BYA5 SECTION 14.2Genetic and environmental factors influence variation between individuals |
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Normal distribution about the mean |
- Mode (most frequent) = median (mid) = mean (average value)
- Bell-shaped/ even distributions of values above and below mean
- Standard error (SE)
- True mean of SE is ±1.96
- In a number of samples each sample will have its own mean
- Standard error measures how much the value of a sample mean is likely to vary
- The greater the standard error, the greater the variation of the mean
- Standard deviation (σ)
- Measure of the spread of results about the mean of a normal distribution curve
- Thinner bell-shape / smaller standard deviation / less variation
- Same pattern with bigger bell-shape
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The causes of variation |
| Independent assortment of bivalents at the equator during anaphase I |
- Chromosomes of bivalents pulled to opposite poles at random
- 2n different combinations of chromosomes in four haploid cells produced where n is the haploid number of chromosomes
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Crossing over between non-sister chromatids during prophase I |
- At synapsis, non-sister chromatids of homologous pairs cross over at chiasmata
- Homologous chromatids (corresponding pieces of genetic material) break and exchange equivalent segments between maternal and paternal chromatids
- Results in new combination of genes from the two parents
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Fertilization / random fusion of gametes |
- Genetic difference amongst the zygote
- New combinations of alleles
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Gene mutation / increased by environmental factors (eg radiation) |
- Addition / at least one base is added during DNA replication
- Deletion / at least one base is not copied (frameshift)
- Substitution / at least one base is copied wrongly
- Interferences with normal base pairing (A-T;C-G)
- Degenerate code / different triplets can code for same amino acids
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Discontinuous variation
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- Limited number of distinct phenotypes / categories (e.g. blood group)
- Strong genetic factor controlled by alleles on one gene
- Frequency histogram has separate bars
- Unaffected by the environment
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Continuous variation |
- Continuous range of values / class intervals (e.g. human height)
- Alleles on many genes located on different chromosomes /
polygenic inheritance
- Frequency histogram is a smooth (normal distribution) curve
- Phenotype is affected by environmental factors
- Lower skin temperature activates a gene for pigment production
- Diet affects individual's size and health. Malnourishment results in shorter height
- \ genes + environment → phenotype (continuous variation)
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Advantageous of variation to species |
- Allows different adaptations/some better adapted
- Some survive / reproduce / pass on gene/allele
- Allows for changing environment/different environment
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References and Further Reading
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]
