Genetics, Heredity, DNA, and Punnett Squares

Genetics, Heredity, DNA, and Punnett Squares

Genetics questions make up a core part of the Life Science domain (~40% of the Science test).

Key Genetic Terminology

• DNA (Deoxyribonucleic Acid): The double-helix molecule carrying genetic instructions.
• Gene: A segment of DNA that codes for a specific protein or trait.
• Allele: A variant form of a gene. Represented by capital letters (Dominant, $B$) or lowercase letters (Recessive, $b$).
• Genotype: The genetic makeup ($BB$, $Bb$, or $bb$).
  • Homozygous Dominant: $BB$ (two dominant alleles)
  • Heterozygous: $Bb$ (one dominant, one recessive)
  • Homozygous Recessive: $bb$ (two recessive alleles)
• Phenotype: The observable physical trait (e.g., brown eyes vs. blue eyes).

Punnett Square Mechanics

Cross two heterozygous parents ($Bb \times Bb$):

• Genotypic ratio: $1\,BB : 2\,Bb : 1\,bb$ (25% $BB$, 50% $Bb$, 25% $bb$)
• Phenotypic ratio: 75% Dominant trait, 25% Recessive trait.