IGCSE Biology Paper-4: Specimen Questions with Answers 171 - 171 of 279

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Passage

Sickle-cell anemia is an inherited disease.

Show is a photomicrograph of some blood cells from a person who has sickle-cell anemia.

Sickle-Cell Anemia

Question 171 (1 of 4 Based on Passage)

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Some people who have sickle-cell anemia have parents who do not have sickle-cell anemia.

Explain how people with sickle-cell anemia inherit the disease.

Explanation

Sickle Cell Anemia

It occurs due to autosomal codominant (formerly considered recessive) allele Hbs present on chromosome 11. It is caused by the formation of an abnormal hemoglobin-S. In which Glutamic acid, of 6th position in β-chain is replaced by Valine amino acid due to substitution of T by A in the second position of the triplet codon (CTC) on chromosome 11. The codon CTC usually transcribed into GAG (coding for Glutamic acid) . but due to substitutions of T by A The new codon CAC is transcribed into GUG that codes Valine. It is the major effect of allele.

Missense Mutation

Other secondary effects involve formation of sickle-shaped erythrocytes during oxygen deficiency. The cells cannot pass through narrow capillaries resulting the latter become clogged. Spleen and brain get damaged. The homozygotes having only hemoglobin-S usually die before reaching maturity due to erythrocyte distortion but individuals survive.

The Sperm

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