Science · Grades 8–12
Genetics: model crosses and their limits
These questions use one gene with two alleles, A and a, and complete dominance where stated.
Understand the idea
Use an explicit model
These questions use one gene with two alleles, A and a, and complete dominance where stated.
Probability is not a quota
A predicted fraction does not guarantee exact counts in a small family or sample.
Traits can be complex
Real traits may involve many genes and environmental factors.
Worked examples
In the stated two-allele model, which genotype is heterozygous?
Aa contains two different alleles at the modeled locus.
Aa
For Aa × aa, what fraction of offspring is expected to be aa under the simple model?
The first parent contributes A or a equally; the second contributes a.
1/2
Try it yourself
For Aa × Aa, what fraction is expected to be AA?
With A completely dominant, which genotype can show the recessive phenotype?
- AA
- aa
- either AA or Aa
- Aa
Does a 1/4 model probability require exactly one aa offspring out of every four? Explain.
Genotype refers here to
- the habitat temperature
- only the visible appearance
- the age of the organism
- the allele combination at the modeled locus
Answers and marking notes
Answers and marking notes
- 1/4
The four equally likely model outcomes are AA, Aa, aA and aa.
- aa
Two recessive alleles are needed in this model.
- No. It is a probability for each modeled outcome; small samples can differ from the expected proportion.
Credit random variation rather than a fixed sequence.
- the allele combination at the modeled locus
It describes genetic variants, not just appearance.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Preview material · independent educational review required. Grade bands are approximate, not official curriculum alignment.