3.4.1 Causes of variation

Variation comes from genes and environment

1. Genetic variation is caused by differences in the alleles inherited by individuals of the same species.
2. Environmental variation is caused by conditions experienced during life, such as diet, light availability or injury.
3. Many phenotypes, including height and body mass, result from an interaction between genes and the environment.

Continuous variation forms a measurable range

1. Continuous variation produces many intermediate values between two extremes, with no distinct categories.
2. Examples include height, mass, hand span and leaf length.
3. Continuous characteristics are usually affected by many genes and the environment.
4. A large sample may form a normal distribution, with most values near the mean and fewer values at either extreme.
5. Display continuous data in a histogram with touching bars because the measurement scale has no gaps.

Discontinuous variation forms categories

1. Discontinuous variation places each individual into one of a limited number of distinct categories.
2. Examples include ABO blood group and the ability or inability to roll the tongue.
3. Discontinuous characteristics are usually controlled by one gene or a small number of genes, with little environmental influence.
4. Display discontinuous data in a bar chart with gaps because the categories are separate.

Results reveal the pattern of variation

1. Height should produce many intermediate values and may approximate a normal distribution.
2. The discontinuous characteristic should produce separate categories with no intermediate values.
3. A larger random sample reduces the influence of unusual individuals and makes the distribution more representative.

3.4.2 Outcomes of the Human Genome Project

Genome sequencing produced a human reference

1. The Human Genome Project was an international programme that determined the order of bases across the human genome.
2. The project also identified the positions of genes within the sequence.
3. A human genome contains about $3\times 10^9$ base pairs and approximately $20,000$ protein-coding genes.
4. The completed sequence provides a reference genome against which individual genomes can be compared.

Genome data supports medicine

1. Comparing DNA sequences can identify alleles associated with inherited disorders.
2. This information can support diagnosis, carrier testing and estimates of disease risk.
3. Knowledge of a gene and its protein can guide the development of treatments.
4. Personalised medicine uses genomic information to help select a medicine or dose for an individual.
5. A risk allele does not guarantee disease because many conditions are affected by several genes and environmental factors.

Sequence comparisons reveal ancestry

1. More similar DNA sequences suggest that two individuals or populations share a more recent common ancestor.
2. Patterns of sequence similarity can therefore be used to investigate ancestry and past human migration.

Genomic information raises ethical issues

1. A statistical association between an allele and a disease does not always prove that the allele causes the disease.
2. Predictions may be uncertain and can cause anxiety for the person tested.
3. Genomic records require informed consent and secure storage because employers or insurers could misuse the information.

3.4.3 Genetic variation within a population

Mutations create new alleles

1. A mutation is the source of a new allele because it changes the base sequence of DNA.
2. Mutations occur randomly and continuously, rather than appearing because an organism needs a particular phenotype.
3. A population contains different alleles, so its members can have different genotypes and phenotypes.

Sexual reproduction reshuffles alleles

1. During meiosis, homologous chromosomes separate independently and genetic material is reshuffled into genetically different gametes.
2. Random fertilisation combines one gamete from each parent to produce new allele combinations.
3. Sexual reproduction increases variation by rearranging existing alleles, but it does not create new alleles.

Selection changes allele frequencies

1. Most mutations have no observable effect, while a few change the phenotype.
2. An allele that improves survival or reproductive success in a particular environment may be described as advantageous.
3. Individuals with the advantageous allele are more likely to reproduce and pass the allele to offspring.
4. The frequency of the allele can therefore increase over many generations through natural selection.

The environment determines advantage

1. An allele is advantageous only when it improves reproductive success under the current environmental conditions.
2. A change in the environment can alter which phenotypes are favoured.
3. The antibiotic selects resistant bacteria; it does not produce the resistance mutation.

3.4.4 Effects of genetic mutations on phenotype

A base change can alter a protein

1. A mutation changes the base sequence of DNA.
2. A mutation in a coding region may change a codon and therefore alter the amino acid sequence of a protein.
3. A changed amino acid sequence may alter how the polypeptide folds.
4. Different folding can change the protein's three-dimensional shape and its function.
5. If an enzyme's active site changes shape, fewer substrates bind and the reaction rate may decrease.

Non-coding mutations can change gene expression

1. A mutation in non-coding DNA can change whether a gene is switched on or off.
2. It can also change how much protein the cell produces.
3. A change in the amount of protein can alter the phenotype even when the protein's amino acid sequence is unchanged.

Mutations have different effects

1. Most mutations have no effect on phenotype.
2. A coding mutation may have no effect because the altered codon specifies the same amino acid.
3. Some mutations have a small effect, while a rare mutation may have a large harmful or beneficial effect.
4. Whether a mutation is advantageous depends on the environment.

A mutation can produce a new allele

1. A mutation within a gene creates a new allele of that gene.
2. DNA replication copies the allele so that it passes to daughter cells.
3. A mutation in a body cell is not normally inherited by offspring.
4. A mutation in a cell that forms a gamete can be inherited.

Gene and allele must not be confused

1. A gene is a section of DNA that codes for a protein or functional RNA.
2. An allele is a different version of the same gene.
3. A mutation normally creates a new allele rather than a completely new gene.