CIE A Level Biology

Revision Notes

Syllabus Edition

First teaching 2020

Last exams 2024

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17.1.1 Variation: Phenotype

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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 variation

    • Organisms of the same species will have very similar genotypes, but two individuals (even twins) will have differences between their DNA base sequences
    • Considering the size of genomes, these differences are small between individuals of the same species
    • The small differences in DNA base sequences between individual organisms within a species population are called genetic variation
    • Genetic variation is transferred from one generation to the next and it generates phenotypic variation within a species population
    • The following processes cause genetic variation as they result in a new combination of alleles in a gamete or individual:
      • Independent assortment of homologous chromosomes during metaphase I
      • Crossing over of non-sister chromatids during prophase I
      • Random fusion of gametes during fertilization

    • Mutation results in the generation of new alleles
      • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (because the genetic code is degenerate
      • New alleles are not always seen in the individual that they first occur in
      • They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation

    • Genes can have varying effects on an organism's phenotype
      • The phenotype may be affected by a single gene or by several
      • The impact that the gene has on the phenotype may be large, small and/or additive

Sources of genetic variation table

Sources of Genetic Variation Table, downloadable AS & A Level Biology revision notes

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 following:
    • 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

  • 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

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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Jenna

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.