A modified dihybrid F2 ratio of 9:3:4

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A modified dihybrid F2 ratio of 934
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A modified dihybrid F2 ratio of 934
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9 3 4 F2 ratios from previous example
9 A __ C__ agouti (wildtype) 3
aa C__ black 3 A__ ca ca
albino
4 __ __ ca ca 1 aa
ca ca albino This is an
example of epistasis or gene (product)
interaction. One set of genes affects the
expression of another set. In this case, ca
(when homozygous) modifies the expression of the
A allele of another gene
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colorless intermediate
C (ca does not function)

black pigment
A (a does not function)
agouti pattern (wildtype)
Epistasis is most easily understood if we think
about gene products interacting in pathways.
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Complementation Two homozygous recessive mutants
are crossed, and the wildtype phenotype
reappears in the F1 progeny.
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If two recessive mutations complement they are
mutations in DIFFERENT genes even though they may
have the same phenotype.
Complementation can be used to determine if two
(or more) mutations which express the same
phenotype are really variants (alleles) of the
same gene. Typically, as mutations with the
same phenotype are discovered, they can be placed
into complementation groups. The members of
each group do NOT complement each other, but they
DO complement other groups. The number of
different complementation groups containing
mutations which prevent the synthesis of some
product (for example, a pigment) is often a
good approximation of the number of steps in a
particular pathway.
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Note the use of for complementation
(wildtype) and - for noncomplementation
(mutant phenotype).
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• p complements all genotypes but X3. Group I p,
  x3
• 2. c does not complement X2 or X4 X2 does not
  complement X4.
• Group II c, x2, x4
• X1 complements all other genotypes. Group III
  x1.
• What is the minimum number of steps in the
  biosynthetic pathway
• leading to the purple pigment?