7.3.1 Genetic Variation

Sources of Genetic Variation

1. Random Fertilisation:

• During sexual reproduction, gametes (sperm and egg) with different alleles combine randomly, producing offspring with unique combinations of alleles.

2. Meiosis:

• Independent assortment of homologous chromosomes during meiosis means alleles are distributed randomly into gametes.
• Crossing over between homologous chromosomes creates new combinations of alleles.

3. Mutation:

• Changes in the DNA sequence of a gene can lead to the formation of new alleles.
• These mutations can be inherited if they occur in gametes, contributing to genetic variation in the next generation.

Environmental Influences on Phenotypic Variation

• Phenotype is determined by both genetic makeup and environmental conditions.
• Example: Two plants with the same alleles for flower colour may display different colours if one grows in mineral-deficient soil, preventing pigment production.

Normal Distribution in Traits

• When environmental factors influence a polygenic trait (controlled by multiple genes), the variation often follows a normal distribution.
• Example: Adult height in humans. Most individuals are close to the mean height, with fewer individuals at the extremes (very short or very tall).