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Biology · 6. Inheritance and variation

Alleles, genotypes and genetic probability

Use genetic crosses to predict ratios and distinguish probability from certainty.

45 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

What can a predicted genetic ratio say—and not say—about one family?

Curriculum coverage

Vietnam 2018 and Cambridge: alleles, meiosis, genetic crosses and probability.

Model scope and limitations

The single-gene model assumes complete dominance; it does not directly cover polygenic, linked, incompletely penetrant or environmental traits.

Learning objectives

  • Distinguish gene, allele, genotype and phenotype.
  • Construct gametes and Punnett squares.
  • Interpret ratios as probabilities over many offspring, not a fixed family sequence.

Prerequisite knowledge

  • Meiosis and fertilisation
  • Simple probability

Core knowledge

Alleles and genotype

A diploid individual usually has two alleles at a locus, one inherited from each parent.

Segregation

The two alleles segregate into different gametes during meiosis.

Probability

Each fertilisation is a new event; a 3:1 ratio does not guarantee exactly three and one in every four offspring.

Worked example

Aa × Aa gives AA, Aa, Aa and aa, a 1:2:1 genotype ratio; with complete dominance the phenotype ratio is 3:1.

Misconceptions to avoid

  • A 3:1 ratio is a long-run probability, not a guarantee for every four offspring.
  • Dominant does not mean more common, healthier or more beneficial.

Virtual experiment procedure: Mendelian genetics

  1. Choose parent genotypes and list each gamete type.
  2. Combine gametes in a Punnett square and count genotypes and phenotypes.
  3. Run many simulated offspring and compare observed frequency with theoretical probability.

Safety and cautions

  • Do not use a single-gene classroom model for personal diagnosis.
  • Avoid labelling alleles “good/bad”; phenotype depends on biological context.

Evidence to collect

  • Gamete lists, a Punnett square and genotype/phenotype ratios.
  • A comparison of simulated frequency with probability, explaining sample-size variation.
Open the virtual experiment

Diagnostic check

1. What is genotype Aa called?

  1. Heterozygous
  2. Homozygous dominant
  3. Homozygous recessive
Show answer and explanation

Correct answer: Heterozygous

Two different alleles at a locus make a heterozygous genotype.

2. Which gametes can an Aa individual produce at this locus?

  1. A and a
  2. Only Aa
  3. AA and aa
Show answer and explanation

Correct answer: A and a

Each gamete receives one allele from the locus.

3. What does a predicted 25% ratio mean?

  1. Probability for each fertilisation
  2. Guaranteed in the first four children
  3. Only in generation four
Show answer and explanation

Correct answer: Probability for each fertilisation

Probability does not prescribe the sequence in a small family.

Application practice

1. What genotype ratio results from Aa × Aa?

  1. 1 AA : 2 Aa : 1 aa
  2. 3 AA : 1 aa
  3. 1 Aa : 1 aa
Show answer and explanation

Correct answer: 1 AA : 2 Aa : 1 aa

The four combinations are AA, Aa, Aa and aa.

2. With complete dominance, what is the recessive-phenotype probability in Aa × Aa?

  1. 25%
  2. 50%
  3. 75%
Show answer and explanation

Correct answer: 25%

Only aa shows the recessive phenotype: 1/4 = 25%.

3. When may a simple Mendelian ratio not apply?

  1. Polygenic or linked traits
  2. Every completely dominant trait
  3. Whenever the Punnett square is correct
Show answer and explanation

Correct answer: Polygenic or linked traits

The model must change when independent, single-gene or complete-dominance assumptions fail.

Academic references

  1. OpenStax Biology 2e — peer-reviewed foundational biology

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer