Causes of variation and continuous vs discontinuous
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of genetic inheritance, mutation, and the role of environmental influences in phenotypic expression. You can test your knowledge on these below.
What is a gene?
A sequence of DNA that codes for a protein or polypeptide.
What is an allele?
A different form of a gene found at the same locus on a homologous chromosome.
What happens during meiosis that increases genetic variation?
Crossing over and independent assortment of chromosomes.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains causes of variation and continuous vs discontinuous or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Causes of Genetic Variation
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Genetic Factors
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Mutations: Spontaneous changes in DNA that may create new alleles.
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Meiosis:
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Crossing over: Exchange of alleles between homologous chromosomes, creating new combinations of alleles in the gametes.
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Independent assortment: Random distribution of maternal and paternal chromosomes, creating new combinations of chromosomes in the gametes.
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Random fertilisation: Any sperm can fertilise any egg, introducing a variety of allele combinations.
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Environmental Factors
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The environment can influence gene expression.
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Traits such as height or body mass are affected by nutrition, lifestyle, climate, etc.
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These do not change the DNA sequence, but can affect phenotype.
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Combined Genetic & Environmental Factors
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- Most variation in a population results from a combination of genetic and environmental influences.
Continuous vs Discontinuous Variation
|
Feature |
Continuous Variation |
Discontinuous Variation |
|
Description |
Shows a full range of phenotypes between two extremes |
Phenotypes fall into distinct categories |
|
Controlled by |
Many genes (polygenic) |
Usually a single gene (monogenic) |
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Influenced by |
Both genes and the environment |
Mainly genetic |
|
Example |
Height, body mass, skin colour |
Blood group, flower colour |
Examples of Variation Types
Continuous Variation Example – Human Height
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Controlled by many genes (polygenic).
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Strongly influenced by nutrition and health during development.
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Results in a range of values across the population.
Discontinuous Variation Example – ABO Blood Group
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Determined by a single gene with three alleles (IA, IB, IO).
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Unaffected by environmental factors.
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Individuals fall into distinct categories: A, B, AB, or O.
Key Terms
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Variation: Differences between individuals in a population.
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Polygenic inheritance: A trait controlled by multiple genes.
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Monogenic trait: A trait controlled by a single gene.
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Continuous variation: A range of phenotypes between two extremes.
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Discontinuous variation: categorical phenotypes.
No answer provided.
Exam Tip
Most phenotypes are influenced by both genes and the environment but only the genetic component will be inherited.
No answer provided.
Describe the difference between continuous and discontinuous variation and give one example of each. (4 marks)
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Continuous variation shows a full range of phenotypes between two extremes, with no distinct categories.
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It is controlled by many genes (polygenic) and often affected by the environment.
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Example: Human height.
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Discontinuous variation involves phenotypes that fall into distinct groups, with no intermediates.
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It is usually controlled by a single gene (monogenic).
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Example: Blood group in humans.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!