. Therefore, people start to come out with the hypothesis that is enhanced selective miscarriage of 21 trisomic conceptions by older mothers. This hypothesis claims that the increase of mother’s age is directly proportional to the enhanced selective miscarriage of trisomy 21. The non-disjunction seems to occur more frequently for older mother. During the meiosis, the older female’s gamete cell will miscarry two chromosome 21. Fertilization between this abnormal ovum with the sperm will cause the Down Syndrome.
 

 

2. Less Effectiveness for Oocyte Selection in Older Mothers

Another hypothesis is the decrease of effectiveness for oocyte selection in older mother. Oocyte selection is the process that determines which oocyte can enter the next stage of development. Only the normal oocyte will be selected and the abnormal oocyte will be eliminated. If the effectiveness of oocyte selection decrease because of the older age, the abnormal oocyte might be selected. This will cause the non-disjunction occur during the meiosis.
 

3. Sperm Aging Hypothesis

Some scientists raised up the sperm aging hypothesis claiming that the non-disjunction occurs because of the influence of sperm aging in male genital tract.
 

4. Chiasma Hormonal Hypothesis

Some scientists prove that there is an interaction between the hormonally governed rate of meiosis and the timing of chiasma terminalisation. The changing hormone levels during the menstrual cycle not only stimulate recommencement of meiosis in ovum, but also control the rate of meiosis through a limiting substance. The hormone levels and the length of the cycle will change with advancing age of the mother. Therefore the meiosis will slow down and chiasma frequencies decline. As we know, chiasmata is important for the meiotic chromosome bivalents to held together during meiosis. It will cause the premature separation during terminalisation. This can lead to Down Syndrome.
 

5. Consanguineous Parents Hypothesis

Some people said that the Down Syndrome is related to the consanguineous parents. Consanguineous parents are the parents who are blood-related.
 

In conclusion, there are many other hypotheses to explain about the causes of Down Syndrome. However, none of which was so far fully convincing. But what we need to aware of is the advancing age of woman. I will recommend you to have your child before 30 years old for woman to avoid from the risk of having Down Syndrome baby.
 

Do you have any knowledge about the cause of Down Syndrome? Share with us.

It is quite a long time I do not write about the genetic disorder in term of Cytogenetics. This time I would like to introduce the Down Syndrome, the most common genetic disorder in cytogenetic level.

Down syndrome is a common genetic chromosomal syndrome among the population in the world. It is about 1 in 800 liveborns in the population with Down syndrome. This syndrome starts to be described by a physician named John Langdon Down who published an article in 1866. He stated that there are some children with common characteristics but distinct from other children with mental retardation. He described this syndrome as “Mongoloids”. He used this unfortunate name just because of those children looked like people from Mongolia. The “Mongoloids” was dropped from scientific use since 1960s to stop the ethic insult.

In cytogenetics, about 95% of all patients with Down syndrome have a 47, +21 karyotype. Among these cases there is a small group with familial translocation involving a chromosome 21 and another chromosome with balanced rearrangement. There are some very rare instances of direct transmission of the additional 21 from a Down syndrome mother or father to a Down syndrome child. This is a kind of birth defect syndrome just like Trisomy X and Mosaicism. There are many hypotheses explain how the Down Syndrome occur.

Down syndrome patient looks almost alike to each other. We can simply identify the Down syndrome patient by just looking to their physical outlook. However, a confident clinical diagnosis might be difficult early after birth, especially in prematures. Some of the useful diagnostic signs are brachycephaly (flat-head), small ears, Brushfield spots (brown spots on the periphery of the iris) and low iliac and acetabular index in pelvic radiographs. Congenital malformations are frequent in Down syndrome patients too. Thyroid dysfunction is also significantly associated with Down syndrome and might be the cause for developmental delay. Mentally retardation is the most common feature among the Down syndrome patients.

Generally, female menarche occurs at normal time and pregnancies are common among the Down syndrome patients. However, there is hypogenitalism and hypogonadism among the male patients. Therefore, the male with Down syndrome usually is infertile.

In conclusion, Down syndrome patients need to be taken care as there are various specific problems throughout their life. I will write more about the guideline for optimal medical care on these Down syndrome patients later.

Have you seen any Down syndrome patient? Be patient with them.