Comprehension
Mendelian Disorders
Read the following passage carefully and answer the given questions.
Genetic disorders may be grouped into two categories – Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in a single gene. These disorders are transmitted to the offspring on the same lines as in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. The Mendelian disorders may be dominant or recessive. By pedigree analysis, one can easily understand whether the trait in question is dominant or recessive. Similarly, the trait may also be linked to the sex chromosome or an autosome.
Question: 1
Which one of the following diseases is an autosomal dominant disorder?
Which one of the following diseases is an autosomal dominant disorder?
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Genetic Inheritance Summary:
$\bullet$ Autosomal Dominant: Myotonic dystrophy, Huntington's chorea.
$\bullet$ Autosomal Recessive: Sickle-cell anaemia, Phenylketonuria, Thalassemia, Cystic fibrosis.
$\bullet$ X-linked Recessive: Haemophilia, Colour blindness.
$\bullet$ Autosomal Dominant: Myotonic dystrophy, Huntington's chorea.
$\bullet$ Autosomal Recessive: Sickle-cell anaemia, Phenylketonuria, Thalassemia, Cystic fibrosis.
$\bullet$ X-linked Recessive: Haemophilia, Colour blindness.
Updated On: Sep 4, 2026
- Sickle-cell anaemia
- Myotonic dystrophy
- Phenylketonuria
- Thalassemia
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The Correct Option is B
Solution and Explanation
Concept:
Mendelian disorders follow predictable inheritance patterns (autosomal dominant, autosomal recessive, X-linked recessive, or X-linked dominant) depending on the chromosomal location of the gene and the dominance relationship of the mutant allele.
Step 1: Classifying the Inheritance Patterns of the Listed Disorders:
- Myotonic dystrophy: An autosomal dominant disorder characterized by progressive muscle wasting, weakness, and delayed muscle relaxation after contraction. Because it is dominant, a single copy of the mutant allele on an autosome is sufficient to manifest the disease phenotype.
- Sickle-cell anaemia: An autosomal recessive disorder caused by a mutation in the $HBB$ gene on chromosome 11.
- Phenylketonuria (PKU): An autosomal recessive metabolic disorder caused by a mutation in the phenylalanine hydroxylase gene on chromosome 12.
- Thalassemia: An autosomal recessive blood disorder causing reduced synthesis of globin polypeptide chains.
Step 2: Final Selection:
Among the given options, Myotonic dystrophy is the autosomal dominant disorder.
Hence, Option (B) is correct.
Mendelian disorders follow predictable inheritance patterns (autosomal dominant, autosomal recessive, X-linked recessive, or X-linked dominant) depending on the chromosomal location of the gene and the dominance relationship of the mutant allele.
Step 1: Classifying the Inheritance Patterns of the Listed Disorders:
- Myotonic dystrophy: An autosomal dominant disorder characterized by progressive muscle wasting, weakness, and delayed muscle relaxation after contraction. Because it is dominant, a single copy of the mutant allele on an autosome is sufficient to manifest the disease phenotype.
- Sickle-cell anaemia: An autosomal recessive disorder caused by a mutation in the $HBB$ gene on chromosome 11.
- Phenylketonuria (PKU): An autosomal recessive metabolic disorder caused by a mutation in the phenylalanine hydroxylase gene on chromosome 12.
- Thalassemia: An autosomal recessive blood disorder causing reduced synthesis of globin polypeptide chains.
Step 2: Final Selection:
Among the given options, Myotonic dystrophy is the autosomal dominant disorder.
Hence, Option (B) is correct.
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Question: 2
Which one of the following substitutions is a cause of sickle-cell anaemia?
Which one of the following substitutions is a cause of sickle-cell anaemia?
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Sickle-cell Point Mutation:
$\bullet$ DNA: $GAG \rightarrow GTG$
$\bullet$ mRNA: $GAG \rightarrow GUG$
$\bullet$ Amino Acid (Position 6 of $\beta$-chain): $\text{Glutamic acid (Glu)} \rightarrow \text{Valine (Val)}$.
$\bullet$ DNA: $GAG \rightarrow GTG$
$\bullet$ mRNA: $GAG \rightarrow GUG$
$\bullet$ Amino Acid (Position 6 of $\beta$-chain): $\text{Glutamic acid (Glu)} \rightarrow \text{Valine (Val)}$.
Updated On: Sep 4, 2026
- Substitution of valine by aspartic acid at the sixth position of the beta globin chain.
- Substitution of glutamic acid by alanine at the sixth position of the beta globin chain.
- Substitution of glutamic acid by valine at the sixth position of the beta globin chain.
- Substitution of glycine by methionine at the sixth position of the beta globin chain.
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The Correct Option is C
Solution and Explanation
Concept:
Sickle-cell anaemia is a classical example of a molecular point mutation (single base substitution) that alters the primary structure of a protein and changes its quaternary behavior under hypoxic conditions.
Step 1: Molecular Basis of Sickle-Cell Anaemia:
- In the normal $\beta$-globin gene ($Hb^A$), the sixth codon on the sense strand of DNA is GAG, which transcribes into mRNA as GAG, coding for the hydrophilic amino acid Glutamic acid (Glu).
- In the mutant sickle-cell gene ($Hb^S$), a point mutation substitutes a single base pair: adenine ($A$) is replaced by thymine ($T$) in DNA ($GAG \rightarrow GTG$).
- The resulting mutant mRNA codon becomes GUG, which translates into the hydrophobic amino acid Valine (Val) at the sixth position of the $\beta$-globin polypeptide chain.
- Under low oxygen tension, this substitution causes mutant hemoglobin ($Hb^S$) molecules to polymerize into long insoluble crystalline fibers, distorting biconcave red blood cells into rigid, sickle-shaped erythrocytes.
Step 2: Conclusion:
Sickle-cell anaemia is caused by the substitution of Glutamic acid by Valine at the sixth position of the beta-globin chain.
Hence, Option (C) is the correct statement.
Sickle-cell anaemia is a classical example of a molecular point mutation (single base substitution) that alters the primary structure of a protein and changes its quaternary behavior under hypoxic conditions.
Step 1: Molecular Basis of Sickle-Cell Anaemia:
- In the normal $\beta$-globin gene ($Hb^A$), the sixth codon on the sense strand of DNA is GAG, which transcribes into mRNA as GAG, coding for the hydrophilic amino acid Glutamic acid (Glu).
- In the mutant sickle-cell gene ($Hb^S$), a point mutation substitutes a single base pair: adenine ($A$) is replaced by thymine ($T$) in DNA ($GAG \rightarrow GTG$).
- The resulting mutant mRNA codon becomes GUG, which translates into the hydrophobic amino acid Valine (Val) at the sixth position of the $\beta$-globin polypeptide chain.
- Under low oxygen tension, this substitution causes mutant hemoglobin ($Hb^S$) molecules to polymerize into long insoluble crystalline fibers, distorting biconcave red blood cells into rigid, sickle-shaped erythrocytes.
Step 2: Conclusion:
Sickle-cell anaemia is caused by the substitution of Glutamic acid by Valine at the sixth position of the beta-globin chain.
Hence, Option (C) is the correct statement.
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Question: 3
Which one of the followings is an example of an inborn error of metabolism and inherited as autosomal recessive trait?
Which one of the followings is an example of an inborn error of metabolism and inherited as autosomal recessive trait?
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Phenylketonuria (PKU) Highlights:
$\bullet$ Autosomal recessive inborn error of metabolism.
$\bullet$ Enzyme deficiency: Phenylalanine hydroxylase.
$\bullet$ Metabolic block: Phenylalanine $\not\rightarrow$ Tyrosine.
$\bullet$ Results in: Phenylpyruvate accumulation, mental retardation, excretion in urine.
$\bullet$ Autosomal recessive inborn error of metabolism.
$\bullet$ Enzyme deficiency: Phenylalanine hydroxylase.
$\bullet$ Metabolic block: Phenylalanine $\not\rightarrow$ Tyrosine.
$\bullet$ Results in: Phenylpyruvate accumulation, mental retardation, excretion in urine.
Updated On: Sep 4, 2026
- Phenylketonuria
- Thalassemia
- Haemophilia
- Colour Blindness
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The Correct Option is A
Solution and Explanation
Concept:
Inborn errors of metabolism are congenital metabolic disorders caused by single-gene defects that result in deficient or non-functional enzymes along essential metabolic pathways.
Step 1: Biochemical Pathology of Phenylketonuria (PKU):
Phenylketonuria is an autosomal recessive inborn error of metabolism.
The affected individual lacks the functional hepatic enzyme phenylalanine hydroxylase.
Normally, this enzyme converts the dietary essential amino acid phenylalanine into tyrosine.
Due to enzyme deficiency, phenylalanine accumulates in the body and is converted into phenylpyruvic acid and related keto-derivatives.
Accumulation of these toxic metabolites in the brain causes severe mental retardation and impaired neurological development.
Excess phenylpyruvic acid is also excreted in the urine because of poor renal reabsorption.
Step 2: Evaluating the Other Disorders:
- Thalassemia: An autosomal recessive quantitative disorder of globin chain synthesis, not an enzymatic inborn error of metabolism.
- Haemophilia: An X-linked recessive blood clotting factor deficiency.
- Colour Blindness: An X-linked recessive defect in red/green retinal cone photoreceptors.
Step 3: Final Answer:
Hence, Phenylketonuria is the correct answer (Option A).
Inborn errors of metabolism are congenital metabolic disorders caused by single-gene defects that result in deficient or non-functional enzymes along essential metabolic pathways.
Step 1: Biochemical Pathology of Phenylketonuria (PKU):
Phenylketonuria is an autosomal recessive inborn error of metabolism.
The affected individual lacks the functional hepatic enzyme phenylalanine hydroxylase.
Normally, this enzyme converts the dietary essential amino acid phenylalanine into tyrosine.
Due to enzyme deficiency, phenylalanine accumulates in the body and is converted into phenylpyruvic acid and related keto-derivatives.
Accumulation of these toxic metabolites in the brain causes severe mental retardation and impaired neurological development.
Excess phenylpyruvic acid is also excreted in the urine because of poor renal reabsorption.
Step 2: Evaluating the Other Disorders:
- Thalassemia: An autosomal recessive quantitative disorder of globin chain synthesis, not an enzymatic inborn error of metabolism.
- Haemophilia: An X-linked recessive blood clotting factor deficiency.
- Colour Blindness: An X-linked recessive defect in red/green retinal cone photoreceptors.
Step 3: Final Answer:
Hence, Phenylketonuria is the correct answer (Option A).
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Question: 4
$\alpha$-thalassemia is controlled by the genes:
$\alpha$-thalassemia is controlled by the genes:
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Thalassemia Genetics:
$\bullet$ $\alpha$-Thalassemia: Genes HBA1 and HBA2 on Chromosome 16 (4 alleles total).
$\bullet$ $\beta$-Thalassemia: Gene HBB on Chromosome 11 (2 alleles total).
$\bullet$ $\alpha$-Thalassemia: Genes HBA1 and HBA2 on Chromosome 16 (4 alleles total).
$\bullet$ $\beta$-Thalassemia: Gene HBB on Chromosome 11 (2 alleles total).
Updated On: Sep 4, 2026
- HBB and HB1 on chromosome 11 of each parent
- HAA1 and HAB1 on chromosome 16 of each parent
- HBA1 and HBA2 on chromosome 16 of each parent
- HBB and HAA on chromosome 11 of each parent
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The Correct Option is C
Solution and Explanation
Concept:
Thalassemia is an autosomal recessive blood disorder categorized into $\alpha$-thalassemia and $\beta$-thalassemia based on which globin chain of the hemoglobin tetramer ($\alpha_2\beta_2$) has reduced synthesis.
Step 1: Genetics of $\alpha$-Thalassemia vs $\beta$-Thalassemia:
- $\alpha$-Thalassemia: The synthesis of $\alpha$-globin chains is impaired. It is controlled by two closely linked genes, HBA1 and HBA2, located on chromosome 16 of each parent (providing four alleles in a diploid cell). The condition is caused by the deletion or mutation of one or more of these four alleles; severity increases with the number of affected genes.
- $\beta$-Thalassemia: The synthesis of $\beta$-globin chains is impaired. It is controlled by a single gene, HBB, located on chromosome 11 of each parent.
Step 2: Evaluating the Given Options:
- Option (A) & (D): Mention chromosome 11 and gene HBB, which controls $\beta$-thalassemia.
- Option (B): Uses incorrect gene designations (HAA1/HAB1).
- Option (C): Correctly specifies genes HBA1 and HBA2 on chromosome 16.
Step 3: Final Answer:
Therefore, $\alpha$-thalassemia is controlled by the genes HBA1 and HBA2 on chromosome 16 of each parent (Option C).
Thalassemia is an autosomal recessive blood disorder categorized into $\alpha$-thalassemia and $\beta$-thalassemia based on which globin chain of the hemoglobin tetramer ($\alpha_2\beta_2$) has reduced synthesis.
Step 1: Genetics of $\alpha$-Thalassemia vs $\beta$-Thalassemia:
- $\alpha$-Thalassemia: The synthesis of $\alpha$-globin chains is impaired. It is controlled by two closely linked genes, HBA1 and HBA2, located on chromosome 16 of each parent (providing four alleles in a diploid cell). The condition is caused by the deletion or mutation of one or more of these four alleles; severity increases with the number of affected genes.
- $\beta$-Thalassemia: The synthesis of $\beta$-globin chains is impaired. It is controlled by a single gene, HBB, located on chromosome 11 of each parent.
Step 2: Evaluating the Given Options:
- Option (A) & (D): Mention chromosome 11 and gene HBB, which controls $\beta$-thalassemia.
- Option (B): Uses incorrect gene designations (HAA1/HAB1).
- Option (C): Correctly specifies genes HBA1 and HBA2 on chromosome 16.
Step 3: Final Answer:
Therefore, $\alpha$-thalassemia is controlled by the genes HBA1 and HBA2 on chromosome 16 of each parent (Option C).
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Question: 5
Failure of segregation of chromatids during cell division results in:
Failure of segregation of chromatids during cell division results in:
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Chromosomal Errors Comparison:
$\bullet$ Failure of chromatid segregation $\rightarrow$ Aneuploidy (gain/loss of individual chromosomes: $2n \pm 1$).
$\bullet$ Failure of cytokinesis after telophase $\rightarrow$ Polyploidy (gain of entire sets of chromosomes: $3n, 4n$).
$\bullet$ Failure of chromatid segregation $\rightarrow$ Aneuploidy (gain/loss of individual chromosomes: $2n \pm 1$).
$\bullet$ Failure of cytokinesis after telophase $\rightarrow$ Polyploidy (gain of entire sets of chromosomes: $3n, 4n$).
Updated On: Sep 4, 2026
- Apomixis
- Aneuploidy
- Polyploidy
- Parthenocarpy
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The Correct Option is B
Solution and Explanation
Concept:
Chromosomal aberrations and numerical abnormalities arise due to mitotic or meiotic non-disjunction during cell division cycles.
Step 1: Mechanism of Aneuploidy vs Polyploidy:
- Aneuploidy: The failure of sister chromatids (or homologous chromosomes) to segregate properly during anaphase of cell division (non-disjunction) results in the gain or loss of one or a few individual chromosomes in daughter cells ($2n+1, 2n-1, 2n+2$, etc.). Examples include Down syndrome (trisomy 21), Turner syndrome ($45, XO$), and Klinefelter syndrome ($47, XXY$).
- Polyploidy: The failure of cytokinesis (cytoplasmic division) after telophase results in an increase in a whole set of chromosomes ($3n, 4n$, etc.), a phenomenon frequently observed in plants.
Step 2: Evaluating the Other Terms:
- Apomixis: A form of asexual reproduction in flowering plants that mimics sexual reproduction by producing seeds without fertilization.
- Parthenocarpy: The development of fruit without fertilization, resulting in seedless fruits (e.g., banana).
Step 3: Final Answer:
The failure of segregation of chromatids during cell division results in Aneuploidy.
Hence, Option (B) is the correct answer.
Chromosomal aberrations and numerical abnormalities arise due to mitotic or meiotic non-disjunction during cell division cycles.
Step 1: Mechanism of Aneuploidy vs Polyploidy:
- Aneuploidy: The failure of sister chromatids (or homologous chromosomes) to segregate properly during anaphase of cell division (non-disjunction) results in the gain or loss of one or a few individual chromosomes in daughter cells ($2n+1, 2n-1, 2n+2$, etc.). Examples include Down syndrome (trisomy 21), Turner syndrome ($45, XO$), and Klinefelter syndrome ($47, XXY$).
- Polyploidy: The failure of cytokinesis (cytoplasmic division) after telophase results in an increase in a whole set of chromosomes ($3n, 4n$, etc.), a phenomenon frequently observed in plants.
Step 2: Evaluating the Other Terms:
- Apomixis: A form of asexual reproduction in flowering plants that mimics sexual reproduction by producing seeds without fertilization.
- Parthenocarpy: The development of fruit without fertilization, resulting in seedless fruits (e.g., banana).
Step 3: Final Answer:
The failure of segregation of chromatids during cell division results in Aneuploidy.
Hence, Option (B) is the correct answer.
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