Punnett Squares

1. Punnett Squares Kelly Coleman High School Biology

2. Objective: • After this lesson you will be able to predict the outcome of genetic crosses using a Punnett Square

3. How are traits inherited? • We know offspring look like parents • Early scientists thought offspring had an average of the parents’ traits • A tall parent and a short parent have medium height offspring

4. Gregor Mendel • Studied seven traits in garden peas • Various forms of traits showed up in the same ratio with each cross 3:1

5. Gregor Mendel • Mendel knew traits came from parents • A plant has two pieces of information, one from each parent • Different combinations of information lead to different appearance of the offspring

6. Genes and Alleles • If one parent plant has Ss on it’s genetic code for pea shape, offspring might get an S or an s • To keep track of possible combinations we use a Punnett Square

7. Punnett Square Process • Read question and write parent’s genotypes • Draw Punnett Square with parent genotypes • Drop parent genotype down and across • Summarize possible offspring genotypes

8. Demonstration • What will result from mating two heterozygous tall pea plants?

9. Group Practice • What will result from mating a heterozygous tall pea plant and a homozygous tall pea plant?

10. Individual Practice • What will result from mating two heterozygous round pea plants?

11. Extended Practice • Having six fingers is a rare dominant trait (D). Homozygous recessive people have five fingers(dd). • A.) What are the chances that a child will be born with six fingers if both parents are homozygous recessive? • B.) What are the chances that a child will be born with six fingers if one parent has five fingers and the other parent is heterozygous?

12. Conclusion • How could knowing what offspring might come from two parents be useful?