# DNA, Genetics and Inheritance
DNA carries the genetic instructions for all living organisms. Understanding DNA structure, protein synthesis, and how characteristics are inherited is central to GCSE Biology.
1. DNA Structure
- DNA: deoxyribonucleic acid
- Double helix structure
- Made of nucleotides (sugar + phosphate + base)
- Four bases: A-T (adenine-thymine) and C-G (cytosine-guanine)
- Complementary base pairing holds strands together
DNA is found in the nucleus on chromosomes. Humans have 23 pairs of chromosomes (46 total).
2. Genes and the Genome
- Gene: section of DNA that codes for a specific protein
- Genome: entire set of genetic material in an organism
- Genome importance: understanding genetic disorders, evolutionary relationships, personalised medicine
3. Protein Synthesis
- Transcription: DNA unzips; mRNA copy made from one strand
- mRNA leaves nucleus → goes to ribosome
- Translation: ribosome reads mRNA codons (3 bases = 1 amino acid)
- Amino acids joined in specific order → protein folds into 3D shape
DNA → mRNA → protein
4. Inheritance Key Terms
| Term | Definition |
|---|---|
| Allele | Different version of a gene |
| Dominant | Expressed when one or two copies present (capital letter, e.g. B) |
| Recessive | Only expressed when two copies present (lowercase, e.g. b) |
| Homozygous | Two same alleles (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
| Genotype | The alleles an organism has |
| Phenotype | The physical characteristic expressed |
5. Punnett Squares
Cross: Bb × Bb
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Ratio: 3 dominant : 1 recessive Probability of recessive phenotype: 1/4 (25%)
6. Genetic Disorders
| Disorder | Inheritance | Details |
|---|---|---|
| Cystic fibrosis | Recessive (ff) | Thick mucus in lungs; both parents must be carriers (Ff) |
| Polydactyly | Dominant (Dd or DD) | Extra fingers/toes; only one allele needed |
7. Sex Determination
- XX = female; XY = male
- 50% chance of boy or girl
- Sex-linked conditions (e.g. colour blindness) carried on X chromosome
8. Practice Questions
- Describe the structure of DNA.
- Cross two heterozygous parents (Bb × Bb) and give the ratio.
- What is the difference between genotype and phenotype?
- Explain why cystic fibrosis can appear in children of two unaffected parents.
- How is sex determined in humans?
Want to check your answers and get step-by-step solutions?
Summary
- DNA: double helix; A-T, C-G base pairing
- Gene: section of DNA coding for a protein
- Protein synthesis: DNA → mRNA → protein
- Punnett squares predict offspring genotypes/phenotypes
- Cystic fibrosis: recessive; polydactyly: dominant