Lecture 9 – Modifications of Mendelian ratios (continued)
I. Interactions between mutations in different genes can modify 9:3:3:1 phenotypic ratios
A. Interactions between genes in different pathways
1. corn snake – normal has orange and black (camouflaged) pattern
- pattern due to two pigments, produced by two different pathways
- what does this do to dihybrid ratios?
orange X black
o+/o–; b+/b– X o+/o–; b+/b–

o+/–; b+/–
o+/–; b–/b–
o–/o–; b+/–
o–/o–;b–/b–
B. Interactions between mutations in different genes in the same pathway.
1. either mutation causes same phenotype
- eg. pea flower color
white X white
c+/c–; p+/p– X c+/c–; p+/p–

c+/–; p+/–
c+/–; p–/p–
c–/c–; p+/–
c–/c–;p–/p–
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2. mutations cause different phenotypes
- eg. blue-eyed Mary (Collinsia parviflora)
magenta X white
w+/w–; m+/m– X w+/w–; m+/m–

w+/–; m+/–
w+/–; m–/m–
w–/w–; m+/–
w–/w–;m–/m–
3. one mutation reverses effect of other mutation
- eg. flyeye color
red eye X purple eye
pd+/pd–; su+/su– X pd+/pd–; su+/su– 
pd+/–; su+/–
2
pd+/–; su–/su–
pd–/pd–; su+/–
pd–/pd–;su–/su–
4. different genetic interactions may produce different ratios
- can get 12:3:1, 9:6:1, etc.
- each of these suggests the underlying genotype involves two alleles of
two genes
II. So how do you determine genotype when all you have are phenotypes?
A. eg. 1: Two true breeding strains of frogs, one with green spots one with red.
green X red
orange X orange
44 red
34 green
113 orange
What are the genotypes of the F2 offspring?
B. eg. 2: Two true breeding strains of frogs, one with yellow spots one with blue.
yellow X blue
green X green
117 yellow
127 blue
236 green
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What are the genotypes of the F2 offspring?
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