Variation: Phenotype (Cambridge (CIE) A Level Biology)

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 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 fertilisation

  • 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

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