Traits from both parents are genetically passed to their children during a process called crossing over. Tell your students to make two 12-centimeter tubes each of red and blue modeling clay. Pinch the tubes of each color together at the center to resemble an "X." Each color represents a replicated chromosome (sister chromatids). Together, they are a homologous pair. Lay the homologous pair side by side to make a tetrad. Mark and label with masking tape three chromosomes on each chromatid at 2 centimeters (A), 4 centimeters (B) and 10 centimeters (C) from the top. Take one C chromosome from a red chromatid and switch places with a C chromosome from a blue chromatid to simulate crossing over.
Help you students examine the frequency of certain inherited traits within the class. At the top of a chart, write "Attached earlobes" on the left side and "Free earlobes" on the right. Underneath each of the two previous headings, write "PTC taster" and "Non-PTC taster." Underneath each of the previous four headings, write "Tongue toller" and "Non-tongue roller." Connect the headings of each line to the ones below them using lines as though you are making a factor tree. Have each student taste the PTC paper, try to roll their tongue and have a classmate check their earlobes. Have each student enter their name underneath the appropriate heading.
To investigate the chromosome combinations that produce a boy or a girl, give each student two paper cups. Label one "ova" and the other "sperm." Have them put two kidney beans (X chromosomes) in the ova cup and one kidney bean and one lima bean (Y chromosome) in the sperm cup. Use a compass to draw two, 2-inch diameter circles on paper. Without looking into the cups, have them take one bean out of each cup and put them in one of the circles. The other two beans go into the second circle. The circle with two kidney beans represents a girl and the other, a boy.
Tell the students the colors green, red and yellow represent different alleles that control the color of a fish. Green (G) is dominant to all colors, red (R) is recessive to green, but co-dominant to yellow (Y) and yellow is recessive to green and co-dominant to red. A combination of red and yellow results in an orange fish. Have the students make a chart showing the combinations of genes needed to produce a fish that is green, red, yellow or orange. Ask them if two red fish can have a green offspring, two orange fish can have a red offspring and two green fish can have an orange offspring.