Inheritance, Variation and Evolution
Inheritance, variation and evolution covers how genetic information passes from parents to offspring through DNA and genes, why individuals of the same species vary in their characteristics, and how populations of species change, or evolve, over long periods of time through natural selection. It also covers how humans deliberately change organisms through selective breeding and genetic engineering, and how new evidence has changed scientific ideas about how life on Earth is classified.
Method
- Distinguish key vocabulary precisely, since marks are often lost through vague terms: a gene is a section of DNA that codes for a protein; an allele is a different version of the same gene; a genotype is the alleles an organism has; a phenotype is the physical characteristic that results.
- For genetic-cross questions, draw a genetic diagram (a Punnett square): write the parents' genotypes, list the possible gametes, combine them in the grid, then read off the ratio of genotypes and phenotypes in the offspring.
- Learn that variation between individuals can be genetic, caused by different alleles including new alleles from mutation, environmental, caused by the conditions an organism lives in, or a combination of both, and be ready to classify a given example as one of the three.
- Learn natural selection as a chain of reasoning: variation exists in a population, individuals with characteristics suited to the environment are more likely to survive and reproduce, the alleles for those characteristics are passed to the next generation, and over many generations the population changes.
- Compare selective breeding, where humans choose which organisms with a desired characteristic breed together over many generations, with genetic engineering, where a single gene is transferred directly into an organism's genome to give it a new characteristic in one generation, including one advantage and one concern for each.
- For evidence questions, be ready to explain what fossils can and cannot show, and to describe antibiotic resistance in bacteria as a real, current example of natural selection in action.
Worked example
In Labrador retrievers, the allele for black fur (B) is dominant over the allele for brown fur (b). Two dogs that are both heterozygous (Bb) are bred together. Using a genetic diagram, determine the ratio of black to brown puppies expected.
- Write the genotypes of the two parents: Bb and Bb.
- List the possible gametes each parent can produce: B or b from each parent.
- Draw a Punnett square and combine the gametes to find the offspring genotypes: BB, Bb, Bb, bb.
- Read off the phenotypes: BB and Bb are both black, since B is dominant, and only bb is brown.
- Final answer: the expected ratio is 3 black to 1 brown.
Practice questions
Try each question, then tap to reveal the answer.
Q1Define the term allele.Show answer
Answer: A different version of the same gene.
Q2Define phenotype.Show answer
Answer: The observable physical characteristic of an organism, resulting from its genotype and, sometimes, the environment.
Q3A plant grown in the shade is shorter than a genetically identical plant of the same species grown in full sunlight. State whether this difference is genetic variation, environmental variation, or both.Show answer
Answer: Environmental variation, since the plants are genetically identical and the difference is caused entirely by the different growing conditions.
Q4In pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). A homozygous tall plant (TT) is crossed with a homozygous short plant (tt). State the genotype and phenotype of all the offspring.Show answer
Answer: All offspring are Tt (heterozygous) and all are tall, since T is dominant.
Q5State one difference between selective breeding and genetic engineering.Show answer
Answer: Selective breeding works by choosing and breeding organisms with an existing desired characteristic over many generations, whereas genetic engineering directly inserts a specific gene into an organism's genome, producing the change in a single generation.
Q6Give one piece of evidence that supports Darwin's theory of evolution by natural selection.Show answer
Answer: Fossil evidence showing gradual change in characteristics over geological time (also accept: observed antibiotic resistance in bacteria, or DNA sequence evidence showing degrees of relatedness between species).
Q7Explain, using the idea of natural selection, why a population of insects can become resistant to a pesticide over time.Show answer
Answer: Variation already exists in the population, so a few individuals happen to carry alleles giving some resistance; these resistant individuals are more likely to survive being sprayed and to reproduce, passing the resistance alleles to their offspring, so the proportion of resistant individuals increases over generations.
Q8Name the three-domain system of classification, and name the scientist most associated with proposing it.Show answer
Answer: Archaea, Bacteria and Eukarya; proposed by Carl Woese.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
State two examples of variation between individuals of the same plant species that are caused mainly by the environment rather than by genes.
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Polydactyly, having extra fingers or toes, is caused by a dominant allele, D. A man who is heterozygous for polydactyly (Dd) has children with a woman who does not have polydactyly (dd). (a) Using a genetic diagram, find the probability that a child of theirs has polydactyly. (b) State this probability as a percentage.
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A species of beetle exists in both a green form and a brown form. A new species of bird moves into the beetles' habitat and starts to prey on them. Over many generations, the proportion of green beetles in the population decreases while the proportion of brown beetles increases, because the beetles live among brown leaf litter. Explain this change in the population using the theory of natural selection.
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