Inheritance, variation & evolution
DNA and inheritance, genetic crosses and Punnett squares, variation and natural selection, evolution and classification.
Learn
Inheritance, variation & evolution, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Describe DNA, the genome, chromosomes, genes and alleles
DNA is the molecule that carries genetic instructions, coiled into chromosomes inside the nucleus. A gene is a short section of DNA that codes for a protein, and the whole set of an organism's genes is its genome. Different versions of the same gene are alleles - the eye-colour gene has alleles for brown and for blue, for instance.
Use dominant and recessive alleles with Punnett squares
For most genes you inherit one allele from each parent. A dominant allele shows even with one copy (written with a capital, B); a recessive allele shows only when both copies match (bb). A Punnett square crosses the parents' alleles to predict the offspring - cross Bb with Bb and you predict 3 brown to 1 blue, a 1 in 4 chance of blue.
Compare sexual and asexual reproduction
Sexual reproduction mixes genes from two parents, so offspring vary - useful for surviving change. It uses gametes (sperm and egg) made by meiosis, each with half the chromosomes. Asexual reproduction has one parent and produces genetically identical clones, which is fast and reliable in a stable environment, as seen in bacteria and strawberry runners.
Explain variation and how natural selection works
Individuals in a species vary because of their genes, their environment, or both. Where variation is inherited, those with features best suited to the environment are more likely to survive and breed, passing those alleles on - natural selection. Over many generations this shifts a population, which is why fast rabbits leave more offspring than slow ones.
Describe evolution and the evidence for it
Evolution is the gradual change in inherited features of a population over time through natural selection, first set out by Darwin. Given long enough, it can produce new species. Fossils show how life has changed over millions of years, and the rapid emergence of antibiotic-resistant bacteria is evolution happening fast enough to watch.
Describe selective breeding, genetic engineering and classification
Selective breeding is choosing parents with wanted features over generations - bigger crops, gentler dogs - a kind of human-driven selection. Genetic engineering transfers a gene directly, such as inserting the human insulin gene into bacteria. Classification groups organisms by their features into a hierarchy, from kingdom down to species, showing how life is related.
Inheritance, variation & evolution key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Gene
- A small section of DNA on a chromosome that codes for a particular sequence of amino acids, making a specific protein.
- Allele
- A version of a gene. You inherit one allele of each gene from each parent.
- Dominant allele
- An allele that produces its characteristic when only one copy is present.
- Recessive allele
- An allele that produces its characteristic only when two copies are present, one from each parent.
- Genotype
- The alleles an organism carries for a gene, such as Bb or bb.
- Phenotype
- The characteristics an organism actually shows, produced by its genotype and its environment.
- Homozygous
- Carrying two identical alleles of a gene (BB or bb).
- Heterozygous
- Carrying two different alleles of a gene (Bb).
- Variation
- The differences in characteristics between individuals in a population, caused by genes, the environment, or both.
- Natural selection
- The process by which individuals with characteristics best suited to their environment are more likely to survive and reproduce, passing those characteristics on.
- Evolution
- A change in the inherited characteristics of a population over time, through natural selection, which may result in a new species.
Practice
Try a Inheritance, variation & evolution question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
In mice, brown fur (B) is dominant to white fur (b). Two heterozygous brown mice (Bb) are crossed. What is the probability that an offspring has white fur? Give your answer as a percentage.
- 100%
- 50%
- 75%
- 25%
Show the answer and the working
Answer: 25%
A Bb × Bb cross gives offspring in the ratio 1 BB : 2 Bb : 1 bb, so 1 in 4, or 25%, are white (bb).
- Each Bb parent can pass on B or b.
- The Punnett square gives BB, Bb, Bb and bb.
- Only 1 of the 4 boxes is bb (white), so the probability is = 25%.