Edexcel IGCSE Biology: Double Science 复习笔记 Types of Variation

Edexcel IGCSE Biology: Double Science 复习笔记 Types of Variation

Types of Variation

 

  • Variation is defined as differences between individuals of the same species
  • Variation can be divided into two types depending on how you are able to group the measurements:
    • Continuous variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale
      • Examples include height, mass, finger length etc. where there can be many ‘in-between groups

       

    • Discontinuous variation is when there are distinct differences for a characteristic
      • 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'

       

     

  • When graphs of these data are plotted, continuous variation gives smooth bell curves (a result of all the small degrees of difference), whereas discontinuous gives a ‘step–like’ shape

 

 

 

Continuous-variation

 

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

 

 

 

Discontinuous-variation

 

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

 

 

Phenotypic Variation

 

  • Phenotypic variation can be caused in two main ways:
    • It can be genetic - controlled entirely by genes
    • Or it can be environmental - caused entirely by the environment in which the organism lives

     

 

 

 

Genetic variation

  • Meiosis creates genetic variation between the gametes produced by an individual
  • This means each gamete carries substantially different alleles
  • During fertilization, any male gamete can fuse with any female gamete to form a zygote
  • This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
  • Zygotes eventually grow and develop into adults
  • Examples of genetic variation in humans include:
    • Blood group
    • Eye colour
    • Gender
    • Ability to roll tongue
    • Whether ear lobes are free or fixed

     

 

 

Earlobes

 

Whether earlobes are attached (lobeless) or free (lobed) is an example of genetic variation

 

 

 

Environmental variation

  • Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle
  • In this instance ‘environmental’ simply means ‘outside of the organism’ and so can include factors like climate, diet, culture, lifestyle and accidents during lifetime
  • Examples 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

     

 

 

Genetic and environmental causes

 

  • Discontinuous variation is usually caused by genetic variation alone
  • 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

     

  • Another way of looking at this is that although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop

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