DNA, Genetics and Inheritance

DNA

A polymer of two strands in a double helix, found in chromosomes in the nucleus.

• A gene = a section of DNA coding for a specific protein.
• The genome = an organism's entire genetic material.

Key terms

• Allele – a version of a gene.
• Dominant (capital, e.g. B) – shows with just one copy.
• Recessive (lowercase, e.g. b) – shows only with two copies (bb).
• Genotype – the alleles present (e.g. Bb). Phenotype – the physical characteristic.
• Homozygous – two same alleles; heterozygous – two different.

Punnett square (Bb × Bb)

      B     b
   ┌─────┬─────┐
 B │ BB  │ Bb  │
   ├─────┼─────┤
 b │ Bb  │ bb  │
   └─────┴─────┘

Ratio 3 dominant : 1 recessive (3:1).

Inherited disorders

• Polydactyly – dominant allele.
• Cystic fibrosis – recessive allele (need two copies).

Sex determination

Females XX, males XY — a 50:50 ratio shown by a Punnett square.

Exam tip

Always draw a Punnett square — it shows working and prevents ratio mistakes. Capital letter = dominant.