Variation in phenotype
Phenotype refers to the traits an organism actually displays. Members of the same species can differ in these traits, producing phenotypic variation within a population.
Differences between phenotypes may arise from genetic differences, from the conditions experienced by an organism, or from an interaction between genotype and environment.
Genetic influence
Individuals may inherit different alleles or combinations of alleles, resulting in differences between their phenotypes.
Example: different combinations of the three alleles at the ABO gene produce the different human blood groups.
Environmental influence
Organisms with identical genetic information can develop different phenotypes when they experience different conditions.
Example: genetically identical plants may reach different heights when their growing conditions differ.
Genotype and environment
The significance of a particular genotype can depend upon the environment in which the organism lives.
Example: heterozygosity for the sickle-cell allele can provide resistance to malaria.
Which variation can be inherited?
Genetic differences can be transmitted to offspring because DNA carrying those differences may be present in the gametes used during fertilisation.
A phenotypic difference produced only by environmental conditions is not transmitted in the same way because that environmental effect does not change the genetic information carried in the gametes.
Exam Tip: If asked why genetic variation can be inherited whereas environmentally produced variation cannot, link inheritance explicitly to DNA in gametes. Do not simply state that an inherited characteristic “runs in families”.