Complex Inheritance

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Complex Inheritance
2-Locus interactions, Epistasis Sept. 18, 2003
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Mendels Traits in Peas

• Traits were
• Clearly distinguishable pairs of phenotypes
• Traits appeared in all plants
• Phenotypes distinguished varieties of peas
• Traits were stable in hybrids and subsequent
  generations

2-3
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Grain Phenotypes in Corn
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Epistasis in Biochemical Pathways

• In Corn, the aleurone pigment is formed by the
  following pathway.
• C R A
  Pr
• pc cl1 cl2 red
• cc, CI rr aa pr
  pr
• The endosperm color pathway is
• Y
• white yellow
• yy

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Monohybrid -- Independent Segregation
alleles areparticulate
allelessegregate
2-7
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Dihybrid Cross Complementation

• AAcc X aaCC
• (Yellow) (Yellow)
• AaCc
• (Purple)
• When two mutant strains are crossed and the F1
  hybrids show normal function, we say that the two
  strains complement each other, and the mutant
  alleles are at different loci so they are
  non-allelic and the loci are non-homologous.

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Pleiotropy of Y-locusfunction in endosperm
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Developmental Timing of Ac activity is affected
by epistatic loci
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DNA chip
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Test Cross ABc / abC X abc / abc
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A C B
rBC 0.144
rAC 0.100
B C 69 67 136
A C48 44 92
A C, B C 6 2 8
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Crossing Over in One Sex

• reciprocal test crosses
• ABc / abC ? X abc / abc ?
• ABc / abC ? X abc / abc ?
• if crossing over only in female,
• compare the results of above crosses

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Complementation

• Suppose you have two yellow strains of corn.
• How do you determine the locus at which they are
  mutant (C, R, or A)?

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ComplementationA Test for Allelism

• Convention -- All loci are dominant and
  functional unless specified otherwise.
• Consider crossing the two strains under two sets
  of assumptions
• First, assume they are at the same locus.
• Second, assume they are at different loci.

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Failure of Complementation

• If the two strains are mutant at the same locus
• The alleles fail to complement.

r1r1 X r2r2 r1/r2
All of the offspring are expected to be mutant.
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Complementation

• Now lets cross yellow corn varieties, assuming
  that do not share the same locus
• cc X aa
• This is an incomplete way to write the cross, but
  is how it would be if we didnt know they were
  different loci.
• Each locus should be represented in each
  individual. Using the convention of Dominant and
  Functional,
• AAcc X aaCC

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Linkage

• Loci number in tens of thousands.
• Chromosomes number in the several to dozens.
• Therefore, many loci are on the same chromosome.
• Chromosomes segregate independently in Meiosis
  I, and alleles at the loci included are carried
  along they are linked.

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Consider two loci on different chromosomes
Segregation AB, Ab, aB, ab If kinetochores
attach to spindle fibers at random
½ A and ½ B ¼ AB or ½ A and ½ b ¼ Ab or ½ a
and ½ B ¼ aB or ½ a and ½ b ¼ ab
½ A or ½ a ½ B or ½ b
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Consider two loci on the same chromosome
Segregation AB and ab At meiosis I there will be
only AB and ab products.
½ A and 1 B ½ AB or ½ A and 0 b 0 Ab or ½
a and 0 B 0 aB or ½ a and 1 b ½ ab
½ A or ½ a ½ B or ½ b
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If there is a New Process, called Crossing
Over
r
Segregation AB and ab At meiosis I there may be
all four gametic products.
½ A and ½ (1-r) B ¼ (1-r) AB or ½ A and r/2 b
r/4 Ab or ½ a and r/2 B r/4 aB or ½ a and
½ (1-r) b ¼ (1-r) ab
½ A or ½ a but ½ B or ½ b
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Mendels 2nd Law Fails

• Model is revised to allow a new process
• Model is complicated with a new variable
• New loci and organization of the cell is
  required
• New conditions are required for cell to function
• New traits result for the organism
• Is this list of incremental change parallel to
  the actual origin of this complexity, or just a
  conceptual conveyance?

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How can we generalize this new model to many
loci?

• Move from 2 loci linked to 3 loci linked
• With 3 loci, what is relationship between
  crossovers in adjacent regions?

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Multiple Crossovers
IF Pco 1 and co 2 Pco 1 X Pco 2
then they are independent events. OTHERWISE
the process of crossing over in two regions are
functionally related this is more complex than
if they are independent. Parsimony requires the
new model uses independent events if refuted,
the alternative model is they are not independent.