A Throat Swab, a Lab Dish and a Family Earwax Puzzle

Imani's Visit to the Lab

1Imani Sorensen woke up with a sore throat, so her dad took her to the clinic at Riverside Hospital. The people here are made up. Dr. Kwame Lindgren swabbed the back of her throat to check for strep, a throat infection caused by bacteria. Imani hopes to become a doctor, so he arranged for her to visit the hospital lab the next afternoon to see how the test works.

2In the lab, scientist Farah Quist showed Imani a dish of nutrient gel that she had rubbed with the swab and kept warm overnight. Small round dots called colonies covered the gel. Ms. Quist explained that each colony began as a single bacterium. A bacterium reproduces asexually: it copies its genetic material and then splits into two cells. Under good conditions in the lab, many kinds of bacteria can divide about every 30 minutes. Table 1 shows how one colony on a dish like this can start.

3Ms. Quist added that every cell in a colony gets all of its genetic material from one parent cell. Apart from rare changes called mutations, the cells in a colony are copies of each other.

4During the checkup, Dr. Lindgren had also looked in Imani's ears. He noticed that Imani has dry, flaky earwax, while both of her parents have wet, sticky earwax. He explained that earwax type is controlled mainly by one gene with two alleles. The allele for wet earwax (W) is dominant, and the allele for dry earwax (w) is recessive.

5Humans reproduce sexually. A child gets one allele of each gene from the mother's egg cell and one from the father's sperm cell. Which of a parent's two alleles ends up in a given egg or sperm is a matter of chance. That evening, Imani asked her relatives about their earwax and made Table 2.