Lesson 11: Genetics & DNA

Genetics & DNA Reference

DNA Structure

DNA (deoxyribonucleic acid) is a double helix. Each strand has a sugar-phosphate backbone with nitrogenous bases. Base pairing: A-T, G-C. The sequence of bases encodes genetic information.

Central Dogma

DNA → RNA → Protein

• Transcription: DNA → mRNA (in nucleus)
• Translation: mRNA → Protein (at ribosome)
• Each codon (3 bases) codes for one amino acid.

Mendelian Genetics

• Alleles: alternative forms of a gene (e.g., A, a).
• Dominant: expressed when at least one copy present (A).
• Recessive: expressed only when homozygous (aa).
• Genotype: allele combination (AA, Aa, aa).
• Phenotype: observable trait.

Punnett Squares

Used to predict offspring genotype ratios. Monohybrid cross (Aa × Aa): 1 AA : 2 Aa : 1 aa (genotype 1:2:1; phenotype 3:1 dominant:recessive).

Mutations & Genetic Disorders

• Point mutation: one base changed.
• Chromosomal mutation: deletion, insertion, inversion, translocation.
• Examples: sickle cell anemia (recessive), Huntington's (dominant), Down syndrome (trisomy 21).