Test Crosses
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Lesson overview
Predicting outcomes of a cross with a homozygous recessive parent.
Go to lesson quiz →Exam guidance · WAEC WASSCE Paper 1 + Paper 2 Lens
Pacing
Objective speed on Paper 1 + structured step-by-step method marks on Paper 2 Theory/Practical.
How WAEC marks this
Examiners award method marks (M1) for stating the formula/definition and accuracy marks (A1) ONLY when SI units are written.
Watch out for
Writing bare numbers without working or omitting SI units loses up to 50% of Paper 2 marks even when the final value is right.
Lesson content
Overview — Test Crosses
Predicting outcomes of a cross with a homozygous recessive parent. This lesson is at the advanced level — by the end, you'll be able to recognise this question type, apply a reliable method, and avoid the traps that cost marks.
Key Concept
A test cross is a specific type of monohybrid cross used to reveal an unknown genotype: an organism showing the dominant phenotype is crossed with a homozygous recessive individual (genotype aa).
Why It's Tested This Way
Test crosses at Advanced level combine genetics reasoning with real scientific method — this is tested because it shows you understand genetics as something SCIENTISTS ACTUALLY USE to solve real unknown-genotype problems, not just an exam exercise.
Method
The RESULT of the test cross tells you the unknown parent's genotype: all-dominant offspring means AA; a roughly 1:1 split means Aa.
Worked Example
Worked example: An unknown-genotype black guinea pig is crossed with a brown (bb) guinea pig. If half the offspring are brown, the black parent must have been Bb (heterozygous).
Second Worked Example
Second worked example: An unknown-genotype tall pea plant crossed with a short plant (tt) produces all-tall offspring — telling us the unknown parent was homozygous dominant (TT).
Common Traps
Common trap: forgetting WHY a test cross works — the homozygous recessive parent can only contribute a recessive allele, so any recessive offspring must have got their dominant parent's hidden recessive allele.
Lesson Summary
Remember: (1) identify what is being asked, (2) apply the WAEC strategy, (3) check against the traps. Complete the 10-question quiz below to unlock the next lesson — you need at least 60% to pass.
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