Lesson 6 of 7 / Inheritance and genetic problems
Infer ratios from a pathway
Why can four genotype classes collapse into three phenotypes?
In this lesson: Solve epistasis problems from interactions between loci.
About 8 min
The key ideaA 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.
Is this true? "Epistasis means an allele moves to a different chromosome."
It describes an interaction in phenotype, not a physical relocation of the gene.