Unformatted text preview: LECTURE  9
1. Dihybrid crosses
2. Branch diagrams
3. Binomial Expansion Dihybrid Cross Rr
R RR Rr
r Rr rr Y y Y YY Yy
y Yy yy Branch Diagrams Rr
r Rr rr
r Rr rr Y y y Yy yy
y Yy yy Dihybrid Testcross INCOMPLETE
DOMINANCE Probabilities and Binomial Expansion A
A Aa a
Aa ¼ albino a Aa aa ¾ normal pigmentation If the heterozygous patients (Aa) have 3 children,
what is the probability that 1 child has albinism?
There are 3 possible scenarios (events):
Child 1 Child 2 Child 3 Albino Pigmented Pigmented Pigmented Albino Pigmented Pigmented Pigmented Albino Probability for event 1 (first child is albino
AND second child is pigmented AND third
child is pigmented) to occur is the product of
the individual probabilities (muliplication
rule)
1/4x3/4x3/4=9/64 : event 1
Similarly, for events 2 and 3, the
probabilities are:
3/4x1/4x3/4=9/64: event 2
3/4x3/4x1/4=9/64: event 3 Only one of these scenarios will actually
occur, event 1 OR event 2 OR event 3.
Hence, the probability of having one child with
albinism and two without (in any order) is
determined by adding the probabilities of
events 1, 2 and 3 (addition rule).
9/64 + 9/64 + 9/64 = 27/64 Binomial Expansion 1
1
1
1
1
1 2
3 4
5 1
1
3
6 10 1
4 10 1
5 1 Pascal's triangle is a geometric arrangement
of the binomial coefficients in a triangle. When progeny of crosses segregate into
two distinct classes, we can calculate the
binomial probability of any particular
combination in another way using the
following formula: s individuals fall into the class with probability a
t individuals fall into the class with probability b
In the albinism example (two heterozygous parents
with three children, one of whom has albinism), s (=
1) would be the number of children with albinism, with
probability a (= ¼)
t (= 2) would be the number of children with normal
pigmentation with a probability b (= ¾) For our other albinism example:
five children three of whom have albinism: ...
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This note was uploaded on 01/25/2011 for the course BICD 100 taught by Professor Nehring during the Spring '08 term at UCSD.
 Spring '08
 Nehring
 Genetics

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