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Sex Determination

Sex determination is the process by which the sex of an offspring is decided at fertilisation.

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Student-friendly explanation

In humans, the mother has XX chromosomes and the father has XY chromosomes. The mother always contributes an X chromosome through the egg, while the father can contribute either X or Y through the sperm. If the sperm carries X, the child is XX and develops as female. If the sperm carries Y, the child is XY and develops as male.

How to write this in exams

  1. 1

    Start with the exact idea

    Sex determination is the process by which the sex of an offspring is decided at fertilisation.

  2. 2

    Then show how to use it

    1. Recall the mother's sex chromosomes. 2. Recall the father's sex chromosomes. 3. Note that the egg always has X. 4. Check whether the sperm has X or Y. 5. Write XX or XY and state the sex.

  3. 3

    Add one concrete example

    A human baby receives one sex chromosome from each parent. An X-bearing sperm produces a girl child, and a Y-bearing sperm produces a boy child.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is to say the mother decides the sex because the child develops in her body. That is incorrect in human sex determination.

Definition

Sex determination is the process by which the sex of an offspring is decided at fertilisation.

Example

A human baby receives one sex chromosome from each parent. An X-bearing sperm produces a girl child, and a Y-bearing sperm produces a boy child.

Rule to remember

Human sex chromosomes: female = XX, male = XY; egg always carries X, sperm carries X or Y.

Memory hook

Mother gives X, father gives X or Y.

Examples and method

Worked example

If an X-bearing sperm fertilises the X-bearing egg, the zygote becomes XX and the child is female. If a Y-bearing sperm fertilises the same egg, the zygote becomes XY and the child is male.

Method to apply

1. Recall the mother's sex chromosomes. 2. Recall the father's sex chromosomes. 3. Note that the egg always has X. 4. Check whether the sperm has X or Y. 5. Write XX or XY and state the sex.

Diagram support

A simple X-Y chromosome chart or fertilisation table helps students see why the father decides the sex.

How CBSE asks it

Explain why the father determines the sex of the child in humans, or write the chromosome combinations for male and female.

Avoid common mistakes

Common confusion

Students often think the mother decides the sex of the baby. That is wrong because the father's sperm can carry either X or Y.

Common wrong answer

A common wrong answer is to say the mother decides the sex because the child develops in her body. That is incorrect in human sex determination.

Exam tip

Always write XX for female and XY for male in humans. Mention that the father's sperm determines the final combination.

Quick check

In humans, which parent can contribute either X or Y chromosome to decide the baby's sex?

The father contributes either an X or a Y chromosome through the sperm, so the father's sperm decides the baby's sex at fertilisation.

Answer writing and exam use

1-mark answer

Sex determination is the process by which the sex of an offspring is decided at fertilisation.

2-mark answer

Sex determination is the process by which the sex of an offspring is decided at fertilisation. Human sex chromosomes: female = XX, male = XY; egg always carries X, sperm carries X or Y. A human baby receives one sex chromosome from each parent. An X-bearing sperm produces a girl child, and a Y-bearing sperm produces a boy child.

3-mark answer

In humans, the mother has XX chromosomes and the father has XY chromosomes. The mother always contributes an X chromosome through the egg, while the father can contribute either X or Y through the sperm. If the sperm carries X, the child is XX and develops as female. If the sperm carries Y, the child is XY and develops as male. Human sex chromosomes: female = XX, male = XY; egg always carries X, sperm carries X or Y. If an X-bearing sperm fertilises the X-bearing egg, the zygote becomes XX and the child is female. If a Y-bearing sperm fertilises the same egg, the zygote becomes XY and the child is male. Explain why the father determines the sex of the child in humans, or write the chromosome combinations for male and female. A common wrong answer is to say the mother decides the sex because the child develops in her body. That is incorrect in human sex determination.
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