GCSE Biology · Biology
Genetics and inheritance: a clear study guide
Genetics questions use a small set of linked ideas. Keep the hierarchy clear: DNA is the molecule, genes are sections of DNA, and chromosomes are long coiled DNA molecules in the nucleus.
DNA, genes and chromosomes
A gene is a section of DNA that contains instructions for a characteristic, often by coding for a protein. Chromosomes are long DNA molecules found in the nucleus, and body cells contain pairs of chromosomes.
Different versions of a gene are called alleles. A person's combination of alleles contributes to their genotype, while the observable characteristic is the phenotype.
- DNA contains genetic information.
- A gene is a section of DNA.
- Alleles are different versions of a gene.
- Phenotype is the observed characteristic.
Using genetic diagrams
Write the parent genotypes, list the possible gametes, then combine one gamete from each parent in a Punnett square. State the possible genotypes and phenotypes and give probabilities as fractions, decimals or percentages when asked.
A dominant allele is expressed when at least one copy is present. A recessive characteristic is expressed only when the individual has two copies of the recessive allele.
Variation and selection
Variation can be genetic, environmental or a combination of both. Mutations create new alleles, and natural selection can change the frequency of characteristics in a population over generations. Use the stages in order and avoid saying that an individual changes because it needs to.
Quick check
What is an allele?
Answer: A different version of a gene.
When is a recessive phenotype expressed?
Answer: When the individual has two copies of the recessive allele for that characteristic.
Common questions
Is a gene the same as a chromosome?
No. A gene is a section of DNA; a chromosome is a long coiled DNA molecule containing many genes.
What is the difference between genotype and phenotype?
Genotype is the allele combination; phenotype is the characteristic that can be observed or measured.