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medium difficulty7 min

Chromosomal Disorders

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

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

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Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

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 you can reuse

  1. 1Read the chromosome number or constitution.
  2. 2Decide whether the change involves one chromosome or a whole set.
  3. 3If one chromosome is extra or missing, write aneuploidy.
  4. 4Match trisomy 21, XXY, or XO with the correct syndrome.

Visual tip

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

How is aneuploidy different from polyploidy?

Think first, then show answer
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.

Common wrong answer

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

Don't mix this up

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

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.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

Define the disorder type, identify aneuploidy or polyploidy, write the chromosome constitution, name the syndrome, and add key features only after the cause.
  1. 1

    Enough for 1 mark

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

  2. 2

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    Chromosomal disorders differ from single-gene Mendelian disorders because the cause is an abnormal chromosome set, not only a change in one gene.

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