GCSE BIOLOGY • SPECIFICATION 1.6

Inheritance & Evolution

Aligned to AQA GCSE Biology (8461/8464). Wording consistent with AQA mark schemes.

DNA, Genetics, and the mechanisms of biological change over time.

1. DNA and the Genome

Exam Definition (Genome):

"The entire genetic material of an organism."

Chromosomes are long, coiled molecules of DNA found in the nucleus. DNA is a polymer made up of two strands forming a double helix.

TermAQA Definition
GeneA small section of DNA on a chromosome that codes for a specific sequence of amino acids to make a specific protein.
AlleleA different version of a gene.

Importance of Understanding the Genome:

  • Search for genes linked to different types of disease.
  • Understanding and treatment of inherited disorders.
  • Tracing human migration patterns from the past.
2. Genetic Inheritance & Punnett Squares
Allele Definitions:
  • Dominant: Expressed in the phenotype if at least one copy of the allele is present.
  • Recessive: Only expressed if two copies are present.
Genotype Keywords:
  • Homozygous: Two of the same alleles (e.g. BB).
  • Heterozygous: Two different alleles (e.g. Bb).
  • Phenotype: The physical characteristic expressed.

The Probability Rule

When completing a Punnett square, provide the ratio or percentage. Note: Each fertilization is an independent event.

Example: Cystic Fibrosis is caused by a recessive allele (f). Calculate the probability of two carriers (Ff) having a child with the disorder.

Parents: Ff x Ff
Offspring: FF, Ff, Ff, ff
Probability = 25% (1 in 4 or 0.25).
3. Variation and Natural Selection

Natural Selection (4-Mark Logic Chain):

  1. Individuals within a species show variation caused by mutation in genes.
  2. Those with characteristics best suited to the environment are more likely to survive and breed.
  3. The alleles for these characteristics are passed on to the next generation.
  4. Over many generations, the population becomes better adapted to its environment.

Evaluation: Selective Breeding vs Genetic Engineering

Selective Breeding: Humans breed for desired traits. Cons: Inbreeding makes populations prone to disease/defects.

Genetic Engineering: Inserting a gene from one organism into another. Cons: Ethical concerns exist; long-term effects on ecosystems and human health are not fully known.

4. Evolution and Speciation

Evolution is a change in the inherited characteristics of a population over time which may result in the formation of a new species.

Mechanism of Speciation:

Speciation occurs when populations become isolated and cannot interbreed. Mutations accumulate in different gene pools until the populations are so different they can no longer produce fertile offspring.

[Image of geographic isolation and speciation]

Q: Explain how MRSA evolved (4 Marks).

  1. Mutations occur in bacteria during reproduction.
  2. Some mutations make them resistant to antibiotics.
  3. Non-resistant bacteria are killed; resistant ones survive and reproduce.
  4. The resistant allele is passed on; its frequency in the population increases.
5. Classification

Linnaeus classified organisms by structure. Modern systems use genetic analysis to show evolutionary relationships.

Kingdom → Phylum → Class → Order → Family → Genus → Species

The Three-Domain System (Carl Woese)

Due to genetic analysis, organisms are now divided into:

  • Archaea: Primitive bacteria from extreme environments.
  • Bacteria: True bacteria.
  • Eukaryota: Protists, fungi, plants, and animals.

Final Exam Guardrail

• Dominant Allele: Expressed in the phenotype even if only one copy is present.
• Mutations: Always state that mutations occur in genes or DNA.
• Natural Selection: It is the allele that is passed on to the next generation.
• Speciation: Requires isolation and the inability to interbreed to produce fertile offspring.

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