11-2 Probability and Punnett Squares

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11-2 Probability and Punnett Squares
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Mendel realized that the principles of
probability could be used to explain the results
of genetic crosses.He repeated a particular
cross and obtained the same results.
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Probability

• The likelihood that a particular event will occur

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A coin flip

• Possible outcomes
• Heads
• Tails
• Either outcome are equal.

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One coin flip, makes 1 chance in 2This is ½ or
50
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The probability of flipping three heads in a row

• ½ X ½ X ½ 1/8
• 1 chance in 8 flips

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The principle of probability can be used to
predict the outcome of genetic crosses.
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Punnett Square

• The gene combinations that might result from a
  genetic cross.

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F1 Parents are shown along the top and left side
of the square.
It also contains the possible gene combination
for F2 offspring and the allele
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The punnett square can be used to predict and
compare the genetic variations that will result
from a cross.
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Homozygous

• Organisms that have two identical alleles for a
  particular trait TT or tt.
• Org. for true breeding in particular trait

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Heterozygous

• Organisms that have two different alleles for the
  same trait. Tt
• Organisms are hybrid for a particular trait.

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Phenotype

• Plants have the same physical characteristics.

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Genotype

• Genetic makeup,
• Genotype of one third of the tall plants is TT
• Two thirds of the tall plant is Tt.

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Plants have the same phenotype but different
genotype.
What would happen if you cross this two plants?
Short or Tall??
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T t
T TT 25 Tt 25
T TT 25 Tt 25
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Probability and Segregation

• ¼ of the F2 plants have two alleles for tallness
• 2/4 or ½ have one allele for tallness
• ¼ one allele for shortness
• ¾ are going to be tall and ¼ short
• Ratio 31

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Predicted Ratio

• For each seven crosses ¾ of the plants showed the
  trait controlled by the Dominant.
• ¼ showed the trait controlled by the recessive
  allele.

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• The larger the number of offspring, the closer
  the resulting number will get to expected values.

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Homework

• To turn in
• Make a punnett square with your family.
• Use only one trait, like eye color, tallness,
  skin color, etc.
• Add pictures to illustrate.

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T t
t Tt 25 tt 25
t Tt 25 tt 25