Science project

Gender Probability: Male and Female Chromosomes

Men have sperm and women have eggs, or ova. When a sperm and an ovum combine, you get a zygote: a cell that is a combination of these two cells. This is the beginning of a new human life, and the moment that this happens is called fertilization.

Ova all have X chromosomes.  Half of the sperm have Y chromosomes and the other half have X chromosomes. Girls have two X chromosomes. If a sperm with an X chromosome fertilizes the ovum, the fetus will be female. Boys have an X and a Y chromosome. If a sperm with a Y chromosome fertilizes the ovum, the fetus will be male.


Find the probability of a baby's gender.


  • Permanent Marker
  • Masking tape
  • 2 paper cups
  • 3 green marbles
  • 1 red marble
  • Friend


  1. Mark one cup “ova”, and put 2 green marbles into that cup.
  2. Mark the other cup “sperm” and put on green marble and one red marble into that cup.
  3. Make a table with the different options: two green (GIRL), one red and one green (BOY).
  4. Look away and choose one marble from each cup. What are the chances that it will be a boy or a girl? If you choose one marble from each container again and again, how many boys and how many girls will you get? Have your friend tally the results in the table, then put the marbles back into the cups.
  5. Do this 30 times. How many boys did you get? How many girls?


As your numbers increase, you’ll get closer to having half girls and half boys. How close did you get?


Since sperm are equally divided into X and Y chromosome sperm, the chances of having a boy or a girl should be equal. So why do some families have all girls or all boys?

Each time a sperm meets an ovum, there is a 50% chance that it will make a boy and a 50% chance that it will make a girl. It doesn’t matter what happened the time before that: each time an ovum is fertilized, this makes a new zygote that could be a boy or a girl.

As numbers increase, the law of large numbers starts to become easy to see. If you take two or three families you know, they may not have equal numbers of boys and girls. However, if you take 200 random families, they will likely have an almost-equal number of girls and boys.

Try this out, using families at your school or other community group as an example. Does the law of large numbers work?

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