THE EXAMS POINTA monohybrid cross: Aa × Aa
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A monohybrid cross: Aa × Aa
01 · AA
One of four equally likely genotype combinations.
02 · Aa
Two of four combinations are heterozygous.
03 · aa
One of four combinations is homozygous recessive.
Connect the branches
With equal segregation, random fertilisation and complete dominance, the expected genotype ratio is 1:2:1 and phenotype ratio is 3:1.
Avoid the exam trap
A probability ratio is not an exact quota for every small family. Incomplete dominance changes the phenotype ratio.
Test yourself: Under complete dominance, what is the probability of the recessive phenotype?
One quarter, because only aa expresses the recessive phenotype under these assumptions.
A concise revision aid with simplified connections. It supports the full chapter and primary references; it does not replace them.