Meiosis: all gametes are genetically different from each other.

The idea

Gametes are sex cells, such as sperm and eggs in animals, or pollen and eggs in flowering plants. They are produced by meiosis in reproductive organs. During this process, genetic information is copied, and the cell then divides twice to form four gametes. Each gamete receives a single set of chromosomes, halving the normal chromosome number. Because the chromosomes are randomly distributed during these two divisions, every resulting cell carries a unique combination of alleles. Consequently, no two sex cells produced by the same parent are identical.

For example: A human parent produces sperm or eggs containing 23 chromosomes, but the specific alleles in each gamete vary, explaining why siblings inherit distinct characteristics.

What earns the marks

• State that gametes are produced by meiosis in reproductive organs.
• Describe how the parent cell divides twice to form four gametes.
• Explain that meiosis halves the chromosome number, leaving each gamete with a single set of chromosomes.
• Identify that the random distribution of chromosomes during division results in unique combinations of alleles in each gamete.
• Use the example of human gametes containing 23 chromosomes to illustrate the halving of the genetic material.

Common mistakes and how to put them right

• Confusing meiosis with mitosis.

  Put it right by stating that meiosis produces four non-identical gametes with half the chromosome number, whereas mitosis produces two genetically identical body cells.

• Thinking all gametes from one parent carry the exact same alleles.

  Put it right by explaining that while they carry the same genes, the random distribution of chromosomes means they carry different combinations of alleles.

• Believing that the chromosome number halves because the DNA is not copied before division.

  Put it right by clarifying that genetic information is copied first, but the cell then divides twice, resulting in half the original chromosome number.

Exam tips

• When comparing cell divisions, explicitly state that meiosis leads to variation because the resulting cells are non-identical, unlike mitosis.
• Remember that you do not need to know the complex stages of meiosis; focus on the copying of genetic material followed by two divisions.