• Genotype is a description of the genes an individual possesses for a particular characteristic.
• This is expressed via protein synthesis.

Asexual species

• Asexual reproduction leads to clones
• Mutations may develop new genotypes
• Conjugation tubes in bacteria

Sexual species

• Sexual reproduction leads to variation amongst all individuals in a species
• Through meiosis – independent assortment of chromosomes and crossing over
• Through random fertilisation of gametes.
• Through mutations

• Phenotype is a description of how the characteristic is expressed.
• There may also be environmental factors affecting how the phenotype is expressed.
• There are few characteristics for which the environment has no effect:
  • Blood group, sex, tongue rolling, eye colour.
• The following are examples of characteristics that may be controlled by genes but also influenced by the environment:
  • Skin colour, hair colour, weight, facial features, height.
• Factors include:
  • Nutrition, exercise, injury, light intensity, soil pH

• Characteristics that are only controlled by genetic factors tend to show discontinuous variation
• The characteristic concerned is one of two or more types with no intermediate forms.
• There are clear-cut differences in the forms of the characteristics.
• Normally controlled by a single gene with 2 or more alleles
• Example 1:Tall or dwarf pea plants
• If the phenotype can be affected by the environment, there may be a normal distribution for each characteristic form.

• E.g. Human blood group

• Characteristics that are affected by the environment tend to show continuous variation
• Marginal variation between individuals for a characteristic.
• Gradation from one extreme to another.
• All intermediates could exist.
• Determined by several genes – polygenic inheritance
• For example: Height in humans
• Individual genes contribute to phenotype, each one has small effect
• Environment also has considerable effect and may extend range
• Low probability of inheriting a combination of genes that lead to extremes.
• Data forms a normal distribution – a ‘bell’ shaped graph.
• The larger the sample size, the more symmetrical the graph.

This is the end of this topic

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