Topics for Today

1
Topics for Today

• Review of Dihybrid Cross - Relationship with
  molecular/chromosomal behavior
• Deductions from Pedigrees
• Calculation of Genetic Probabilities

2
Mendels InterpretationsReinterpretted
Mendels Interpretation Our Interpretation
Traits do not blend but are determined by
unchangeable units
Genes proteins traits
x
3
Mendels InterpretationsReinterpretted
Mendels Interpretation Our Interpretation
Each trait is determined by two units
Two homologous chromosomes
4
Mendels InterpretationsReinterpretted
Mendels Interpretation Our Interpretation
A a
The two units may or may not be identical
Genes come in different forms, alleles, which
make different protein
5
Mendels InterpretationsReinterpretted
Mendels Interpretation Our Interpretation
One character form is recessive to or dominant
over another
P gt p
p gt P
6
Mendels InterpretationsReinterpretted
Mendels Interpretation Our Interpretation
The two character forms carried by a heterozygote
are passed to progeny with equal likelihood Law
of Segregation
7
Mendels InterpretationsReinterpretted
Mendels Interpretation Our Interpretation
Different traits assort independently Law of
Independent Assortment
8
Different traits assort independentlyLaw of
Independent Assortment
9
Different traits assort independentlyLaw of
Independent Assortment
10
Deductions from Pedigrees

• Pedigree with ephemeral trait (Fig. 2)
• Pedigrees with other kinds of traits (next week)
• Genetic counseling (later today)

11
Deductions from PedigreesHow is the trait
inherited?
A- A-
A-?
A-?
Try dominant
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Deductions from PedigreesHow is the trait
inherited?
aa aa
aa aa aa aa aa
aa aa aa aa aa aa aa
aa aa
aa A- aa A- aa
Any problem?
13
Deductions from PedigreesHow is the trait
inherited?
aa aa
Try recessive
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Deductions from PedigreesHow is the trait
inherited?
A- A-
A- A- A- A- A-
A- A- A- A- A- A- A-
A- A-
A- aa A- aa A-
Can we get more?
15
Deductions from PedigreesHow is the trait
inherited?
Which one gave a?
Which one gave a?
A- A-
A- A-
A- A- A- A- A-
A- A-
A- A- A- A- Aa Aa A-
A- aa A- aa A-
Can we get more?
16
Deductions from PedigreesHow is the trait
inherited?
Which one gave a?
Which one gave a?
A- A-
What about outsiders?
What about outsiders?
A- Aa A- Aa A-
A- A- A- A- Aa Aa A-
A- aa A- aa A-
Can we get more?
17
Deductions from PedigreesHow is the trait
inherited?
A- A-
AA Aa A- Aa AA
A- A- A- A- Aa Aa AA
A- aa A- aa A-
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Genetic Counseling
Make the problem concrete
Will our children be normal ? ? ?
Whats the probability that a child of III.5 x
III.6 will have CS?
Whats the probability that a child of III.5 x
III.6 will be aa?
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Genetic Counseling
A- aa
Parse the problem (start simple)
AA A- A- A- AA
A- A- A- A- A- A- A-
Child will be aa if
AND III5 a AND III6 a
III5 is Aa AND III6 is Aa
Probability
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Genetic Counseling
A- aa
Solve each segment
AA A- A- A- AA
A- A- A- A- A- A- A-
Child will be aa if
AND III5 a AND III6 a
III5 is Aa AND III6 is Aa
Probability
1/2
1/2
II2 is Aa AND II2 a
1/2
1
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Genetic Counseling
A- aa
Solve each segment
AA A- A- A- AA
A- A- A- A- A- A- A-
Child will be aa if
AND III5 a AND III6 a
III5 is Aa AND III6 is Aa
Probability
1/2
1/2
1/2
1/2
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Genetic Counseling
A- aa
Put partstogether
AA A- A- A- AA
A- A- A- A- A- A- A-
Child will be aa if
AND III5 a AND III6 a
III5 is Aa AND III6 is Aa
Probability
1/2
1/2
1/2
1/2
Add?
union mutually exclusive more possibilities
Multiply?
intersection independent fewer possibilities
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Union of possibilities
Probability that progeny of Aa x Aa has A
phenotype
Gets A from female OR gets A from male
Rule of additionunionmutually exclusive
Gets aA OR AA OR Aa
P(A-) 1/4 1/4 1/4 3/4
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Intersection of possibilities
Probability that progeny of Aa x Aa has a
phenotype
Gets a from female AND gets a from male
Rule of multiplicationintersectionindependent
Gets a from female AND gets a from male
P(aa) 1/2 x 1/2 1/4
25
Genetic Counseling
A- aa
Put partstogether
AA A- A- A- AA
A- A- A- A- A- A- A-
Child will be aa if
AND III5 a AND III6 a
III5 is Aa AND III6 is Aa
x
x
x
Probability
1/2
1/2
1/2
1/2
Add?
union mutually exclusive more possibilities
Multiply?
intersection independent fewer possibilities
1/16
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Example illustrating Rule of Complementation

• Suppose there are two genes (A, B) that are
  required for dark hair

• A defect in any one of them will produce light
  hair

• What is the probability that a person will have
  light hair?

Make problem concrete
Light hair if A- OR B-
P(A- OR B-) P(A-) P(B-)
Parse problem
Is Rule of Addition valid here?
Is possession of A- and possession of B- mutually
exclusive?
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How to Calculate P(A- OR B-)Probability of light
hair
P(A-)
P(B-)
P(A-) P(B-)?
Not mutually exclusive.P(A-B-) added twice
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How to Calculate P(A- OR B-)Probability of light
hair
P(A-) P(B-)?
Not mutually exclusive.P(A-B-) added twice
P(A-) x P(B-)?
Gives intersection, not union
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How to Calculate P(A- OR B-)Probability of light
hair
P(A-)
P(B-)
P(A-) probability of possessing defective allele
of gene A
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How to Calculate P(A- OR B-)Probability of light
hair
not A-
P(not A-) 1 - P(A-) probability of not
possessing defective allele of gene A
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How to Calculate P(A- OR B-)Probability of light
hair
P(A-)
P(B-)
P(B-) probability of possessing defective allele
of gene B
32
How to Calculate P(A- OR B-)Probability of light
hair
not B-
P(not B-) 1 - P(B-) probability of not
possessing defective allele of gene B
33
How to Calculate P(A- OR B-)Probability of light
hair
not A-
P(not A-) 1 - P(A-) probability of not
possessing defective allele of gene A
34
How to Calculate P(A- OR B-)Probability of light
hair
not A-ANDnot B-
P(not A- and not B-) 1 - P(A-) x 1 - P(B-)
probability of not possessing either defective
allele
35
How to Calculate P(A- OR B-)Probability of light
hair
A- OR B-
not A-ANDnot B-
P(A- or B-) 1 - 1 - P(A-) x 1 - P(B-)
probability of possessing either defective allele
36
How likely to get hemophilia?

• There are five known alleles for the clotting
  factor protein Factor VIII (H1-, H2-, H3-, H4-,
  H5-).

• Three of them (H3-, H4-, H5-) cause hemophilia.

H3- hemophilia H4-
hemophilia H5-
hemophilia H4
???

• What is the probability that a person will have
  one of the three defective alleles and thus get
  hemophilia?

P(H3-, H4-, OR H5-)
P(H3-) P(H4-) P(H5-)
Rule of addition
Union?
but is possession of the alleles mutually
exclusive?
37
Union of possibilities
P(A-) 0.4 0.4 0.1 0.9