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Difficulty: MediumCell Biology, Genetics, and Evolution

Consider the following human genetic disorders:
1. Turner Syndrome
2. Haemophilia
3. Down Syndrome
4. Sickle Cell Anaemia

Which of the following pairs comprises disorders caused exclusively by chromosomal numerical aberrations (aneuploidies) rather than single-gene Mendelian mutations?

  1. A
    Haemophilia and Sickle Cell Anaemia
  2. B
    Down Syndrome and Sickle Cell Anaemia
  3. Turner Syndrome and Down SyndromeAnswer
  4. D
    Turner Syndrome and Haemophilia

Answer

Turner Syndrome and Down Syndrome
The pair comprising Turner Syndrome and Down Syndrome is correct because both conditions originate from chromosomal non-disjunction during cell division, leading to whole-chromosome numerical alterations (aneuploidy). Turner Syndrome is characterized by the loss of one X chromosome (45,XO45, XO), while Down Syndrome results from an extra copy of chromosome 21 (Trisomy 21).

Step-by-Step Solution

1
Differentiate between chromosomal aneuploidies and Mendelian disorders.
Chromosomal aneuploidies result from the gain or loss of complete chromosomes due to non-disjunction during meiosis, whereas Mendelian disorders are determined by mutation in a single gene locus.
Accurate categorization requires identifying the scale and mechanism of the genetic defect.
2
Analyze the genetic basis of each listed condition.
Turner Syndrome is a sex-chromosome monosomy (45,XO45, XO). Down Syndrome is an autosomal trisomy (47,XX+2147, XX+21 or 47,XY+2147, XY+21). Haemophilia is an X-linked recessive single-gene defect. Sickle Cell Anaemia is an autosomal recessive point mutation (GAG→GTGGAG \rightarrow GTG in the β\beta-globin gene).
Evaluating each disorder reveals whether the etiology is numerical chromosomal alteration or a monogenic mutation.
3
Select the pair that contains only chromosomal numerical aberrations.
The combination of Turner Syndrome (monosomy) and Down Syndrome (trisomy) exclusively satisfies the criterion.
Both conditions involve abnormal chromosome counts rather than altered nucleotide sequences of individual genes.

Key Concept

Classification of Genetic Disorders: Chromosomal Aneuploidies versus Mendelian Monogenic Mutations
Estimated Time:1m 0s
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Topics

Cell Biology, Genetics, and Evolution

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