Variation (OCR GCSE Combined Science A (Gateway))

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 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 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