Inheritance, variation and evolution — section overview

B6 is the genetics and evolution section — covering how characteristics are inherited, how species change over time, and the evidence for evolution.

DNA and the genome

• DNA (deoxyribonucleic acid) — double helix; made of nucleotides containing a sugar, phosphate and base (A, T, C, G)
• Gene — a section of DNA that codes for a protein
• Chromosome — long strand of coiled DNA; humans have 23 pairs (46 total)
• Genome — all the DNA of an organism

Base pairing: A–T; C–G

Protein synthesis: DNA → mRNA (transcription) → protein (translation at ribosome)

Inheritance

Alleles: different versions of a gene. Each individual has two copies.

• Dominant allele B — expressed even if only one copy present
• Recessive allele (b) — only expressed when two copies are present

Genotype: alleles an individual has (BB, Bb, bb) Phenotype: physical characteristic expressed

Punnett square: tool for predicting offspring ratios.

Monohybrid cross: crossing for one gene.

	B	b
B	BB	Bb
b	Bb	bb

Result: 3 dominant : 1 recessive phenotype ratio (for two carriers Bb × Bb).

Sex determination: XX = female; XY = male. 50% probability of each sex per pregnancy.

Inherited conditions:

• Cystic fibrosis — recessive; chloride ion channels affected
• Polydactyly — dominant; extra fingers/toes
• Sickle cell anaemia — recessive; abnormal haemoglobin

Variation

Genetic variation: differences caused by different alleles — determined by inheritance + mutation Environmental variation: differences caused by environment (e.g. language, scars, plants growing in shade) Continuous variation (e.g. height) vs discontinuous variation (e.g. blood type)

Evolution and natural selection

Darwin's natural selection:

1. Variation exists in a population
2. More offspring produced than can survive
3. Organisms with advantageous variations survive and reproduce
4. Advantageous alleles passed to offspring → frequency increases over generations

Evolution = change in allele frequency in a population over time.

Evidence for evolution: fossils, comparative anatomy, DNA/protein similarity, antibiotic resistance, direct observation.

Classification

Carl Woese three-domain system: Archaea, Bacteria, Eukarya

Binomial nomenclature: Genus species (e.g. Homo sapiens)

Common exam mistakes in B6

1. Dominant does not mean most common — a rare disorder can be dominant
2. Two recessive alleles for recessive trait — carrier (Bb) does not show the trait
3. Lamarck vs Darwin — Lamarck: acquired characteristics inherited (WRONG); Darwin: natural selection (CORRECT)