7.1.5 Investigating Genetic Ratios

Key Steps in Investigating Genetic Ratios

1. Determine the Parental Genotypes

• Identify the genotypes of the parents based on the given traits (e.g. heterozygous, homozygous dominant, or recessive).

2. Use a Punnett Square

• Create a Punnett square to predict the ratios of offspring genotypes and phenotypes.

3. Expected Ratios

• Monohybrid crosses (one gene): typically result in a 3:1 phenotypic ratio (dominant: recessive) when heterozygous parents are crossed.
• Dihybrid crosses (two genes): typically result in a 9:3:3:1 phenotypic ratio when both parents are heterozygous for both traits.

4. Collect Observed Data

• Perform experiments, such as breeding trials, to record the actual traits observed in offspring populations.

5. Compare Expected and Observed Ratios

• Differences may occur due to chance, small sample sizes, or environmental factors.

6. Chi-Squared Test

• Use the chi-squared statistical test to compare observed data with expected ratios:
• A small chi-squared value indicates close agreement with predictions.
• A large chi-squared value suggests other factors may influence inheritance.

Tips for Success in Genetic Ratio Questions

• Always state whether traits are dominant or recessive before solving.
• Check that the sample size is large enough for reliable ratios.
• Understand how to set up and interpret a chi-squared test to evaluate results.