Punnett Squares Kelly Coleman High School Biology

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

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

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

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

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

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

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

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

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

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?

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