Boy or Girl: What are the Chromosome Combinations that Produce a Boy or a Girl?

Problem

What are the chromosome combinations that produce a boy or a girl?

Materials

• masking tape
• marking pen
• 2 coffee cups
• 3 red kidney beans
• 1 white lima bean
• drawing compass
• sheet of typing paper

Procedure

1. Use the tape and marking pen to label one cup Ova and the other cup Sperm.
2. Place 2 red kidney beans in the cup labeled Ova and 1 red kidney bean in the cup labeled Sperm.
3. Add the white lima bean to the sperm cup.
4. Set the cups on a table
5. Use the compass to draw two circles with a 2-inch (5-cm) diameter on the paper.
6. Place the paper on the table near the cups.
7. Without looking into the cups, take 1 bean out of each and place the 2 beans in one of the circles drawn on the paper.
8. Repeat step 7, placing the beans in the other circle.

Results

Each circle has 2 beans in it. One circle has 2 red beans and one circle has a red and a white bean.

Why?

The sex of a baby is due to two sets of instructions. These instructions are in the sex chromosomes, known as X and Y. Females have two X chromosomes, and males have an X and a Y. An ovum (female sex cell, or egg) and a sperm (male sex cell) have one sex chromosome each.

Ova have only X chromosomes, while half of the sperm have X chromosomes and half have Y chromosomes. The joining of an ovum and a sperm is called fertilization, and the fertilized egg is called a zygote. If an ovum is fertilized by a sperm carrying a Y chromosome, the XY combination produces a boy. If the ovum is fertilized by a sperm carrying an X chromosome, the XX combination produces a girl.

The red beans in this experiment represent X chromosomes and the white bean a Y chromosome. The combination of 2 red beans indicates a girl, and a red-and-white combination a boy. The sex chromosome from the sperm determines the sex of the baby. The combinations of the beans in the experiment represent one boy and one girl.

Let's Explore

Are the odds of a baby being a boy or a girl always 50-50? If three families each have four children, will each family have two boys and two girls? Repeat the experiment, placing 12 red kidney beans in the ova cup. Since there is usually one egg but many available sperm to combine with it, place 12 red kidney beans and 12 lima beans in the sperm cup. Draw 12 circles on a sheet of poster board as shown in the diagram. Take 1 bean out of each cup and place the pairs in one of the circles. Fill the four circles for the first family, then fill them for the other two families. Science Fair Hint: Glue the beans to the poster board and record how many boys (XY) and girls (XX) are in each family. Create a Legend. Use the poster board and Legend as part of a project display.

Show Time!

Would the chance of having a boy or a girl be 50-50 if more children were born? Use a pen, ruler, and paper to prepare a chart, like the one shown at the bottom of the page, for 15 families.

Stretch a small hand towel out on a table. Hold 6 pennies in your hand and shake them back and forth several times, then toss the coins together over the towel. NOTE: The towel will prevent the coins from rolling off the table. Count the number of heads and tails, and write the numbers in the columns for Family 1. Toss the coins 14 more times, recording the results for each family. Use the results to determine whether each of the 15 families shows a ratio (a numerical comparison of two different things) of exactly half boys and half girls. Does the total number of girls and boys in all the families show a ratio of 50-50?

Check it Out!

Find out more about fertilization in humans. Where does it occur? Where are sperm and ova produced? For information, see pages 185-189 of Janice VanCleave's The Human Body for Every Kid (New York: Wiley, 1995).

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