Punnett squares
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Punnett squares can be used to predict the possible genotypes and phenotypes of offspring from a genetic cross between two parents. Mendel performed dihybrid crosses involving two traits, like seed shape and color, and found independent assortment of traits in a 9:3:3:1 ratio. A Punnett square represents the possible combinations of alleles from each parent that are passed on to offspring. It accurately predicts the probability but not absolute outcomes of genetic crosses.
Punnett squares
- 1. Punnett Squares MENDEL LOOKED AT Monohybrid crosses: ONE TRAIT Dihybrid crosses: TWO TRAITS AT THE SAME TIME MENDEL WANTED TO SEE IF TRAITS ARE INHERITED TOGETHER OR INDEPENDENTLY
- 2. PUNNETT SQUARES A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING GENERATIONS 1 ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S
- 3. PUNNETT SQUARES CROSS TT X Tt
- 4. CONT’D T T X T t T T T t T T T T T t T t
- 5. Practice with Punnett Squares 1. A round seeded plant (RR) is crossed with a wrinkle seeded plant (rr). What are the phenotypes of the offspring? Offspring phenotypes: all would be round
- 6. Punnett square practice 2. If purple flowers are dominant to white flowers, two heterozygous purple flowered pea plants are crossed. What are the phenotypes of their offspring and in what proportion? Use F for allele letter. Ratio of phenotypes 3:4 purple 1:4 white Ratio of genotypes 1:4 homozygous dominant 2:4 heterozygous 1:4 homozygous recessive
- 7. Punnett square practice 3. A plant with green seeds (yy) is crossed with a heterozygous plant (Yy). List the following: create a table to show a Punnett square ratio of phenotypes ratio of genotypes
- 8. In dragons... Wings are a dominant trait (W), but some dragons are born wingless (w). 1. What are the chances (ratio or percent) that two heterozygous dragons have a baby that is wingless? 2. If a wingless dragon is crossed with one that is heterozygous, how many of its offspring will also be wingless? Punnett square practice…
- 9. Why does the Punnett square work? It all goes back to meiosis.. each side represents a sperm or egg. The boxes simply give you the statistical chance that a certain sperm will fertilize a certain egg. Consider a pea plant that is both round and tall = RrTt When this plant's cells go through MEIOSIS, the alleles segregate - each sperm receives a random combination... These possible combinations include: RT, Rt, rT, rt
- 10. Mendel's Law of Independent Assortment – Illustrated by the DIHYBRID cross The second law describes the outcome of dihybrid (two character) crosses, or hybrid crosses involving more than one trait. A dihybrid is an individual that is a double heterozygote (e.g., with the genotype RrYy - round seed, yellow seed). What are the gametes that can be produced by this individual?
- 11. DIHYBRID CROSS MENDEL TOOK TWO TRUE BREEDING PLANTS FOR 2 DIFFERENT TRAITS (ROUND/WRINKLED SEEDS and YELLOW/GREEN SEEDS) 1 ST GENERATION WHAT WOULD THE OFFSPRING PHENOTYPES BE IF HE CROSSED A TRUE BREEDING ROUND W/ A TRUE BREEDING WRINKLED (ROUND IS DOMINANT) ALL THE OFFSPRING ARE ROUND
- 12. DIHYBRID CROSS – 1 ST GENERATION CONT’D SO WHAT DO YOU THINK HAPPENED WHEN HE CROSSED TRUE BREEDING ROUND/YELLOW (RRYY) SEEDS WITH TRUE BREEDING WRINKLED/GREEN SEEDS (rryy) ALL THE F1 WERE ROUND AND YELLOW
- 13. DIHYBRID CROSS – 2 ND GENERATION TOOK THE F1 PLANTS AND BRED THEM TOGETHER (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW) 2 ND GENERATION FOUND ROUND/YELLOW - 9 FOUND ROUND/GREEN - 3 FOUND WRINKLED/YELLOW - 3 FOUND WRINKLED/GREEN - 1 ( 9 : 3 : 3 : 1 RATIO)
- 14. EXPLANATION OF 2 ND GENERATION MENDEL CAME UP W/ 2 ND LAW – THE LAW OF INDEPENDENT ASSORTMENT GENES FOR DIFFERENT TRAITS ARE INHERITED INDEPENDENTLY FROM EACH OTHER THIS IS WHY MENDEL FOUND ALL THE DIFFERENT COMBONATIONS OF TRAITS
- 15. DIHYBRID CROSSES A LITTLE DIFFERENT HhGg X HhGg MUST FIND OUT ALL THE POSSIBLE ALLELE COMBINATIONS Must combine one g with one h huge is dominant to small, green is dominant to blue
- 16. H h G g X H h G g 1. HG 2. Hg 3. hG 4. hg FOIL – FIRST, OUTSIDE, INSIDE, LAST BOTH PARENTS ARE THE SAME
- 17. NOW LET’S DO A DIHYBRID CROSS H h G g X H h G g HG Hg hG hg HG Hg hG hg HHGG HHGg HhGG HhGg HHGg HHgg HhGg Hhgg HhGG HhGg hhGG hhGg HhGg Hhgg hhGg hhgg
- 18. WHAT ARE THE PHENOTYPIC RATIO’S? H h G g X H h G g (huge/green X huge/green) HG Hg hG hg HG Hg hG hg HHGG HHGg HhGG HhGg HHGg HHgg HhGg Hhgg HhGG HhGg hhGG hhGg HhGg Hhgg hhGg hhgg huge/green = 9 small/green = 3 huge/blue = 3 small/blue = 1
- 19. PROBABILITY WILL REAL LIFE FOLLOW THE RESULTS FROM A PUNNETT SQUARE? NO!!!!!! – A PUNNETT SQUARE ONLY SHOWS WHAT WILL PROBABLY OCCUR IT’S A LOT LIKE FLIPPING A COIN – YOU CAN ESTIMATE YOUR CHANCES OF GETTING HEADS, BUT REALITY DOESN’T ALWAYS FOLLOW PROBABILITY