Inheritance, Variation, and Evolution
DNA, genetics, natural selection, evolution, and classification.
In this topic
DNA and Genetics
DNA (deoxyribonucleic acid) is a polymer made of two strands forming a double helix. It contains the genetic code for making proteins.
A gene is a small section of DNA on a chromosome that codes for a particular protein (and therefore a characteristic).
The genome is the entire set of genetic material in an organism. The Human Genome Project mapped all human genes.
Key terms: - Allele: different versions of the same gene - Dominant: the allele that is expressed when at least one copy is present (capital letter, e.g., B) - Recessive: only expressed when two copies are present (lowercase, e.g., b) - Homozygous: two identical alleles (BB or bb) - Heterozygous: two different alleles (Bb) - Genotype: the alleles present (e.g., Bb) - Phenotype: the physical characteristic shown (e.g., brown eyes)
Key Points
- Humans have 23 pairs of chromosomes (46 total)
- Sex chromosomes: XX = female, XY = male
- Punnett squares show possible offspring genotypes
- Some disorders are inherited: cystic fibrosis (recessive), polydactyly (dominant)
Example Questions
3Two parents are both heterozygous for a dominant trait (Bb). Draw a Punnett square and state the probability of their offspring being homozygous recessive.
Punnett square: BB, Bb, Bb, bb. Probability of bb = 1/4 = 25%
[3 marks]
3Cystic fibrosis is caused by a recessive allele (f). Two healthy parents have a child with cystic fibrosis. What are the parents' genotypes?
Both parents must be carriers (Ff). They are phenotypically healthy (have at least one dominant allele F) but both carry the recessive allele f. The child with cystic fibrosis is ff, having inherited one f from each parent.
[3 marks]
Evolution and Natural Selection
Evolution is the gradual change in inherited characteristics of a population over many generations.
Natural selection (Darwin's theory): 1. Variation exists within a population 2. Organisms with beneficial characteristics are more likely to survive (survival of the fittest) 3. These organisms are more likely to reproduce and pass on their alleles 4. Over many generations, the beneficial alleles become more common
Evidence for evolution: - Fossil record shows how organisms have changed over time - Antibiotic-resistant bacteria demonstrate evolution in action - DNA analysis shows genetic similarities between species
Key Points
- Natural selection requires variation, competition, and inheritance
- Mutations introduce new alleles into a population
- Selective breeding is artificial selection by humans
- Speciation occurs when populations become so different they can no longer interbreed
Example Questions
4Explain how bacteria become resistant to antibiotics through natural selection.
Random mutations occur in bacterial DNA, some causing antibiotic resistance. When antibiotics are used, non-resistant bacteria are killed, but resistant ones survive. The resistant bacteria reproduce rapidly, passing on the resistance gene. Over time, the population becomes mostly resistant. This is why antibiotics should be used correctly and courses completed.
[4 marks]