Review of Complementation
Key Premise: In some cases the same variant phenotype can be due to homozygous recessive
genotypes at distinct genes.
e.g. aa BB and AA bb might have the same phenotype
So when we have two (or more) independent pure-breeding strains with the same recessive
phenotype, we want to find out whether they are all homozygous for recessive alleles of the same
gene, or homozygous for recessive alleles of different genes:
e.g. are all strains aa, or some aa and some bb, some cc etc.?
A Complementation Test (Cross the strains to each other in pairs) allows us to decide:
• If the F1 shows the dominant WT phenotype (complementation), the crossed strains were
homozygous for recessive alleles of different genes: e.g. one is aa BB, other is AA bb so the F1 is Aa
Bb
• If the F1 shows the recessive variant phenotype (no complementation), the crossed strains were
homozygous for recessive alleles of the same gene (e.g. both aa so the F1 is aa)
Same Gene
Strain 1
Strain 2
Same Gene
Strain 1
Strain 2
Different Genes
Strain 1
Strain 2
aa BB x aa BB
AA bb x AA bb
aa BB x AA bb
aa BB
AA bb
Aa Bb
Homozygous
Recessive
Homozygous
Recessive
Heterozygous
Variant Phenotype
Variant Phenotype
WT Phenotype
No complementation
No complementation
Complementation
To identify genes involved in congenital deafness, you have isolated 3 true-breeding
strains of deaf mice.
What should your first set of crosses be? What are you checking for?
You cross them in all possible pair-wise combinations and obtain deaf or WT progeny
as shown:
1
2
3
Among these strains, how many different genes
are homozygous for recessive deafness alleles?
1
deaf
2
WT
deaf
3
deaf
WT
deaf
Epistasis example
• Pure breeding white squash crossed with pure breeding green squash
• F1 generation yields all white squash progeny
• F2 generation yields 12 white squash, 3 yellow squash, and 1 green squash
• Explain what is happening in the F2 generation
Three point cross and mapping example
BCD
bcd
Bcd
bCD
BcD
bCd
BCd
bcD
400
400
1
1
68
68
31
31
Meiosis and non-disjunction