What you'll learn
- Why organisms of the same species are not all identical.
- How genetic and environmental causes influence phenotype.
- Why most characteristics involve multiple genes, not just one gene.
- How the Human Genome Project can be used in medicine, and why there are ethical issues.
Start with the language of genetics
Variation means differences between individuals. For example, people vary in height, blood group, eye colour, body mass, fingerprints and ability to digest lactose.
A species is a group of organisms that can usually breed together to produce fertile offspring. A population is a group of organisms of the same species living in the same area at the same time.
Inside body cells, DNA is the chemical that carries genetic information. DNA is arranged into chromosomes, and sections of DNA are called genes. A gene is a section of DNA that affects a characteristic, often by coding for a protein. Different versions of the same gene are called alleles.
Genotype and phenotype
A genotype is the alleles an organism has. A phenotype is the observable characteristics of an organism, such as height, blood group or flower colour.
Phenotype: genes and environment together
Your phenotype is affected by your genotype, but it is often also affected by your environment — the conditions around you and experiences during life.
A useful GCSE shorthand is:
phenotype=genotype+environment\text{phenotype} = \text{genotype} + \text{environment}phenotype=genotype+environment
This is not a calculation. It means both inherited genetic information and life conditions can influence what an organism is like.
Most features involve multiple genes
Some characteristics are mainly controlled by one gene, but most phenotypic features are the result of multiple genes. When many genes affect one characteristic, this is called polygenic inheritance.
Height is a good example: many genes affect growth, and environmental factors such as diet and health also matter. This is why height shows a wide range rather than just a few neat categories.
The big idea
Most characteristics are not controlled by one gene on its own. They are usually affected by several genes, and often by the environment too.
Genetic variation
Genetic variation means differences between individuals caused by differences in their DNA or alleles.
There are two important causes:
- Mutation — a change in the DNA base sequence. Mutations can create new alleles.
- Sexual reproduction — reproduction involving two parents, where sex cells called gametes join during fertilisation. This produces new combinations of alleles.
Mutation
A mutation is a change in the DNA base sequence. Mutations are the original source of new genetic variation in a population.
Sexual reproduction does not usually create brand-new alleles. Instead, it shuffles alleles into new combinations. Over many generations, mutations add new alleles to a population, and sexual reproduction mixes them.
Create and shuffle
Mutations create new alleles. Sexual reproduction shuffles alleles into new combinations.
Because mutations have built up over time, there is usually extensive genetic variation within a population of a species.
Environmental variation
Environmental variation means differences caused by an organism’s surroundings or experiences. These are sometimes called acquired characteristics, because they are gained during life rather than inherited through DNA.
Examples include:
- scars caused by injury
- muscle size affected by exercise
- plant height affected by light, water and mineral ions
- body mass affected by diet and activity
- language or skills learned during life
Many characteristics are affected by both genes and environment. For example, body mass is influenced by inherited factors, but also by diet, exercise and health.
Acquired characteristics
A scar, tattoo or learned skill is not passed on genetically to offspring. It may affect the phenotype of the individual, but it does not change the alleles in their gametes.
Deciding what caused a difference
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If two siblings have different blood groups, the difference is genetic because they inherited different combinations of alleles from their parents.
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If one identical twin has a scar and the other does not, the difference is environmental because it was caused by an injury after birth.
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If plants of the same species grow taller in bright light than in shade, the difference is environmental, although genes still help set the plant’s possible growth range.
Continuous and discontinuous variation
Some variation falls into clear categories. This is discontinuous variation. Blood group is an example because you belong to one category, such as A, B, AB or O, with no values in between.
Other variation shows a range of values. This is continuous variation. Height is an example because people can have many possible heights between the smallest and largest values.
Continuous variation is often caused by multiple genes and environmental factors working together.
Investigating variation
In a practical investigation, you might measure a feature such as leaf length, hand span or height in a sample of organisms.
For reliable data:
- measure the same feature in the same way each time
- use suitable units, such as millimetres or centimetres
- use a large enough sample
- calculate a mean for measured data
- use a bar chart for categories and a histogram or line graph for continuous measurements
Using variation data
A student measures the shell width of five snails from the same species: 18 mm, 20 mm, 21 mm, 19 mm and 22 mm.
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Shell width is measured on a scale, so it shows continuous variation rather than separate categories.
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Add the measurements:
- Divide by the number of snails:
- Since only five snails were measured, this mean is only an estimate for the population. A larger random sample would give a more reliable estimate.
Mutations and phenotype
Mutations are important because they create new genetic variation, but they do not always change what an organism looks like or how it functions.
Most genetic mutations have no effect on the phenotype. Some mutations have a small effect. Rarely, a single mutation has a significant effect on the phenotype, especially if it changes an important protein.
A mutation may be:
- neutral, with no noticeable effect
- harmful, if it disrupts an important gene
- beneficial, if it gives an advantage in a particular environment
Mutation always means harmful
Most mutations do not affect the phenotype. A mutation is simply a DNA change; its effect depends on where it happens and what it changes.
Classifying mutation effects
A mutation changes a gene used to make a pigment-producing enzyme, and the enzyme no longer works.
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The mutation is in a gene involved in making a protein, so it has a realistic chance of affecting the phenotype.
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The protein affected is an enzyme needed for pigment production, so a faulty enzyme could change the organism’s colour.
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This mutation is likely to have a significant effect on phenotype because one DNA change has disrupted an important process.
The Human Genome Project
A genome is all the DNA in an organism. The Human Genome Project was an international project to work out the DNA base sequence of the human genome and identify the positions of many human genes.
It produced a reference human genome that scientists can compare other DNA sequences against.
Medical applications
The Human Genome Project can help medicine by allowing scientists and doctors to:
- identify genes and alleles linked with inherited disorders
- estimate a person’s risk of developing some diseases
- diagnose some genetic conditions more accurately
- offer genetic counselling for families
- develop targeted medicines that work better for people with particular genetic variants
- study cancer mutations and choose treatments that target those mutations
- research new drugs by understanding which genes and proteins are involved in disease
Limitations and ethical issues
Genome information is powerful, but it must be used carefully.
For many diseases, having a certain allele only changes the risk. It does not guarantee that a person will or will not develop the disease. Many conditions are affected by multiple genes and environmental factors.
There are also ethical concerns, including:
- privacy of genetic information
- consent before testing or sharing data
- possible discrimination by employers or insurers
- anxiety caused by knowing disease-risk information
Using genome information in medicine
A person has a DNA test showing a variant linked with a higher risk of an inherited disease.
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Doctors compare the person’s DNA sequence with a reference genome to identify the variant.
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The result is interpreted as increased risk, not a certain diagnosis, because other genes and environmental factors may also affect the phenotype.
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The information could be used to offer earlier screening, lifestyle advice, genetic counselling or targeted treatment if suitable.
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The result should be handled with consent and privacy because genetic information can affect the person and their relatives.
In the exam
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When asked about causes of variation, clearly separate genetic variation from environmental variation.
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For mutations, remember the pattern: most have no effect, some have a small effect, and rarely one mutation has a significant effect.
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If a question says “discuss” the Human Genome Project, include both a medical benefit and a limitation or ethical issue.
Check yourself
- Why do mutations create new genetic variation, while sexual reproduction mainly shuffles existing alleles?
- Give one characteristic affected mostly by genes, one mostly by environment, and one affected by both.
- How could a reference human genome help doctors without perfectly predicting whether someone will get a disease?
