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DIHYBRID CROSSES

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									    Journal: Use punnett squares to answer the
       following questions. Use your notes and
        punnett practice from last class for help.
1. Cross a heterozygous brown furred male with
   homozygous recessive white furred female.
   a. What percentage of offspring will be brown?
   b. What percent will be white?
   c. What is the ratio of brown to white?
   d. What percentage of offspring will be BB?
   e. What percentage of offspring will be Bb?
   f. What percentage of offspring will be bb?
   g. Which is dominant brown fur or white?
   h. Give an example of a genotype from this
            scenario.
   i. Give an example of a phenotype from this
            scenario.
•   In pea plants, purple flowers are dominant over
    white flowers. Cross a homozygous dominant
    plant with a homozygous recessive plant.
    What percentage of offspring will be purple?
    What percent will be white?
•   Cross two offspring produced in #3. What are
    the genotypes and phenotypes produced?
    Express this in ratios and percentages.
      GENETICS
        DAY 3


  DIHYBRID CROSSES &
EXCEPTIONS TO MENDELIAN
       GENETICS
            DIHYBRID CROSSES
• Cross involving two pairs
  of alleles. Uses 16 boxes
• EX: round seeds R are
  dominant over wrinkled
  seeds (r). Yellow seeds
  (Y) are dominant over
  green seeds (y).
    – Parent generation =
      RRYY x rryy
• F1 generation produces
  all dominant traits.
    – RrYy
• F2 generation produces a
  mix of traits
    – RrYy x RrYy
•Ratio is 9:3:3:1
     EXCEPTIONS TO MENDELIAN
            GENETICS
• INCOMPLETE
  DOMINANCE
• Neither allele is
  completely dominant
• Both alleles combine
  equally to give a new
  trait.
• Called hybrids
• EX: red flower crossed
  with white flower
  produces pink flower.
• CODOMINANCE
• Both alleles are
  expressed
• EX: cross a black
  chicken with a white
  chicken, offspring will
  be black and white
  checkered.
           DAILY QUIZ #3
1. Cross out quiz #3 on the front of your
   quiz sheet.
2. On the back of your quiz sheet, complete
   the following dihybrid cross:

              GgRr x GgRr
            JOURNAL #3
• John has red hair and blue eyes. His two
  brothers and one sister have brown hair
  and brown eyes. Explain how this is
  possible. Use punnett squares to show
  how he might have inherited each trait.

								
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