9.7.2 Pleiotropy: Multiple Phenotypic Effects

Question 1

peiotropy

Answer 1

• Review: In polygenic inheritance, many genes affect a single phenotypic trait.
  • In pleiotropy, one gene affects many phenotypic traits. Pleiotropy commonly occurs in human genetic disorders.
  • Examples of disorders with pleiotropic effects include:
  · sickle cell anemia
  · cystic fibrosis
  · Tay-Sachs

Question 2

note

Answer 2

• Review: In polygenic inheritance, many genes affect a
  single phenotypic trait.
• In pleiotropy, one gene affects many phenotypic traits. This phenomenon commonly occurs in human genetic disorders.
• Question: Why do genetic disorders run in ethnic groups?
• Different genetic disorders are characteristic of different ethnic groups that have been genetically isolated until recently. As more and more ethnic groups intermarry, allelic frequencies for disorders should decrease.
• Recessive versus dominantly inherited disorders. A
  dominant allele is fully expressed in a heterozygote. A
  recessive allele is completely masked in a heterozygote.
• A recessively inherited disorder occurs when individuals with two defective alleles express the disease (hh). The alleles result in a nonfunctioning enzyme. Heterozygotes (Hh) have one normal enzyme and do not display the disease.
• A dominantly inherited disorder occurs when one defective allele results in a phenotypic effect (H_). Heterozygotes (Dd) have one defective allele and display the disease.
• An example of a dominantly inherited disorder is
  Huntington’s disease. A dominant allele (H_) causes
  deterioration of the nervous system. Individuals with
  Huntington’s disease generally develop symptoms between the ages of 35 and 45, and the disease is eventually fatal.

Question 3

sickle cell anemia

Answer 3

• One example of pleiotropy involves the disease
  sickle cell anemia. The disease occurs in 1/400
  African-Americans and is caused by a defect in the
  hemoglobin protein of red blood cells. The H allele results in production of normal hemoglobin. The defective h allele causes red blood cells to sickle. Heterozygous Hh individuals have at least one normal allele and are phenotypically normal (although a few do show signs of the disease). Individuals that are homozygous for the h allele have sickle cell anemia.
• Sickled red blood cells cause a “domino effect” of other problems. The abnormal cells accumulate in the spleen, causing physical weakness. The break down of these red blood cells can contribute to anemia, heart failure, and physical weakness. The abnormal cells clump and clog the blood vessels, which can lead to heart failure and brain damage

Question 4

cystic fibrosis

Answer 4

• caused by a defect in a transport protein of
  the cell membrane. It affects 1/2500 people of European descent.
• Proteins play an important role in keeping the linings of the lungs and other organs hydrated. Normally the protein secretes chloride ions (Cl-) outside of the cell. As the negative ion concentration increases, water leaves the cell by osmosis and hydrates the lining. Defective proteins do not transport chloride, and consequently the linings of the lungs dry out.
• The defect has a number of other pleiotropic effects. The airways of cystic fibrosis patients become obstructed with thick mucus and are vulnerable to bacterial infections. Consequently, the general health of the person declines

Question 5

Tay-Sachs

Answer 5

• prevalent among people of eastern European
  Jewish origin. The disease manifests itself during infancy. Tay-Sachs is caused by a defect in an enzyme that breaks down lipids in the brain. As the lipids in the brain accumulate, brain cells die. The disease causes mental retardation, failure to develop control of the muscles, and blindness. Children with this disease usually die in early childhood.

Question 6

In pleiotropy,

Answer 6

• a single gene causes multiple phenotypic effects.

Question 7

The condition of albinism results from an inability to deposit melanin in the tissues. A recessive allele is responsible for the condition. A woman who does not display albinism marries a man who is similarly unaffected. They have one albino child and three unaffected children. What are the genotypes of the mother and father?

Answer 7

• Aa × Aa

Question 8

The disease, cystic fibrosis, has been determined to be the most common lethal genetic disease in the United States. The disease is caused by a defect in a membrane protein that transports chloride ions between certain cells and extracellular fluid. Which of the following represents the most likely pleiotropic effects?

Answer 8

• Mucus build-up in the pancreas, lungs, and digestive tract.

Question 9

Which of the following explains why some genetic disorders are more common within one race or ethnic group?

Answer 9

• Until recently, ethnic and racial groups have been genetically isolated.

Question 10

All of the following exhibit pleiotropism except:

Answer 10

• Bombay phenotype

Question 11

Which statement best describes a lethal recessive genetic disorder?

Answer 11

• A lethal recessive disorder can cause death in an individual who does not receive medical care.