9.7.3 Sickle Cell Anemia: The Case against Dominant and Recessive

Question 1

note

Answer 1

• Dominant/recessive relationships can become more complex when you look at the molecular level.
• A recessive allele that deforms the hemoglobin protein causes sickle cell anemia.
• The type of inheritance that governs sickle cell anemia can appear different at different levels of organization.
- Establishing dominant/recessive relationships can become complex when you look at the molecular level.
- An amino acid substitution in one of the four chains of the hemoglobin molecule causes sickle cell anemia. The
substitution changes the shape of the protein, causing the cell to take on a sickle shape.
- Inheritance patterns can become complicated at the molecular level.
- Let “S” stand for the allele that causes the amino acid
substitution and “N” stand for the normal allele. In the
heterozygote, both hemoglobin genes are expressed at the molecular level, a situation of codominance. But the
hemoglobin molecule also displays a mixture of the defective and normal traits, a case for blending inheritance. At the level of the red blood cell, the normal allele dominates, meaning that the alleles are related in a dominant/recessive manner. Individuals that are SS die, meaning that it is a lethal recessive. Finally, the heterozygous condition is considered over-dominant with respect to survivorship, because individuals are protected by a different disease—malaria.

Question 2

Which statement best describes overdominance?

Answer 2

• A heterozygote for a particular trait has a selective advantage over either homozygote.

Question 3

Why can the sickle cell allele and the normal allele be considered codominant on the cellular level?

Answer 3

• When both alleles are present, the individual has both normal and sickled red blood cells.

Question 4

Which of the following statements concerning sickle cell anemia is true?

Answer 4

• Sickle cell anemia occurs as a result of a change in one amino acid of the beta chains of hemoglobin.

Question 5

In tropical Africa, heterozygotes for the sickle cell trait have an advantage over noncarriers.

Answer 5

• true

Question 6

What does the phrase “lethal recessive” mean in relation to sickle cell anemia?

Answer 6

• Without medical treatment sickle cell anemia is a fatal disorder in the homozygous condition.

Question 7

The disorder sickle cell anemia is caused by a recessive allele. Individuals of the genotype HbAHbA are unaffected. Heterozygotes, HbA HbS, show few outward symptoms, and homozygotes HbSHbS have sickle cell anemia. Suppose a woman whose mother is unaffected and father has sickle cell anemia marries a man who is heterozygous for the allele. What is the probability that the couple’s first child will be homozygous for the HbS allele?

Answer 7

• 1/4