Pathogenesis of Down Syndrome – Genetics&Birth Defects Example

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"Pathogenesis of Down Syndrome" is a wonderful example of a paper on genetics and birth defects. This is a genetic condition in which a person has 47chromosomes in the cells instead of the usual 46. It causes human birth defects due to trisomer 21 and consequently, it interferes with the brain and body normal development (Prasher, 2006). It is therefore responsible for mental retardation causing mental disability, dysmorphic facial features, and other phenotypic or physical traits characteristics for the syndrome. Gene is the basic unit of DNA. The 22 pairs of chromosomes are identical in both males and females but the sex chromosome has X and Y chromosomes.

A pair of chromosomes is similar and carries the same information. However, there may be variations of that gene called alleles (Prasher, 2006). There are two types of cell divisions. The first one is mitosis and is responsible for human growth. The second one is meiosis. In this division the resulting number of chromosomes is half that of the parent. During meiosis, dysfunction occurs. Pairs of chromosomes split and go to different locations of the new cell (Prasher, 2006).

However, errors may occur during cell division. One pair may go to one spot instead of dividing. The resulting cells will have 22 and 24 chromosomes respectively. The abnormality is transferred to the offspring through reproduction (Cohen, Nadel and Madnick, 2002). DiagnosisDiagnosis of Down syndrome is done either during pregnancy or shortly after birth. At birth, a doctor checks the heart for heart murmur using a stethoscope (Cohen, Nadel, and Madnick, 2002). The murmur is due to the turbulent blood flow through the valves of the atria and ventricles.

Basically, valves cause a unidirectional blood to flow (Prasher, 2006). Murmurs occur in two ways. One is when the blood leaks back at the valve. The other occur when there is stenosis; abnormal narrowing of blood vessel near the heart. Murmur may be systolic or can be diastolic. A more prominent murmur can be felt with the palm of the hand over the heart. Presystolic murmur can be noticed if there is a murmur prior to systole (Cohen, Nadel and Madnick, 2002). Echocardiogram It is the use of a high pitch sound wave through Doppler ultrasound to produce a moving image of the heart and can produce a more detailed picture of the x-ray images.

It provides different forms of imaging. M-mode type of echo is a single dimension imaging and can display a more accurate measurement of the heart (Cherney, 1994). A two dimension echo can display a more detailed image showing a cross section of the heart, valves and the exit of the blood vessel from the heart (Cherney, 1994). There are different types of echocardiogram.

The most common is Transthoracic echocardiogram where a transducer is moved to different places around the heart to look for heart murmurs. Another one is Transesophageal Echocardiogram where a scope is inserted down through the throat. A scope has a device on the other end produce sound waves which produce an echocardiogram of the heart (Cherney, 1994). A Doppler examination is performed using an ultrasound beams to evaluate the blood flow as they flow through the heart. The signals will appear on the screen as clear colored images (Cherney, 1994).

Audible sound signal with a pulsating sound can be sampled. The doctor obtains the results from a sonographer and interprets the results. A transducer instrument; which releases high-frequency sound waves, is placed on the chest and directed at the heart. The resulting images are also taken from beneath (Cherney, 1994). The echo of sound wave is picked by a transducer and is transmitted as electrical impulses. The impulse is converted to moving pictures of two or three dimension depending on the part or side of the heart that is required to be examined and the machine being used (Cherney, 1994).

The doctor may find hard to obtain the required result for analysis. This may be due to interference from lungs, ribs, or body tissues which may prevent sound wave and echoes from producing a clear picture of the heart function. To enable a better view of the heart, a small amount of liquid is injected through an IV. The use of a special echocardiography probes for invasive test is not necessary (Cherney, 1994). Karyotyping This is done at the fetal stage.

This is done by maternal blood testing or ultrasound test. It involves the use of amniocentesis to obtain fetal cells which are cultured, stained and viewed under microscope. The number and the structural changes of the chromosomes are identified. Karyotyping involves two types of testing (Prasher, 2006). They include; Chorionic villus sampling and Amniocentesis. In Chorionic villus sampling a sample of a placenta examined at or after the 10th week of pregnancy. A needle is passed through the wall of the abdomen or bypassing a small tube through the cervix (Prasher, 2006). Amniocentesis process involves sampling of the amniotic fluid surrounding the fetus withdrawn through a long catheter needle inserted into the uterus through the abdomen.

Chromosomes of the fetus are then analyzed (Goldstein & Reynolds, 2005). The test is done after 15 weeks of gestation. An amniocentesis is an invasive type of prenatal test, which involves the use of a long catheter needle that is used to pierce the abdominal area through to the uterus to collect a small sample of amniotic fluid (Goldstein & Reynolds, 2005).

A high level ultrasound is used to help guide the doctor find a suitable pocket of fluid where a small sample of amniotic fluid will be taken (Prasher, 2006). The bone marrow is obtained through a process called bone marrow biopsy. A laboratory specialist grows a culture on a dish. The sample from the growing sample is stained and examined by use of microscope to determine the number of chromosomes. Some abnormalities can be noticed through the arrangements and the number of chromosomes. These include deletions, translocations and trisomies (Prasher, 2006). Electrocardiogram (EKG/ECG)This is a test down on the chest to electrical action of the heart.

The patient lies down and patches of electrodes are attached to several part of the body. Sometimes hair is shaved so that the patches can be attached easily to the skin (Gillberg & Coleman, 2000). The patches are connected to a machine through a wire which turns the heart's electrical signals into wavy lines. The line produced is printed on a paper which is then reviewed by a doctor (Goldstein & Reynolds, 2005).

The machine is very sensitive and therefore there should be no movements including muscle tremors such as shivering, because the results can be altered as the recording is taking place. Stress test is also done by during an exercise or under minimal stress to monitor changes in the heart (Gillberg & Coleman, 2000). X-rayThe test is done by passing electromagnetic radiation from a radiographer through the body and a computer or special type of film is used. Symptoms of Down Syndrome examined carefully. (Gillberg & Coleman, 2000). For Down Syndrome a doctor looks for Atlanto-axial instability (AAI) a condition in which the flexibility increases between the first two bones of the neck.

AAI affects the joints and especially the cervical vertebrae. The diagnosis involves physical examination and x-rays. They may be pressure on spinal cord while x-rays images of neck and the neck from side and forward are taken. In AAI there is a space between the two cervical of more than 5mm (Gillberg & Coleman, 2000). Preimplantation Genetic DioganosisThis where genetic testing is done at an embryonic stage before implantation takes place for parents who have carrier recessive disorders.

In such cases PGD technology is used. Vitro fertilization is done where the growing embryo is tested for abnormalities. The test is done before the implantation take place. Defects or abnormalities are detected before they are transferred to the uterus for development. All of these diagnoses have small or no risk of miscarriage. They can be done during pregnancy and therefore anyone can get the information early in the pregnancy. Echocardiogram has no radiation and has better pictures as compared to the x-ray test.

The type test to be taken depends on an individual. Prenatal diagnostic tests are available and they are more conclusive. than prenatal screening tests. Prenatal tests are useful in terms of identifying and treating health problems that could endanger both the mother and unborn child. Both of the chorionic villus sampling and amniocentesis can cause complications like infection, miscarriage and heavy bleeding and therefore great care should be taken when performing the test.


G. Christopher , M. Coleman, 2000, The biology of the autistic syndromes Clinics in developmental medicine, Cambridge University Press.

L. R. Cherney, 1994, Clinical management of dysphagia in adults and children Rehabilitation Institute of Chicago publication series, Aspen Publishers.

W. I. Cohen, L. Nadel, M. E. Madnick, 2002, Down syndrome: visions for the 21st century, John Wiley and Sons.

V. P. Prasher , 2006, Down Syndrome and Alzheimer's disease: biological correlates, Radcliffe Publishing.

S. G., Cecil R. Reynolds, 2005, Handbook of neurodevelopmental and genetic disorders in adults, Guilford Press.

Medical news today, What Down's syndrome - Diagnosis, retrieved on

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