Lesson 6 of 7 / Inheritance and genetic problems
Infer ratios from a pathway
Why can four genotype classes collapse into three phenotypes?
I can solve epistasis problems from interactions between loci.
I can link genotype to phenotype.Syllabus
Syllabus 9477, Core 2(w). Link genotype to phenotype.
I can solve dihybrid problems involving dominance, codominance, multiple alleles, sex linkage, autosomal linkage and epistasis.Syllabus
Syllabus 9477, Core 2(x). Solve dihybrid problems involving dominance, codominance, multiple alleles, sex linkage, autosomal linkage and epistasis.
I can infer epistatic interactions and phenotype ratios from pathway information.Syllabus
Syllabus 9477, Core 2(aa). Infer epistatic interactions and phenotype ratios from pathway information.
About 8 min
Make a guess. You are not marked.
A dihybrid cross gives 9 purple : 3 brown : 4 white. What could explain only three phenotypes?
Show the answer
One gene masks the effect of the other.
If one enzyme is missing, the next step has nothing to act on. aaB_ and aabb both look white.
The key idea
A gene at one locus can mask the phenotypic effect of another; derive the ratio from the supplied biology instead of memorising it.
Build from gametes
Which step is blocked by the genotype?
A_B_ gives purple. A_B_ is purple (9/16); A_bb remains brown (3/16); aaB_ and aabb lack the first product and are white (4/16).
The ratios refer to AaBb x AaBb with independent loci and complete dominance at each locus. Gene interaction changes phenotype grouping; it does not change the underlying 9:3:3:1 genotype-class probabilities. A and B label functional enzyme-producing alleles in this model.
Explanation
Epistasis is an interaction in which a genotype at one locus masks or modifies the phenotypic expression of another locus. It differs from dominance, which describes the relationship between alleles at the same locus.
Consider a pigment pathway: enzyme A makes a brown intermediate from a colourless precursor, and enzyme B converts brown to purple. One functional dominant allele at each locus is sufficient for its enzyme. A_B_ is purple, A_bb is brown, and aa__ is colourless because the upstream step fails regardless of B.
In AaBb x AaBb with independent assortment, the underlying genotype groups are 9 A_B_, 3 A_bb, 3 aaB_ and 1 aabb. Applying the pathway combines the last two into colourless, giving 9 purple : 3 brown : 4 colourless. The ratio follows from the pathway, not from a new way of segregating chromosomes.
Other pathways produce other groupings. If both enzymes are required to make any coloured product and either recessive homozygote blocks it, only A_B_ is coloured, giving 9 coloured : 7 uncoloured. The syllabus emphasises interpreting information, not memorising a catalogue of special ratios.
Step by step
- 1
Read the biochemical rule
Identify which step each allele controls.
- 2
Assign each genotype class
Ask whether the relevant product can form.
- 3
Combine matching phenotypes
Add probabilities only after the biological grouping.
Worked example
Work through the evidence
In the brown-to-purple pathway, what is the phenotype of aaBB and why?
One way to explain it
Colourless: without functional A, the brown intermediate is not made, so B has no substrate to convert to purple.
Why this answer works
- The upstream block masks the downstream genotype.
- BB does not bypass the missing biochemical step.
Watch out for this
A student says: "Epistasis means an allele moves to a different chromosome." What is wrong with this?
Show the answer
It describes an interaction in phenotype, not a physical relocation of the gene.
Check your understanding
Which genotype is brown in the stated two-step pathway?
Show the answer
Aabb
A produces the brown intermediate, but bb blocks its conversion to purple.
Exam-style questions
Check your understanding
Which ratio from AaBb x AaBb is NOT explained by epistasis?
Show the answer
1:1:1:1
That is the test-cross ratio for AaBb x aabb. Epistasis merges classes of 9 : 3 : 3 : 1 into fewer groups.
Quick recall
Why can four genotype classes collapse into three phenotypes?