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Genetics Concepts

Terms in this set (13)


Huntington's Disease (HD) is a brain disorder that affects a person's ability to think, talk, and move.

The disease destroys cells in the basal ganglia, the part of the brain that controls movement, emotion, and cognitive ability. HD is caused by a mutation in a gene on chromosome 4. The job of its protein product, huntingtin, is to direct the delivery of small packages (vesicles containing important molecules) to the outside of the cell. Normally, the coding region of this gene contains the DNA sequence "CAG" repeated again and again. The number of times this triplet is repeated varies from person to person, ranging from 10 to 26 times. People with HD have an abnormally high number of these CAG triplets, approximately 40 or more. This likely disrupts the function of the gene's protein product, but how the expansion of the CAG repeat causes disease is unknown. Somehow the brain cells of HD patients accumulate clumps of protein that become toxic, resulting in cell death. Some patients lose more than 25% of their brain cells before they die.


Huntington's disease affects the part of the brain that controls thinking, emotion, and movement. Most people who have the disease start to see symptoms between the ages of 30 and 50 (but symptoms can appear earlier or later in life). The disease gets worse over time.

Symptoms include poor memory, depression and/or mood swings, lack of coordination, twitching or other uncontrolled movements, and difficulty walking, speaking, and/or swallowing. In the late stages of the disease, a person will need help doing even simple tasks, like getting dressed.


Treatments do not slow the progression of the disease, but they can help make the patient more comfortable. Medications ease feelings of depression and anxiety; others control involuntary movements. Physical or speech therapy helps HD patients lead more normal lives.


The disease was named for Dr. George Huntington, who first described it in 1872.

In the United States, about 1 in every 30,000 people has Huntington's disease.

Sickle cell disease is a disorder that affects the red blood cells, which use a protein called hemoglobin to transport oxygen from the lungs to the rest of the body. Normally, red blood cells are round and flexible so they can travel freely through the narrow blood vessels.

The hemoglobin molecule has two parts: an alpha and a beta. Patients with sickle cell disease have a mutation in a gene on chromosome 11 that codes for the beta subunit of the hemoglobin protein. As a result, hemoglobin molecules don't form properly, causing red blood cells to be rigid and have a concave shape (like a sickle used to cut wheat). These irregularly shaped cells get stuck in the blood vessels and are unable to transport oxygen effectively, causing pain and damage to the organs.


Sickle cell disease prevents oxygen from reaching the spleen, liver, kidneys, lungs, heart, or other organs, causing a lot of damage. Without oxygen, the cells that make up these organs die. For example, the spleen is often destroyed in these patients, leading to some loss of immune function. As a result, these patients often experience frequent infections.

The red blood cells of patients with sickle cell disease don't live as long as healthy red blood cells. So people with this disorder often have low red blood cell counts (anemia), which is why this disease is commonly referred to as sickle cell anemia.

When sickle-shaped red blood cells get stuck in blood vessels, patients can have episodes of pain called crises. Other symptoms include delayed growth, strokes, and jaundice (yellowish skin and eyes because of liver damage).

Organ damage and other complications often shorten patients lives by about 30 years.


Babies and young children with sickle cell disease must take a daily dose of penicillin to prevent potentially deadly infections. Patients also take folic acid, which helps build new red blood cells.

Doctors advise people with sickle cell disease to get plenty of rest, drink lots of water, and avoid too much physical activity.

Blood transfusions are commonly done to provide a patient with healthy red blood cells.

People with more severe cases of the disease can be treated with a bone marrow transplant. This procedure provides the patient with healthy blood stem cells from a donor, ideally a sibling.


Unlike normal red blood cells, which can live for 120 days, sickle-shaped cells live only 10 to 20 days.

In the United States, the disease most commonly affects African-Americans. About 1 out of every 500 African-American babies born in the United States has sickle cell anemia.

Sickle cell disease is most common among people from Africa, India, the Caribbean, the Middle East, and the Mediterranean. The high prevalence of the defective gene in these regions may be due to the fact that carriers of a mutation in the beta-subunit of hemoglobin are more resistant to malaria. Malaria is a disease caused by a parasite transmitted by infected mosquitoes. The sometimes fatal disease, which causes recurring chills and fever, is common in hot climates.

Down syndrome is a developmental disorder caused by an extra copy of chromosome 21 (which is why the disorder is also called "trisomy 21"). Having an extra copy of this chromosome means that individuals have three copies of each of its genes instead of two, making it difficult for cells to properly control how much protein is made. Producing too much or too little protein can have serious consequences. Genes on chromosome 21 that specifically contribute to the various symptoms of Down syndrome are now being identified.


People with Down syndrome have distinct facial features: a flat face, a small broad nose, abnormally shaped ears, a large tongue, and upward-slanting eyes with small folds of skin in the corners.

People with Down syndrome have an increased risk of developing a number of medically significant problems, including respiratory infections, gastrointestinal tract obstruction (blocked digestive tract), leukemia, heart defects, hearing loss, hypothyroidism, and eye abnormalities. They also have moderate to severe intellectual disability; children with Down syndrome usually develop more slowly than their peers and have trouble learning to walk, talk, and take care of themselves.

Because of these medical problems, most people with Down syndrome have a decreased life expectancy. About half live to be 50 years of age.


There is no cure for Down syndrome. But physical therapy and/or speech therapy can help people with the disorder develop more normally. Screening for common medical problems associated with the disorder, followed by corrective surgery, can often improve quality of life. Moreover, enriched environments significantly increase children's capacity to learn and lead meaningful lives.


Down syndrome is really the only trisomy compatible with life. Only two other trisomies have been observed in babies born alive (trisomies 13 and 18), but babies born with these trisomies have only a 5% chance of surviving longer than one year.

In 90% of Trisomy 21 cases, the additional chromosome comes from the mother's egg rather than the father's sperm.

Down syndrome is the most common genetic disorder caused by a chromosomal abnormality. It affects 1 out of every 800 to 1,000 babies.

Down syndrome was originally described in 1866 by John Langdon Down. It wasn't until 1959 that French doctor Jerome Lejeune discovered it was caused by the inheritance of an extra chromosome 21.

Turner syndrome is caused by a missing or incomplete X chromosome. People who have Turner syndrome develop as females. Some of the genes on the X chromosome are involved in growth and sexual development, which is why girls with the disorder are shorter than normal and have incompletely developed sexual characteristics.


Turner syndrome affects growth and sexual development. Girls with this disorder are shorter than normal and may not start puberty when they should. This is because the ovaries (which produce eggs, as well as the sex hormones estrogen and progesterone) don't develop properly.

Turner syndrome usually does not affect intelligence. Common physical symptoms of Turner Syndrome include a stocky build, arms that turn out slightly at the elbow, a receding lower jaw, a short webbed neck, and a low hairline at the back of the neck.

Medical symptoms can include lymphedema (swelling of hands and feet), heart and/or kidney defects, high blood pressure, and infertility (inability to have children).


Hormone replacement therapy is the best way to treat this disorder. Teenagers are treated with growth hormone to help them reach a normal height. They may also be given low doses of androgens (male hormones that females also produce in small quantities) to increase height and encourage hair and muscle growth. Some patients may take the female hormone estrogen to promote sexual development.


Turner Syndrome affects 60,000 females in the United States. This disorder is seen in 1 of every 2000 to 2500 baby girls, with about 800 new cases diagnosed each year.

In 75-80% of cases, the single X chromosome comes from the mother's egg; the father's sperm that fertilizes the egg is missing its sex chromosome.

Turner syndrome is named for Dr. Henry Turner, who in 1938 published a report describing the disorder.

The average height of an untreated woman with Turner syndrome is 4 feet 8 inches.

A female fetus (normally XX) that is missing one of its X chromosomes can survive, but a male fetus (normally XY) cannot. The X chromosome is a long DNA molecule with many genes that are needed for cells to function; it is essential for life. In contrast, the Y chromosome carries few genes and is not essential for life.