AeroLab Reading room

Biology / A2 level

Inheritance and gene expression

Crossing over and the orientation of homologous chromosome pairs generate different allele combinations during meiosis.

Learning objectives

The big idea

Crossing over and the orientation of homologous chromosome pairs generate different allele combinations during meiosis. Fertilisation combines gametes; linked loci do not necessarily assort independently, and recombination frequency informs linkage analysis.

Genetic diagrams model specified assumptions about dominance, sex linkage and gene interactions. A probability describes expected proportions over many offspring rather than a required pattern in each small family. Gene expression and environmental effects connect genotype to phenotype.

Worked example

For two independent events with probabilities 1/2 and 1/4, find the probability of both.

Follow the solution

Independence permits multiplication: 1/2×1/4=1/8. This method must not be assumed for linked events.

Investigate and explain

Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.

Related simplified PhET model. Explain what it represents and what the living system does differently.

Open the interactive investigation

Check your understanding

1. When does crossing over occur in meiosis?

  1. During ordinary cytokinesis only
  2. After fertilisation in every embryo
  3. During prophase I between non-sister chromatids of homologous chromosomes
Check the answer and reasoning

During prophase I between non-sister chromatids of homologous chromosomes

Paired homologues can exchange corresponding DNA regions in prophase I.

2. For two independent events with probabilities 1/2 and 1/4, find the probability of both.

  1. 1/8
  2. 3/4
  3. 1/2
Check the answer and reasoning

1/8

Independence permits multiplication: 1/2×1/4=1/8. This method must not be assumed for linked events.

3. Does a 1/4 expected recessive probability guarantee exactly one affected offspring in every four?

  1. Yes
  2. No
  3. Only with complete dominance
Check the answer and reasoning

No

Each outcome is probabilistic; small samples can differ substantially from the expected proportion.