Inheritance, Variation and Evolution (AQA)
1. DNA and the Genome
DNA (Deoxyribonucleic Acid) is the genetic material found in the nuclei of cells. It carries the instructions needed to make proteins and control cell functions. DNA is a polymer made up of two strands forming a double helix.
Each strand is made of nucleotides, which consist of a sugar, phosphate group, and a base. The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G). The base pairs are A-T and C-G, which form the “rungs” of the DNA ladder.
A gene is a short section of DNA that codes for a specific protein. The genome is the entire genetic material of an organism. The Human Genome Project mapped all human genes, helping scientists understand genetic diseases and develop gene therapies.
2. Protein Synthesis
Protein synthesis occurs in two stages: transcription and translation.
Transcription: The DNA is used to make an mRNA (messenger RNA) copy in the nucleus.
Translation: The mRNA moves to a ribosome, where the code is read in triplets (codons) and translated into a chain of amino acids, which folds into a protein.
Proteins play many roles in the body, such as acting as enzymes, hormones, and structural components (e.g., collagen in skin).
Mutations are changes in the DNA sequence. Most mutations are harmless, but some can lead to altered proteins, causing diseases or advantageous traits.
3. Inheritance
Humans have 23 pairs of chromosomes—one set from each parent. Each gene has different forms called alleles. An individual inherits one allele for each gene from each parent.
Dominant alleles only need one copy to be expressed (e.g., brown eyes).
Recessive alleles need two copies to be expressed (e.g., blue eyes).
Genetic diagrams (Punnett squares) help show the probability of offspring inheriting certain traits.
Homozygous means two identical alleles (e.g., BB or bb), while heterozygous means two different alleles (e.g., Bb).
Some genetic disorders are inherited:
Cystic fibrosis is caused by a recessive allele.
Polydactyly (extra fingers or toes) is caused by a dominant allele.
4. Sex Determination
The 23rd pair of chromosomes determines biological sex:
XX = female
XY = male
The Y chromosome carries genes responsible for male development. A Punnett square can show the 50:50 chance of having a male or female child.
5. Genetic Inheritance and Screening
Genetic screening allows detection of inherited diseases before birth. This can help parents make informed decisions, but it raises ethical issues:
Should parents be allowed to select embryos?
Could screening lead to discrimination?
What about privacy of genetic information?
Embryo screening is especially useful in IVF (in vitro fertilization) to test for genetic disorders.
6. Variation and Evolution
Variation exists among individuals due to:
Genetic causes: Differences in genes inherited from parents.
Environmental causes: Lifestyle, climate, diet, etc.
Both: Most traits are influenced by both genes and the environment.
Continuous variation (e.g., height) shows a range of values and is usually influenced by many genes. Discontinuous variation (e.g., blood group) has distinct categories.
Evolution is the gradual change in species over time. It occurs through natural selection, proposed by Charles Darwin:
Variation exists within a population.
There is competition for resources.
The individuals with the best adaptations survive and reproduce.
Beneficial traits are passed on to the next generation.
Over many generations, this can lead to the development of new species (speciation).
7. Selective Breeding and Genetic Engineering
Selective Breeding
Humans use selective breeding to produce organisms with desirable traits:
High-yield crops
Disease-resistant animals
Domesticated pets
However, selective breeding reduces gene pool diversity, increasing vulnerability to diseases.
Genetic Engineering
This involves changing the DNA of an organism:
Gene is isolated and inserted into the DNA of another organism (often using enzymes and vectors like plasmids or viruses).
Produces genetically modified (GM) organisms.
Uses include:
Insulin-producing bacteria
Golden rice enriched with vitamin A
Disease-resistant crops
While promising, GM raises ethical concerns about safety, environmental impact, and food labeling.
8. Cloning and Biotechnology
Cloning produces genetically identical individuals. Types include:
Tissue culture: Cloning plants from small cell samples.
Cuttings: Simple plant cloning from shoots.
Embryo transplants: Splitting an embryo to implant into different wombs.
Adult cell cloning: Nucleus from an adult cell is inserted into an egg cell with no nucleus, and the embryo is implanted into a surrogate.
Cloning can preserve endangered species but raises ethical concerns about animal welfare and genetic diversity.
Biotechnology includes using biological processes for human benefit, like fermentation (e.g., in making yogurt) or producing insulin via genetically modified bacteria.
9. Evidence for Evolution
Evidence supporting evolution includes:
Fossils: Show how organisms have changed over time.
Antibiotic resistance: Bacteria evolve rapidly; resistant strains survive and reproduce.
Comparative anatomy and genetics: Similarities in DNA and body structures indicate common ancestry.
Extinction happens when species can’t adapt to environmental changes. Causes include:
New predators or diseases
Habitat destruction
Competition
Climate change
10. Classification
Living organisms are classified into groups:
Carl Linnaeus developed the binomial system (Genus species).
Modern classification uses three domains:
Bacteria
Archaea
Eukaryota
Organisms are grouped based on similarities in DNA, biochemistry, and structure.