Edexcel GCSE Biology (1BI0) — Genetics
Look around a crowded room: even though everyone belongs to the exact same species, no two people look entirely identical. This is because there is usually extensive genetic variation within a population of a single species.
While all individuals in a population have the same genes, they possess a wide range of different alleles (versions of those genes). The observable characteristics of an organism, known as its phenotype, are determined by a combination of two factors:
All genetic variation ultimately arises from mutations. While processes like sexual reproduction mix existing alleles together in new combinations, they do not create entirely new genetic information.
A mutation is a rare, random change in the DNA base sequence of a gene or chromosome. This creates a new genetic variant. Mutations occur spontaneously and continuously in all living organisms. However, the rate of mutation can be significantly increased by exposure to:
Crucially, for a mutation to contribute to the genetic variation of the next generation, it must occur in the gametes (sex cells). Mutations in standard body cells are not inherited by offspring.
The vast majority of mutations have absolutely no effect on an organism's phenotype. This often happens if the mutation occurs in a section of DNA that does not code for a protein, or if the changed DNA sequence still codes for the exact same amino acid.
Sometimes, mutations have a small effect, slightly altering a characteristic. For example, multiple small mutations across different genes can lead to slight variations in human eye colour or skin tone. Very rarely, a single mutation will significantly affect the phenotype, such as creating a completely new characteristic or resulting in a non-functional protein. For example, a single base substitution causes sickle cell anaemia by altering the shape of red blood cells.
To explain exactly how a mutation changes a characteristic, you must understand the step-by-step causal link:
Mutations do not just happen in the genes that build proteins; they can also occur in non-coding DNA regions located in front of a gene. These regions control gene expression by determining how easily RNA polymerase can bind to the DNA during transcription.
If a mutation makes it easier for RNA polymerase to bind, more mRNA is transcribed, increasing the quantity of protein produced. If it makes binding harder, less mRNA is transcribed, decreasing the quantity of protein. Rather than changing the shape of the protein, non-coding mutations change the amount of protein, which still directly influences the organism's phenotype.
Students often incorrectly state that sexual reproduction or the environment is the origin of new variation. You must explicitly state that ALL new genetic variation originates from mutations.
When a question asks you to 'State' the level of variation within a species, examiners are specifically looking for the keyword 'extensive'.
To get full marks when explaining how mutations affect phenotype, always write the logical sequence: mutation changed DNA base sequence changed amino acid sequence changed protein 3D shape altered phenotype.
If an exam question asks about a mutated enzyme, you must explicitly mention that the shape of the 'active site' changes so the substrate no longer fits.
Higher Tier students must clearly distinguish between coding mutations (which change the protein's shape) and non-coding mutations (which change the amount of protein produced by affecting RNA polymerase).
Genetic variation
The differences in the DNA sequences (alleles) between individuals within a population.
Population
A group of individuals of the same species living in the same area at the same time.
Alleles
Different versions or variants of a specific gene.
Phenotype
The observable physical characteristics of an organism, determined by its genotype and its interaction with the environment.
Mutations
Rare, random changes in the DNA base sequence of a gene or chromosome.
Genetic variant
A different version of a gene caused by a change in the DNA sequence; often used interchangeably with allele or mutation.
Gametes
Sex cells (such as sperm and egg) that can pass genetic information on to offspring.
Active site
The specific 3D region on an enzyme where the substrate binds to form an enzyme-substrate complex.
RNA polymerase
An enzyme responsible for copying a DNA sequence into an RNA sequence during the process of transcription.
Put your knowledge into practice — try past paper questions for Biology
Genetic variation
The differences in the DNA sequences (alleles) between individuals within a population.
Population
A group of individuals of the same species living in the same area at the same time.
Alleles
Different versions or variants of a specific gene.
Phenotype
The observable physical characteristics of an organism, determined by its genotype and its interaction with the environment.
Mutations
Rare, random changes in the DNA base sequence of a gene or chromosome.
Genetic variant
A different version of a gene caused by a change in the DNA sequence; often used interchangeably with allele or mutation.
Gametes
Sex cells (such as sperm and egg) that can pass genetic information on to offspring.
Active site
The specific 3D region on an enzyme where the substrate binds to form an enzyme-substrate complex.
RNA polymerase
An enzyme responsible for copying a DNA sequence into an RNA sequence during the process of transcription.
