C
CraftExam
medium difficulty7 min

Sex Determination

Sex determination is the biological mechanism by which chromosomal constitution or ploidy decides whether an organism develops as male or female.

In one line: In humans, the sperm decides X or Y; in birds, the egg decides Z or W.

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Understand it

  • Different organisms use different sex-determination systems.
  • In humans and many insects, females are XX and males are XY; the sperm determines the sex of the child because it may carry X or Y.
  • In the XX-XO system, males have only one X chromosome and no Y chromosome.
  • In birds, the system is ZW-ZZ, where females are ZW and males are ZZ, so the egg determines sex.
  • Honey bees show haplodiploidy: fertilised diploid eggs become females, while unfertilised haploid eggs develop into males.
  • Exam questions often test which parent contributes the deciding gamete and how chromosomal notation differs among systems.

Example

In humans, an ovum always carries an X chromosome. A sperm carrying X produces XX female offspring, while a sperm carrying Y produces XY male offspring.

Rule to remember

Humans: XX female and XY male.

Birds: ZW female and ZZ male.

Honey bee: fertilised diploid egg forms female; unfertilised haploid egg forms male.

See it worked out

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

Worked example

  1. 1In a human cross XX x XY, the mother produces X ova and the father produces X or Y sperm.
  2. 2The possible zygotes are XX and XY in approximately equal probability.

Method you can reuse

  1. 1Identify the organism.
  2. 2Write the sex chromosome constitution of male and female.
  3. 3Identify which sex is heterogametic.
  4. 4List gametes.
  5. 5Combine gametes to show possible offspring sex.

Visual tip

A sex-determination chart should label female gametes as X and male gametes as X or Y in humans. For birds, label Z and W correctly because the female is heterogametic.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is the father said to determine the sex of a human child?

Think first, then show answer
The father is said to determine chromosomal sex because his sperm may carry either an X or a Y chromosome. The mother's ovum carries X, so X-bearing sperm gives XX and Y-bearing sperm gives XY.

Common wrong answer

Writing female birds as XX is wrong; in birds, females are ZW and males are ZZ.

Don't mix this up

A common and biologically wrong idea is that the mother determines the sex of a human child. In humans, the sperm contributes either X or Y, so the father determines chromosomal sex.

Exam tip

Write the organism and system before explaining. XX-XY, XX-XO, ZW-ZZ, and haplodiploidy should not be mixed.

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 for a diagram of human sex determination, comparison of XX-XY and ZW-ZZ, explanation of honey bee haplodiploidy, or correction of parent-related misconceptions.

Answer plan

Name the system, write male and female chromosome sets, show gametes, combine them in a small chart, and state which parent or process determines sex.
  1. 1

    Enough for 1 mark

    Sex determination is the biological mechanism by which chromosomal constitution or ploidy decides whether an organism develops as male or female.

  2. 2

    Add for 2 marks

    Example: In humans, an ovum always carries an X chromosome. A sperm carrying X produces XX female offspring, while a sperm carrying Y produces XY male offspring.

  3. 3

    Add for 3 marks

    Different organisms use different sex-determination systems.

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