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Using a Punnett Square

Using a Punnett Square. What is a PUNNETT SQUARE ?. If you know the genes of the parents, a Punnett square can be used to… show the different ways alleles in the sex cells can combine predict the probability of certain traits in offspring. Using a PUNNETT SQUARE.

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Using a Punnett Square

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  1. Using a Punnett Square

  2. What is a PUNNETT SQUARE? If you know the genes of the parents, a Punnett square can be used to… • show the different ways alleles in the sex cells can combine • predict the probability of certain traits in offspring

  3. Using a PUNNETT SQUARE One of Harry Potter’s best friends is Hermione Granger. Hermione is a wizard, but her parents are not. So, how did Hermione get the wizarding trait? We can use a Punnett square to figure it out…

  4. Using a PUNNETT SQUARE A letter is chosen to represent a characteristic, such as magical powers. Having magical powers is a recessive trait, so we’ll use: • Mfor muggle (non-wizard) • mfor wizard

  5. Using a PUNNETT SQUARE Both of Hermione’s parents are muggles and both have a genotype of Mm. Genotypes: Mm x Mm (mom x dad) Phenotypes: both muggles (non-wizards)

  6. Using a PUNNETT SQUARE Cross Hermione’s parents: M m M m

  7. Using a PUNNETT SQUARE The cross looks like this: M m M M M M m m m m m M

  8. Using a PUNNETT SQUARE Each square stands for 25% of the possible offspring. Or, ¼ of the possible offspring. M m M M M M m m m m m M

  9. Using a PUNNETT SQUARE Remember, the recessive trait is having magical powers. What’s the probability of being a wizard? M m M M M M m m m m m M

  10. Expressing PROBABILITIES Can express probability several ways: 3:1 ratio of muggles to wizards 1/4 chance of being wizard 25% chance of being wizard M m M M M M m m m m m M

  11. Expressing PROBABILITIES can use for genotypes 25% - MM 50% - Mm 25% - mm or phenotypes 3/4 - muggles 1/4 – wizards 3:1 muggles to wizards M m M M M M m m m m m M

  12. Practice #1 Mendel crossed a pure-breeding purple flower (PP) with a pure-breeding white flower (pp). Complete a Punnett square for this cross. • Which is the dominant trait? • What are the offspring genotypes? • For the offspring, what fraction will have purple flowers? • What percentage will have genotype Pp?

  13. Widow’s Peak Widow’s Peak No Widow’s Peak

  14. Practice #2 A mom with a widow’s peak (Ww) is crossed (mates) with a dad who does not have a widow’s peak (ww). Complete a Punnett square for this cross. • Which is the dominant trait? • What are the offspring genotypes? • What is the ratio of widow’s peak to no widow’s peak for the offspring? • What is the ratio of Ww to ww for the offspring?

  15. Practice #3 A man and a woman both have a cleft chin. Both have genotypes of Cc. Complete a Punnett square for a cross between them. • Which is the dominant trait? • What are the offspring genotypes? • What fraction of offspring will have a cleft chin? • What percentage will have genotype Cc?

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