Genetic Crosses and Probability
Using genetic diagrams, ratios and probability to predict single-gene inheritance and human sex determination.
How to study for GCSE Combined Science
Build the three sciences as connected disciplines, practise mathematical and practical reasoning, then apply the common core to the exact tier and paper structure of your current specification.
Core concepts
Concept 1
Dominant and recessive describe allele expression in a heterozygote, not strength, frequency or desirability.
Exam cue: Define allele symbols and parental genotypes before constructing the cross.
Concept 2
A genetic cross tracks parental genotypes, possible gametes and offspring combinations.
Exam cue: State genotype probability separately from phenotype probability.
Concept 3
Predicted ratios are probabilities for each independent offspring and may differ from small observed samples.
Targeted study blocks
Tier coverage
Foundation core with Higher-tier extensions
Higher-tier papers can demand less familiar crosses, reverse inference from family evidence and multistep probability reasoning.
Risk pitfalls and guardrails
Changing the probability because earlier offspring had a particular phenotype.
Guardrail: Do not import separate-science content or another route's paper order; use the full physics equations sheet required for current specifications while checking the exact series materials and entered tier.
Inferring a genotype that the phenotype cannot uniquely determine.
Guardrail: Do not import separate-science content or another route's paper order; use the full physics equations sheet required for current specifications while checking the exact series materials and entered tier.
Memory anchors
Dominant allele
An allele expressed in a heterozygous genotype.
Recessive allele
An allele expressed phenotypically only when no dominant allele is present.
Homozygous
Having two identical alleles at a gene locus.
Heterozygous
Having two different alleles at a gene locus.
Genetic probability
The predicted chance of an independently inherited outcome.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
For a gene with alleles B and b, what does heterozygous mean?
What does it mean for allele B to be dominant to allele b?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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