Beyond the Punnett Square

1
Beyond the Punnett Square

• Warmup What is the probability of getting a
  heterozygous F1 when crossing a homozygous
  recessive with a heterozygous?
• Read 138-152

2
Dihybrid Cross

• Like using two punnett square crosses at the same
  time.
• Still follows same rules as regular punnett
  square
• Law of Ind. Assortment sets up the options for
  across top and down side
• Ex Cross RrYy x RrYy
• Each box would get one R and one Y, best done
  using foil method. (RY, Ry, rY, ry)

3
Test Cross

• As mentioned last class, a dominant trait
  expressing hides the genotype (i.e. HH or Hh)
• One way to figure out the genotype is to use a
  test cross.
• Using a homozygous recessive will determine the
  parent genotype
• Do a quick set of punnett squares to see why!
• hint Cross HH x hh and Hh x hh

4
Lucky for Mendel

• Interestingly enough, traits in nature with only
  2 distinct forms are VERY rare.
• When dominance isnt 100 expressed over a
  recessive we get two possibilities
• Incomplete dominance
• Codominance

5
Incomplete Dominance

• Results in a blending of the phenotypes of two
  homozygous parents.
• Each allele is partially expressed
• Example
• Pink roses
• Animal coats/fur
• Human cholesterol levels

6
Codominance

• Instead of a blending like incomplete dominance,
  codominance has full expression by each allele.
• Ex Blood Type A, B, AB, O
• An allele is labeled as IA (dom) or IB (dom) or i
  (rec)
• Each blood type is a combination of those 3
  variations but notice that AB blood is a fully
  expressed IA and IB

7
Polygenetic traits

• Another variation on the simple punnett square is
  the introduction of polygenetic traits.
• Poly means many, so a polygenetic trait is one
  that is spread over many genes.
• Ex Eye color, skin color
• This results in many shades of phenotypes.

8
Test Results
9
Test Corrections

• Because of the low average
• All tests can be corrected
• Corrected tests will receive ½ of all points lost
• i.e. a 30 will gain ½ of the 70 points wrong -gt
  65
• i.e. a 60 will gain ½ of the 40 points wrong -gt
  80
• All 5 questions must be re-answered for
  correction points.
• Test corrections are due in ONE WEEK