Alien Babies!

This is a fun and creative activity that helps explain how sexual reproduction generates genetic variation in offspring. Sadly, I can not claim this activity as my own – I found it on TES a few years ago and have tweaked it only slightly.


Before you start, check the students’ understanding of dominant and recessive alleles, phenotype and genotype. A simple way of demonstrating dominant and recessive alleles is to select two students to stand up and hold mini-white boards on which one has written ‘blue eyes’ (or a lower case b) and the other ‘brown eyes’ (or an upper case B). Ask the student with ‘brown eyes’ to stand in front of the student with ‘blue eyes.’ Highlight the fact that although both alleles / students are present, only the dominant allele is expressed in the phenotype / only one student can be seen.

You will need:

  • Chromosome cards (printed and cut out)
  • Large paper body cells (x2), egg cells (x2) and sperm cells (x2)
  • Genotype / phenotype table
  • Resources for building alien babies:
    • Brown and white marshmallows
    • Pipe cleaners
    • Coloured drawing pins
    • Cocktail sticks
    • Colouring pens
    • Nails
    • Wool or string


  1. Give one set of chromosome cards (male and female) to each pair of students (mummy and daddy aliens).
  2. Ask the students to arrange their chromosomes into homologous pairs inside the nucleus of a body cell.
  3. Turn over the chromosomes so that the letters can not be seen.
  4. Ask the students to randomly assort the chromosomes in each homologous pair into one of two sperm cells or one of two egg cells. Explain that this represents meiosis and that the gametes produced are haploid.
  5. Students should then select one egg cell and one sperm cell and push them together across the table. Highlight that this represents fertilisation and that the new diploid cell is the zygote.
  6. Ask the students to put the chromosomes in the zygote back into homologous pairs.
  7. Students should complete the genotype / phenotype table and use it as a guide to start building their alien babies using the resources provided. Use the cocktail sticks to join the marshmallows together.
  8. Once complete, ask the students to compare their babies. Are there any babies that are identical to one another or their parents? How much genetic material did each parent contribute? Why is it important that meiosis halves the number of chromosomes in each gamete? Ask the students to think about the steps involved in making their alien babies then consider how sexual reproduction generates genetic variation in offspring.

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