National Higher Biology - Variation and Inheritance

Key Concepts

• Variation: The differences among individuals within the same species.
  • Types of Variation:
    • Discrete Variation:
      • Controlled by a single gene (single gene inheritance).
      • Traits fall into distinct categories.
      • Examples: Eye color, attached/unattached earlobes, wet/dry earwax.
    • Continuous Variation:
      • Controlled by multiple genes (polygenic inheritance).
      • Traits have a range and can be measured.
      • Examples: Height, shoe size.

Terminology

• Genotype: The genetic makeup of an organism; the alleles an individual possesses.
• Phenotype: The observable physical appearance of an organism.
• Alleles: Different forms of a gene that produce different phenotypes.
  • Dominant Alleles:
    • Always expressed in the phenotype if present.
    • Represented by a capital letter (e.g., A).
  • Recessive Alleles:
    • Only expressed if the organism is homozygous for the recessive allele.
    • Represented by a lowercase letter (e.g., a).

Genetic Inheritance

• Homozygous: Both alleles are the same (either dominant or recessive).
• Heterozygous: One dominant and one recessive allele present.
• Punnett Squares: Used to predict the genotype and phenotype ratios of offspring from parental crosses.
  • Monohybrid Crosses: Analyze the inheritance of a single trait.

Example Analysis: Pea Plants

• Alleles: A (dominant, purple) and a (recessive, white).
• Cross Examples:
  1. Homozygous Dominant (AA) x Homozygous Recessive (aa):
     • All offspring are heterozygous (Aa) and exhibit the dominant phenotype (purple).
     • 100% purple phenotype.
  2. Heterozygous (Aa) x Homozygous Recessive (aa):
     • Offspring genotype: 50% heterozygous (Aa), 50% homozygous recessive (aa).
     • Phenotype: 50% purple, 50% white.
  3. Heterozygous (Aa) x Heterozygous (Aa):
     • Genotype ratio: 25% AA, 50% Aa, 25% aa.
     • Phenotype ratio: 75% purple, 25% white.

Fertilization and Ratios

• Phenotype ratios are not always perfectly achieved due to the random nature of fertilization.
• Theoretically predicted ratios may vary in practice.

Conclusion

• Understand the differences between discrete and continuous variation and how they are inherited.
• Be familiar with key genetic terms like genotype, phenotype, alleles, and types of inheritance.
• Utilize Punnett squares for predicting outcomes of genetic crosses.
• Recognize that fertilization randomness affects phenotype ratios.

Next Steps

• Complete attached quizzes focusing on terminology.
• Practice calculating genetic crosses and phenotype ratios.