Genetic Diagrams

Genetic Diagrams

What Are Genetic Diagrams?

Genetic diagrams help us predict the probability of inherited traits by showing how alleles (different versions of genes) are passed from parents to offspring.

💡 Key Concept: Everyone inherits two alleles for each gene – one from each parent.

Key Terms to Know

Term

Definition

Gene

A section of DNA that codes for a protein.

Allele

A different version of a gene.

Dominant

An allele that is always expressed (only needs one copy). We use a capital letter to show this.

Recessive

An allele that is only expressed if both copies are recessive. We use a lower case letter to show this.

Homozygous

Two of the same alleles (e.g. AA or aa).

Heterozygous

Two different alleles (e.g. Aa).

Genotype

The combination of alleles an organism has.

Phenotype

The physical characteristics caused by the genotype.

👉 Suggested Image: A key terms flashcard-style diagram or visual table.

Monohybrid Crosses

A monohybrid cross looks at one gene and how it is inherited.

Example: Inheritance of dimples

  • D = dimples (dominant)
  • d = no dimples (recessive)

Cross: Dd × Dd

 

D

d

D

DD

Dd

d

Dd

dd

Result:

  • 75% have dimples (DD, Dd, Dd)
  • 25% have no dimples (dd)

💡 Genotypic ratio: 1 DD : 2 Dd : 1 dd
💡 Phenotypic ratio: 3 dimples : 1 no dimples

Using Genetic Diagrams to Predict Outcomes

✔ Identify the genotype of each parent
✔ Use a Punnett square
✔ Work out the genotype and phenotype ratios of the offspring
✔ Use percentages or fractions if asked for probabilities

Interpreting Genetic Diagrams in Exams

You may be asked to:

  • Complete a Punnett square
  • Identify dominant/recessive traits
  • Describe the genotype/phenotype of offspring
  • Use ratios, percentages, or fractions

💡 Tip: Read the question carefully – it may ask for probability, genotype, or phenotype.

Questions 

  1. What is a genotype?
  2. What is the phenotype of someone with genotype Dd if D = dimples?
  3. What does a Punnett square show?
  4. What is the chance of having a child with no dimples from Dd × Dd?
  5. What do you call an organism with two of the same alleles?

Summary 

  • Genetic diagrams (Punnett squares) show how alleles are inherited.
  • Traits are determined by dominant and recessive alleles.
  • You should be able to predict genotype and phenotype ratios.
  • Know key terms like allele, genotype, phenotype, homozygous, and heterozygous.