Working with probability laws in genetics needs large sample sizes because small samples:

1. Mendel could cultivate large quantities of garden peas simultaneously. 2. That let him conclude that his results did not come about simply by chance. 3. To use probability laws in practice, it is necessary to work with large sample sizes because small sample sizes are prone to deviations caused by chance. 4. The true ratio does not change with sample size; only the reliability of the estimate does. 5. This is why a three-plant sample proves very little about a cross. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 12 'Mendel's Experiments and Heredity', sections 12.1-12.3_