Features controlled by multiple genesSpec 3.19
Some characteristics, like flower colour in Mendel's pea plants, are controlled by a single gene. These are the ones used in monohybrid crosses.
Most phenotypic features are the result of multiple genes rather than single gene inheritance. Several genes work together to produce the feature. Features controlled by many genes, such as height, show a wide range of values rather than a few clear categories.
Causes of variationSpec 3.20
Differences between individuals of the same species are called variation. Variation in phenotype has two causes: genes and the environment. Often it is a combination of both.
| Genetic variation | Environmental variation | |
|---|---|---|
| What it is | Different characteristics as a result of mutation and sexual reproduction | Different characteristics caused by an organism's environment (acquired characteristics) |
| Source | Differences in the alleles inherited | Conditions during life, such as diet, climate, injury or activity |
| Examples | Eye colour, sex | A scar, body mass affected by diet, plant height affected by light |
Continuous and discontinuous variation
- Continuous variation: the feature can take any value in a range, such as height or mass. The results are shown on a histogram.
- Discontinuous variation: the feature falls into distinct categories with nothing in between, such as sex or purple versus white flowers. The results are shown on a bar chart.
Calculating an arithmetic mean
The heights of five seedlings grown in the same conditions were 12 cm, 15 cm, 11 cm, 14 cm and 13 cm. Calculate the mean height.
- Add the values: 12 + 15 + 11 + 14 + 13 = 65 cm.
- Divide by the number of values: 65 ÷ 5 = 13 cm.
Answer: Mean height = 13 cm.
Mutations and genetic variationSpec 3.22, 3.23
A mutation is a change in the base sequence of DNA. There is usually extensive genetic variation within a population of a species, and this variation arises through mutations.
Mutations affect the phenotype in different ways:
- Most genetic mutations have no effect on the phenotype.
- Some mutations have a small effect on the phenotype.
- Rarely, a single mutation will significantly affect the phenotype.
The Human Genome ProjectSpec 3.21
The Human Genome Project was an international research project to map the human genome. It was completed in 2003 and identified the sequence of bases in human DNA and where the genes are.
Potential applications in medicine
- Finding the genes linked to inherited disorders, so that people at risk can be tested and their risk predicted.
- Developing new treatments and drugs, and tailoring treatment to a person's own genes.
- Understanding how genes are involved in disease, which could help prevent or diagnose it earlier.
Concerns
- People may not want to know if they are likely to develop a disease that cannot be cured, which could cause worry.
- Genetic information must be kept private. There are concerns that it could be used unfairly, for example by insurers or employers.
Quick check
State where genetic variation in a population comes from.
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Mutations (and sexual reproduction).
What is environmental variation?
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Different characteristics caused by an organism's environment (acquired characteristics).
What effect do most mutations have on the phenotype?
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No effect.
Give one possible medical use of the Human Genome Project.
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For example, identifying genes linked to inherited disorders so that people can be tested, or developing treatments.