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In this topic of IBDP Biology, we will continue to learn about what variation is and causes of variation.
How do genotypes vary?
- 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
The effect of environmental factors
When it comes to IBDP Biology,
- 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
Variation through Genetic factors
When it comes to IBDP Biology,
- 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
Variation through environmental factors
- 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
