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Chromosomal Disorders

Chromosomal disorders are conditions caused by abnormal chromosome number or structure, often due to errors during cell division such as nondisjunction.

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Student-friendly explanation

Chromosomal disorders differ from single-gene Mendelian disorders because the cause is an abnormal chromosome set, not only a change in one gene. Aneuploidy occurs when one or more chromosomes are gained or lost, usually due to nondisjunction during meiosis. Polyploidy means the presence of extra complete sets of chromosomes and is more common as a tolerated condition in plants than in humans. Down syndrome is commonly associated with trisomy 21. Klinefelter syndrome is associated with an XXY chromosome constitution in males, while Turner syndrome is associated with a single X chromosome in females. Exams focus on cause, chromosome constitution, and key identifying features.

How to write this in exams

  1. 1

    Start with the exact idea

    Chromosomal disorders are conditions caused by abnormal chromosome number or structure, often due to errors during cell division such as nondisjunction.

  2. 2

    Then show how to use it

    Read the chromosome number or constitution. Decide whether the change involves one chromosome or a whole set. If one chromosome is extra or missing, write aneuploidy. Match trisomy 21, XXY, or XO with the correct syndrome.

  3. 3

    Add one concrete example

    Down syndrome is caused by the presence of an extra copy of chromosome 21, giving trisomy 21. Turner syndrome is represented as 45,X, where one sex chromosome is missing.

  4. 4

    Avoid this incomplete answer

    Writing that Turner syndrome is XXY is incorrect; XXY indicates Klinefelter syndrome, while Turner syndrome has only one X chromosome.

Definition

Chromosomal disorders are conditions caused by abnormal chromosome number or structure, often due to errors during cell division such as nondisjunction.

Example

Down syndrome is caused by the presence of an extra copy of chromosome 21, giving trisomy 21. Turner syndrome is represented as 45,X, where one sex chromosome is missing.

Rule to remember

Aneuploidy: 2n + 1 or 2n - 1 type changes. Down syndrome: trisomy 21. Klinefelter syndrome: XXY. Turner syndrome: XO or 45,X representation.

Memory hook

Down is extra 21, Klinefelter is extra X in male, Turner is missing one sex chromosome in female.

Examples and method

Worked example

If a case describes an individual with 47 chromosomes due to three copies of chromosome 21, identify the disorder as Down syndrome caused by trisomy 21.

Method to apply

Read the chromosome number or constitution. Decide whether the change involves one chromosome or a whole set. If one chromosome is extra or missing, write aneuploidy. Match trisomy 21, XXY, or XO with the correct syndrome.

Diagram support

A karyotype-style diagram should highlight the extra chromosome 21 in Down syndrome or the altered sex chromosome set in Klinefelter and Turner syndromes.

How CBSE asks it

Questions may ask students to define aneuploidy, identify syndromes from chromosome constitution, compare Down, Klinefelter, and Turner syndromes, or explain nondisjunction.

Avoid common mistakes

Common confusion

Students often confuse Mendelian disorders with chromosomal disorders. Haemophilia is a single-gene disorder, while Down syndrome is due to abnormal chromosome number.

Common wrong answer

Writing that Turner syndrome is XXY is incorrect; XXY indicates Klinefelter syndrome, while Turner syndrome has only one X chromosome.

Exam tip

For each syndrome, write the chromosomal cause first, then give one or two key features if asked. Do not list features without the chromosome basis.

Quick check

How is aneuploidy different from polyploidy?

Aneuploidy means gain or loss of individual chromosomes, such as trisomy 21. Polyploidy means addition of complete chromosome sets, so the whole set number changes.

Answer writing and exam use

1-mark answer

Define the disorder type, identify aneuploidy or polyploidy, write the chromosome constitution, name the syndrome, and add key features only after the cause.

2-mark answer

Define the disorder type, identify aneuploidy or polyploidy, write the chromosome constitution, name the syndrome, and add key features only after the cause.

3-mark answer

Define the disorder type, identify aneuploidy or polyploidy, write the chromosome constitution, name the syndrome, and add key features only after the cause.
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