OCR A Level Biology

Revision Notes

6.2.2 Variation: Phenotype

Variation: Phenotype

  • The observable characteristics of an organism are its phenotype
  • Phenotypic variation is the difference in phenotypes between organisms of the same species
  • In some cases, phenotypic variation is explained by genetic factors
    • For example, the four different blood groups observed in human populations are due to different individuals within the population having two of three possible alleles for the single ABO gene
  • In other cases, phenotypic variation is explained by environmental factors
    • For example, clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions
  • Phenotypic variation can also be explained by a combination of genetic and environmental factors
    • For example, the recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria
  • The complete phenotype of an organism is determined by the expression of its genotype and the interaction of the environment on this:

Phenotype = Genotype + Environment

Genetic factors

  • Genes can have varying effects on an organism’s phenotype
    • The phenotype may be affected by a single gene or by several genes
    • The effect that the gene has on the phenotype may be large or small
  • Specifically, the different alleles an organism has at a single gene locus can determine the phenotype
    • Remember – diploid organisms will inherit two alleles of each gene, these alleles can be the same or different
  • For example, the F8 gene that codes for the blood-clotting protein Factor VIII
    • The different alleles at the F8 gene locus dictate whether or not normal Factor VIII is produced and whether the individual has the condition haemophilia

Environmental factors

  • The environment that an organism lives in can also have an impact on its phenotype
  • Different environments around the globe experience very different conditions in terms of the:
    • Length of sunlight hours (which may be seasonal)
    • Supply of nutrients (food)
    • Availability of water
    • Temperature range
    • Oxygen levels
  • Changes in the factors above can affect how organisms grow and develop
    • For example, plants with a tall genotype growing in an environment that is depleted in minerals, sunlight and water will not be able to grow to their full potential size determined by genetics (their phenotype is being affected by their environment)
  • Variation in phenotype caused solely by environmental pressures or factors cannot be inherited by an organism’s offspring (only alterations to the genetic component of gametes will ever be inherited)

Examples of how the environment can affect phenotypic variation

  • Diet in animals:
    • The fruit fly (Drosophila melanogaster) is normally grey but there is a genetic mutant that is yellow (a genotypic characteristic that is expressed regardless of the environment)
    • If the larvae of normal grey flies are given a diet of silver salts, they develop the yellow colour regardless of their genotype
    • This means that flies that should be grey (according to their genes) can become yellow due to an environmental factor (their diet)
  • Growing conditions for plants:
    • Plants that are grown in the dark or that cannot access enough magnesium become yellow even though, genetically, they should be green
      • This condition is known as chlorosis and occurs because the synthesis of chlorophyll is slowed down or stops completely
    • Plants that are grown in the dark may also develop long stems with small, curled leaves even though, genetically, they should develop normally
      • This is known as etiolation

Exam Tip

Some questions in the exam may ask you to explain why the variation in phenotype due to genetics is inherited but the variation in phenotype due to environmental factors is not. This is because genetic variation directly affects the DNA of the gametes but variation in phenotype caused by the environment does not.

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