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Punnett squares

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Punnett squares

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

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