Biology A Level Revision (AQA)
Paper 1 - Year 12 / AS Level
Biological Molecules
Cells
Organisms Exchanging Substances
Genetic Information, Variation
Paper 2 - Year 13 / A Level
Energy Transfers In and Between Organisms
Organisms Responses to their Environment
Genetics, Evolution and Ecosystems
The Control of Gene Expression
Some diagrams used in this document are sourced from Cognito (https://cognitoedu.org).
All rights to those diagrams belong to Cognito.
, Biology Revision A Level AQA
Genetics, Populations, Evolution and Ecosystems
Key Terms in Genetics
● Key Terms
- Gene - a section of DNA that codes for a certain protein on a locus of DNA.
- Alleles - different variations of a gene.
- Genotype - genetic makeup of an organism (both physically visible and
non-expressed alleles).
- Phenotype - the observable traits expressed by an organism(determined by the
interaction between its genetic constitution and the environment).
- Dominant Allele - always expressed in the phenotype of an organism.
- Recessive Allele - only expressed in the phenotype if there are two copies of the
allele.
- Co-dominant Allele - both expressed in the heterozygote.
- Homozygote - organisms with two copies of the same allele.
- Heterozygote - organisms that have different alleles for a gene.
Inheritance
● Monohybrid Crosses
- When two parents differ from only one characteristic breed, a monohybrid cross
allows the genotype of the offspring to be predicted.
- Parental genotypes are identified first (e.g., NN = normal wings, nn = short wings).
- Gametes carry one allele each (haploid). True-breeding parents produce gametes
with only their allele (N or n).
- F1 generation: First offspring; crossing heterozygotes (Nn × Nn) gives 25% NN, 50%
Nn, 25% nn.
- F2 generation: Offspring of F1 × F1; genotypes: NN,Nn,nn.
- Ratios: Phenotypic ratio of normal:short wings= 3:1.
Page 1
Paper 1 - Year 12 / AS Level
Biological Molecules
Cells
Organisms Exchanging Substances
Genetic Information, Variation
Paper 2 - Year 13 / A Level
Energy Transfers In and Between Organisms
Organisms Responses to their Environment
Genetics, Evolution and Ecosystems
The Control of Gene Expression
Some diagrams used in this document are sourced from Cognito (https://cognitoedu.org).
All rights to those diagrams belong to Cognito.
, Biology Revision A Level AQA
Genetics, Populations, Evolution and Ecosystems
Key Terms in Genetics
● Key Terms
- Gene - a section of DNA that codes for a certain protein on a locus of DNA.
- Alleles - different variations of a gene.
- Genotype - genetic makeup of an organism (both physically visible and
non-expressed alleles).
- Phenotype - the observable traits expressed by an organism(determined by the
interaction between its genetic constitution and the environment).
- Dominant Allele - always expressed in the phenotype of an organism.
- Recessive Allele - only expressed in the phenotype if there are two copies of the
allele.
- Co-dominant Allele - both expressed in the heterozygote.
- Homozygote - organisms with two copies of the same allele.
- Heterozygote - organisms that have different alleles for a gene.
Inheritance
● Monohybrid Crosses
- When two parents differ from only one characteristic breed, a monohybrid cross
allows the genotype of the offspring to be predicted.
- Parental genotypes are identified first (e.g., NN = normal wings, nn = short wings).
- Gametes carry one allele each (haploid). True-breeding parents produce gametes
with only their allele (N or n).
- F1 generation: First offspring; crossing heterozygotes (Nn × Nn) gives 25% NN, 50%
Nn, 25% nn.
- F2 generation: Offspring of F1 × F1; genotypes: NN,Nn,nn.
- Ratios: Phenotypic ratio of normal:short wings= 3:1.
Page 1
