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Punnett Practice. What does the Punnett square represent?. When written together, the letters on the edges of the square represent the genotypes of the parents (e.g. Tt, yy)
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What does the Punnett square represent? • When written together, the letters on the edges of the square represent the genotypes of the parents (e.g. Tt, yy) • BUT, when placed along the Punnett square, they actually represent the alleles present in the gametes of the parents (e.g. T, t, Y, y) Explain to your partner why the second statement makes sense.
What does the Punnett square represent? Gametes must have half the normal number of chromosomes that body cells have or else the offspring will have too many chromosomes!
Monohybrid Cross T - allele for a tall plant t - allele for a short plant Show the possible offspring for the following cross: TT x Tt
Monohybrid Cross T - allele for a tall plant t - allele for a short plant Show the possible offspring for the following cross: heterozygous X homozygous recessive
Test Cross In dogs, there is a hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog is heterozygous, the owner does not wish to use him for breeding. (Why not?)
Test Cross She can find out the genotype of her dog by breeding him to a deaf female (dd). • Draw the Punnett squares to illustrate these two possible crosses. • In each case, what percentage of the offspring would be expected to be hearing? • How could you tell the genotype of this male dog?
Test Cross • Would you do a test cross if the organism is homozygous recessive? • How can two hearing dogs produce deaf offspring? • What is a test cross?
Dihybrid Cross • When you’re considering two sets of genes at the same time • For an online tutorial, go to: http://www.biology.arizona.edu/mendelian_genetics/problem_sets/dihybrid_cross/03t.html
Dihybrid Cross A dihybrid cross with two heterozygous parents • Gene 1: A-dominant, a-recessive • Gene 2: C-dominant, c-recessive Parents are heterozygous for both genes To set up the Punnett square, think about what combination of alleles can be in the parents’ gametes
Dihybrid Cross AC Ac aC ac AC Ac aC ac
Dihybrid Cross AC Ac aC ac AACC AACc AaCC AaCc AC Ac aC ac AACc AAcc AaCc Aacc AaCC AaCc aaCC aaCc AaCc Aacc aaCc aacc
More Dihybrid Crosses (Part 1) Gene 1: A - dominant; a - recessive Gene 2: C - dominant; c - recessive 1) heterozygous X homozygous recessive 2) heterozygous X homozygous dominant
More Dihybrid Crosses (Part 2) • Read p. 320 Analyzing Data (blood types and Rhesus factor). Answer questions 3 and 4, then solve the problem below. • Mrs. Xu, Mrs. Yang, and Mrs. Zhang all gave birth to baby girls on the same day. Someone later claimed that the hospital mixed up the babies. As a hospital administrator, it is your job to make sure that each pair of parents has the correct baby, so you order blood typing to be done on all the parents and all the babies.
More Dihybrid Crosses (Part 2) Here are the results: Baby 1 O+ Baby 2 AB- Baby 3 B-
More Dihybrid Crosses (Part 2) First, for each parent, think about what possible genotype(s) could give that phenotype. If there is more than one possible genotype, then which of the possible genotypes would give the most variation in terms of possible children? (If you want to make a Punnett square, this is a dihybrid cross.) Which baby belongs to which set of parents? Prepare to defend your answer.
