O Level Revision : Combined Science - Inheritance

Plants of the same species are different. The difference between organisms of the same species is called variation. There are two types of variation: continuous and discontinuous. Variation is either inherited or caused by the environment. Man is interested in variation and seeks to manipulate it to produce good quality crops and livestock.  Selection is the process of eliminating individuals with undesirable characteristics.  Selection can be natural or artificial.



  • Variation describes the differences found between organisms of the same species.
  • Continous variationis whenthere are several intermediates such which are measurable. Examples are body mass, height, number of seeds in a pod, etc.
  • Features that exhibit continuous variation are affected by the environment rather than being inherited.
  • Discontinous variation refers to features that do not have intermediate forms, i.e. the differences are clear cut. Examples are coat colour in cattle, seed texture in peas.
  • Discontinous variation is controlled by genetic factors only.

Factors that cause variation in living organisms:

  1. a) Environmental factors

These are non-heritable factors and may include availability of food, exposure to light, variations in temperature or other atmospheric conditions etc

  1. b) Genetic factors

These are passed from parents to offspring.


Definition: The preferential elimination of individuals with certain genetic compositions, through natural or artificial controlling factors.

Selection can be natural or artificial.

Natural selection

  • Organisms of the same species compete for food, space and mates.
  • In natural selection, the individuals in the species that are able to adapt to the environment are more likely to survive and produce offspring than those with less useful adaptations.
  • This increases the occurrence of useful adaptations over generations, since surviving organisms pass on their characteristics to the next generation.
  • The less well-adapted individuals die or fail to reproduce and their characteristics are not passed on to the next generation.
  • Natural selection is largely due to environmental factors such as adverse weather or seasonal variations.
  • Natural selection is a mechanism for change of characteristics in an organism.

       Artificial selection

  • Artificial  selection is the manipulation of heritable variations in crops and livestock by humans through controlled breeding.
  • The breeders identify organisms with the desired characteristics and allow them to mate and reproduce.
  • This ensures that only desired features are obtained.
  • The qualities are automatically desirable to humans rather than for the use of the organism in natural environment.

       Applications of artificial selection

–     Artificial selection can be used to:

  1. a) increase meat yields                           b)   increase milk yields
  2. c) improve diseases resistance               d)   improve drought tolerance

–     Breeders select animals or plants with the desired characteristics and breed them artificially.

Artificial selection can also be used for aesthetic purposes through breeding on lawns, flowers and other ornamental plants.


  • There are two types of breeding, in-breeding and cross breeding.
  • In-breeding is the mating of organisms that are closely related e.g. brother or sister or father.
  • Cross-breeding is the mating of pure breed animals from different breeds e.g. Mashona cow and Brahman bull.

Advantages of in-breeding

Mating of closely related organisms has the following advantages:

  • Passing on of desirable characteristics in the family continually e.g. milk, meat, disease resistance etc.
  • Assist in the development of uniformity.

Disadvantages of in-breeding

Mating of closely related organisms has the following disadvantages.

  • Undesirable characteristic may surface.
  • Animals may end up with reduced vigour in some traits.

Advantages of cross-breeding

The advantages of cross-breeding include:

  • Hybrid vigour  (heterosis) i.e. it produces offspring superior to their parents.
  • The process combines desirable traits from parents.
  • Offspring shows improved performance quality e.g. meat, milk etc.
  • Brings variety and increased productivity.

Disadvantages of cross-breeding

The disadvantages of cross-breeding include:

  • Crossbreeds have the weakness of the breeds from which they descend.
  • It is difficult to identify organisms with the desired traits.