Read the given passage and answer the following questions.
Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. It is important to mention here that such Mendelian disorders may be dominant or recessive.
What are the alleles that control sickle-cell anaemia?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → HbA and HbS
Sickle-cell anaemia is controlled by a single pair of alleles, HbA and HbS. HbA is the normal allele and HbS is the mutant one. Three genotypes are possible. Only HbSHbS persons show the disease. HbAHbS persons look unaffected but are carriers. In the mutant allele, a single base change (GAG to GUG) in the sixth codon of the β globin gene puts valine in place of glutamic acid. The other symbols in the options are not alleles of this gene.
NCERT Extract:
Mendelian Disorders – “… Sickle-cell anaemia : This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous). The disease is controlled by a single pair of allele, HbA and HbS. Out of the three possible genotypes only homozygous individuals for HbS (HbSHbS) show the diseased phenotype. Heterozygous (HbAHbS) individuals appear apparently unaffected but they are carrier of the disease as there is 50 per cent probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait (Figure 4.15).”