Genetic Disorders In Prenatal Development

Welcome to the NetLetter's Genetic Disorders in Prenatal Development page. This page is here to help define and teach about some of the many genetic disorders that are in circulation today.

Each of us starts off as a single cell inside of our mothers. This cell houses our genetic information, that is, information about the characteristics we will be born with and who we might become in the world. The information that tells our bodies how to develop is in the form of genetic codes. These codes also carry the disorders that our bodies may or may not develop through life. Just because our bodies have the codes does not mean that we will automatically be affected by these disorders with in our lives. Some of us can live a very prosperous life and never know that we are infected with the disorder. In this case the person that does not show the symptoms of the disorder is usually known as a carrier. A carrier is a person that has the genotype to have the disease, but is lacking the dominant gene to infect him or her to the point that symtoms show. Before they will infect their off spring with the disorder, carriers must conceive children with another carrier or person with the full fledge disorder. These abnormalities in the genes can be determined using prenatal tests. For example, amniocentesis is a prenatal procedure in which a sample of the ammniotic fluid is withdrawn and tested to see if the fetus is suffering from any chromosome or metabolic disorders. Some abnormalities may be dormant for awhile, and only become active after the child is born.

Each year in the U.S. 100,000 to 150,000 children are born with a genetic disorder or defect. These infants account for about twenty percent of the infant death rate each year. Statistics such as these lead many want-to-be-parents to genetic counseling, which helps determine their risk for having a child with a genetice disorder.

In this web page, we hope to shed light on some genetic disorders, and help individuals to learn about the causes and cures for them.

Anencephaly

Anencephaly is a neural tube defect which occurs in the third or fourth week of prenatal development. Anencephaly involves the incomplete development of the brain, spinal cord, and the coverings that protect them due to the neural tube not closing at the caphalic (head) end. Babies born with anencephaly are born without a forebrain and a cerebrum, which often leaves the neonate blind, deaf, unconscious, and unable to feel pain.

It is believed that what a mother eats may be a factor in newborns acquiring anencephaly, but there is no scientifically known cause of the disorder. There is also no treatment for it, which leads to the newborns death within a few hours or days after birth. One possible prevention of the disorder is the intake of dietary folic acid by the mother. It is believed to drop the rate of anencephaly from 1 in 1,000 to 1 in 10,000.

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Down syndrome

Down syndrome is a genetic disease, but not necessarily an inherited disease. The most common form of down syndrome is known as Nondisjunctive, or, trisomy 21. This form of Down syndrome affects 95% of the Down syndrome population and occurs because of a faulty cell division. This results in and extra number 21 chromosome.

Translocation is another form of Down syndrome which affects only 3% of the Down syndrome population. Translocation occurs when a portion of the 21st chromosome breaks off during cell division and attaches to a different chromosome. Although in this case there are the normal 46 chromosomes, those with translocation still have the traits of Down syndrome.

In people with Down syndrome, about 2 % have what is called mosaicism. With mosaicism the 21st chromosome doesn't separate until after fertilization which results in having the extra number 21 chromosome in only some of the cells.

More about Down Syndrome

Tay-Sachs disease

Tay-Sachs disease is a hereditary disorder of the nervous system that causes severe brain damage, enlargement of the head, convulsions, blindness, deafness, lack of energy, and eventually death. When children are about six months old they begin to show symptoms, beginning with a reddish dot on the retina of the eye.

Tay-Sachs disease occurs in children who have too little of the enzyme hexosaminidase. This controls the amount of ganglioside that accumulates in the nerve cells. Ganglioside is a lipid produced by normal cell reproduction. Nerve cells that store too much of the lipid become swollen and die. Brain damage can result from a large number of the damaged or dead cells.

Symptoms of the disease were first recorded in the 1800's by Waren Tay, a British physician, and Bernard Sachs, an American physician. In 1969 researchers discovered the disease was caused by a lack of hexosaminidase. They then developed a test to determine hexosaminidase activity in blood and tissues to reveal whether an unborn child has the disease. Tests were also developed to determine if an adult is a carrier of the trait. Both parents must be carriers of the trait for the child to inherit the chance of being infected fully by Tay-Sachs disease.

Thus, one out of every four children born by parents that are carriers will have full Tay-Sachs diseas, and half of the children that are born will be carriers of the disease.

More about Tay-Sachs disease

PKU

Most commonly called PKU, Phenylketonuria is a genetic disease that, if not detected and treated within the first few weeks of life, leads to mental retardation and other neurological problems. Those with PKU must adhere to a very strict diet beginning at birth in order to live a normal and healthy life. This diet is low in protein (including breast milk) because protein has very high amounts of phenylaline which is the part of protein that people with PKU cannot process. Those with PKU need to adhere to the special diet for most, if not all, of their life.

In the past children with PKU were taken off of the diet around the age of 6 because it was believed that once the brain was fully developed no more damage could be done by the disease. It is now known, however, that being taken off the diet at any time can result in, among others, learning disabilities, behavior problems, and drops in IQ points. It is important for those with PKU to stick to the diet specified in the individual's PKU treatment program.

Because PKU is a recessive gene it takes 2 carriers of the gene to produce an offspring with PKU. There is a 25% chance that the offspring of 2 carriers will have PKU. The occurrence of children with PKU in America is about 1 in every 14,000 live births, while the incidence of being a carrier is 1 in 50.

Women with PKU who are planning on having children must watch the level of phenylalanine intake in their diet because a high level of phenylalanine in the blood stream may harm the unborn child. These women should follow a low protein diet because it reduces the risk of mental retardation and other serious problems for the child.

More about PKU

Thalassemia

Thalassemia is an inherited red blood cell disease. It is primarily due to the inability of the red blood cells to produce hemoglobin which carries oxygen to the body. Without hemoglobin, the body's organs do not get the oxygen they need to sustain normal life. The red blood cells become abnormally thin and produce pale skin, tiredness, irritability, and loss of appetite. Most people with thalassemia are classified as having jaundiced. The heart will become enlarged along with the liver and spleen. The disease also weakens bones, especially those in the face. Symptoms of the disease are present at birth or generally appear within six months after.

Thalassemia cannot be cured, but can be treated by blood transfusion and antibiotics. This relieves the symptoms but causes an iron excess in some organs. This iron excess often leads to diabetes and heart failure. Scientists are currently developing drugs to remove the iron deposits from the body.

More about Thalassemia

Sickle-Cell Anemia

Sickle-Cell Anemia is another inherited genetic abnormality in red blood cells. Hemoglobin, which carries oxygen throughout the body, is not properly distributed through out the red blood cells. The infected blood cells take the shape of a sickle or half moon. The infected cells have a short survival time and make the infected person anemic. The disease is also characterized by acute extremity pain.

Sickle-cell anemia also has a symptom less form. This occurs when the tendency to form sickled-cells is present, but there is not enough of the abnormal sickle-cell hemoglobin present in the red blood cells to produce symptoms. An infected child is said to have sickle-cell traits or is known to be a cell carrier. If a child's parents have sickle-cell traits, then the child may develop a sudden and severe form of sickle-cell anemia.

Sickle-cell anemia is caused by an abnormal looping bond between two amino acids which causes the hemoglobin to form in slender strands. Such abnormal hemoglobin strands strung together cause the elongation of the red blood cells. These abnormal bonds form when the oxygen level of the blood stream falls, and the elongated red blood cells that result accumulate in smaller capillaries, especially those in the extremities of the body.

More about Sickle-cell Anemia

References used to build this site can be found in:

Memmler, R. and Rada, R. (1970). The Human Body In Health And Disease , J.B. Lippincott Co.
Fishbein, Morris M.D. (1979). The New Illustrated Medical and Health Encyclopedia, H.S. Stuttman Co., Inc.
Tortora, G. and Grabowski, S. (1996). Principles Of Anatomy and Physiology, Harper Collins College Publishers
Encylcopedia Americana. (1973). American Corporation; Lexington Ave, New York, Vol 24.
The World Book Encyclopedia. (1986). World Book Inc.; Chicago, Illinois,Vol. 18
Baker, G. and Allen, G. (1980) The Study of Biology; Addison-Wesley Publishing Co; Reading, Mass.
Thomas, Clayton M.D. (1972). Taber's Cyclopedic Medical Dictionary; F.A. Davis Company, Philadelphia, Pa.

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