Describe the causes of variation that influence phenotype, including: a…

The idea

Phenotype is the observable characteristic an organism shows, and it is shaped by two broad causes. Genetic variation arises because DNA differs between individuals. Mutation changes the base sequence of a gene, producing a new allele that may alter the protein made. Sexual reproduction reshuffles existing alleles: meiosis produces gametes with different allele combinations, fertilisation pairs one gamete from each parent, and the offspring therefore differ from both parents and from each other. Environmental variation produces acquired characteristics caused by an organism's surroundings, such as a plant growing taller in good light or a person developing a tan in sunshine. Many phenotypes result from both causes acting together, for example height in humans.

What earns the marks

• Genetic variation is caused by differences in alleles arising from mutation and from sexual reproduction.
• Mutation changes the DNA base sequence, creating a new allele that may change the characteristic.
• Sexual reproduction produces genetic variation because meiosis makes genetically different gametes and fertilisation combines alleles from two parents.
• Environmental variation produces acquired characteristics caused by an organism's environment, such as light, diet or temperature.
• Many phenotypes, such as human height, are influenced by both genetic and environmental causes.

Common mistakes and how to put them right

• Treating a tan or a scar as inherited.

  Put it right: These are acquired characteristics caused by the environment, so they are not passed on through genes.

• Saying sexual reproduction creates new alleles.

  Put it right: It reshuffles existing alleles, whereas mutation is the source of new alleles.

• Assuming every characteristic has a single cause.

  Put it right: Most phenotypes, such as height, result from genetic and environmental factors together.

Exam tips

• Read the question stem to see whether it asks for a genetic or an environmental cause before choosing an option.
• Reject options that describe a characteristic acquired during life as if it were inherited.
• Where an option names mutation, check it correctly describes a change in DNA rather than a change in behaviour.