Variation (OCR Gateway GCSE Biology: Combined Science)

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Continuous & Discontinuous Variation

  • Variation is defined as differences between individuals of the same species
  • Phenotypic variation is the difference in features between individuals of the same species, resulting from:
    • The genome; differences caused by genes are known as genetic variation
      • Examples of genetic variation in humans include:
        • Blood group
        • Eye colour
        • Gender
        • Ability to roll tongue
        • Whether ear lobes are free or fixed
    • The interactions between the organism and its environment, this is known as environmental variation
      • Examples of environmental variation in humans include:
        • An accident may lead to scarring on the body
        • Eating too much and not leading an active lifestyle will cause weight gain
        • Being raised in a certain country will cause you to speak a certain language with a certain accent
        • A plant in the shade of a big tree will grow taller to reach more light
  • Phenotypic variation can be divided into continuous or discontinuous depending on how you are able to group the measurements

Continuous variation

  • Continuous Variation is when there are very many small degrees of difference for a particular characteristic between individuals
    • The data are arranged in order and can usually be measured on a scale
    • For example, height, mass, finger length etc. where there can be many ‘in between’ groups
  • Continuous features often vary because of a combination of genetic and environmental causes, for example:
    • Tall parents will pass genes to their children for height
    • Their children have the genetic potential to also be tall
    • However, if their diet is poor then they will not grow very well
    • Therefore, their environment also has an impact on their height 
  • Plotting continuous data onto a graph will give smooth bell curves (a result of all the small degrees of difference)

Height graph

Height is an example of continuous variation which gives rise to a smooth bell-shaped curve when plotted as a graph

Discontinuous variation

  • Discontinuous Variation is when there are distinct differences for a characteristic
    • Data fits into discrete categories with no crossover between categories
  • Discontinuous variation is usually caused by genetic variation alone 
    • For example, people are either blood group A, B, AB or O; are either male or female; can either roll their tongue or not - there are no ‘in-betweens’
    • Plotting discontinuous data onto graphs will give a ‘step – like’ shape

Blood type graph

Blood group is an example of discontinuous variation which gives rise to a step-shaped graph

Mutation & Variation

  • Variation occurs as the result of different alleles in the population
  • These allele variants are introduced into the gene pool as a result of mutations
  • Mutations are changes in the DNA base sequence and can impact the genome in several different ways
    • Some mutations make no difference to the resulting proteins
    • Others may make a difference to the protein but have no effect on the protein function
    • In some cases, mutations alter the protein significantly enough to impact the function of the protein
      • Only this third scenario would have an effect on the phenotype of the organism

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Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.