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Biology & life sciences · 06 / 12

Inheritance: gametes and probability

High school · intermediate · Worked example and 4 exercises with solutions, at your pace.

Need a foundation?Cells and DNA: levels of organisationMitosis and meiosis: maintain or reduce
UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

In a simple diploid model, an individual has two alleles at a locus and each gamete receives one. A cross combines one gamete from each parent. Genotype describes alleles; phenotype describes an observable trait. Complete dominance is a model assumption, not a universal trait rule.

p=1/4
THE PICTURE TO REMEMBERInheritance: gametes and probability
  1. 01Two alleles
  2. 02One per gamete
  3. 03Combine
  4. 04Genotype then phenotype

WORKED EXAMPLE

From principle to reasoning

Aa × Aa gives AA, Aa, Aa, aa: probabilities 1/4, 1/2, 1/4. Complete A dominance gives dominant phenotype probability 3/4.

Solve a problem in this subject

  1. Frame a question

    State the relationship to explain and its scale: molecule, cell, organism, population or geological time.

  2. Describe evidence

    Give a measurable observation, unit, comparison and conditions before explaining it. Observation and inference do different jobs.

  3. Connect a mechanism

    Use a precise concept and label arrows as production, transfer, action or transformation. Sequence alone does not prove causation.

  4. Answer with a limit

    Connect evidence and knowledge to the question. Distinguish the simplified model from living systems and identify a missing condition.

YOUR TURN

01 · Try before checking

What genotypes result from AA × aa?

One hint

The first parent only supplies A gametes.

Reveal and understand the solution

Every model offspring is Aa, not AA even if the phenotype is dominant.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

In a two-allele model, what gametes does Aa produce?

A hint to get started

A gamete receives one allele of this gene.

Step-by-step solution
  1. Allele separation gives A or a.
  2. In the simple Mendelian model each has probability 1/2.
  3. Aa is the parent genotype, not each gamete genotype.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

Cross Aa with aa. Find the probability of aa.

A hint to get started

The second parent always supplies a.

Step-by-step solution
  1. The first supplies A or a with probability 1/2.
  2. Offspring genotypes are Aa or aa.
  3. P(aa) = 1/2 in this model.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

Must a dominant allele be the most common in a population?

A hint to get started

Dominance describes the heterozygote phenotype.

Step-by-step solution
  1. Dominance compares allele expression in an individual.
  2. Frequency measures allele prevalence in a population.
  3. A dominant allele can be rare: these are separate ideas.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

Small families need not match the expected probabilities exactly.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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