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17.1 Variation

Syllabus
9700–2028–2029
Topic
17.1
Level
A2

Phenotypic variation can be genetic, environmental or both

Phenotypic variation means observable differences among individuals of the same species. A difference can result mainly from inherited genetic factors, environmental factors, or interaction between genotype and environment.

Main source Example Causal explanation
genetic ABO blood group inherited alleles determine which antigen phenotype is produced
environmental a scar injury changes the phenotype without changing the inherited allele set
genetic + environmental human height many alleles influence growth potential, while nutrition and health influence how fully that potential is expressed

Phenotype = genotype expressed in an environment is a reasoning framework, not a numerical equation. An environmentally caused phenotype is not automatically inherited, and a genetic influence does not mean the environment has no effect.

Discontinuous categories contrast with continuous ranges

Feature Discontinuous variation Continuous variation
Values distinct categories with no intermediates measurable range with intermediate values
Data qualitative/category counts quantitative measurements
Typical display separate bars with gaps frequency distribution or histogram across intervals
Example ABO blood groups height or body mass
Typical basis one or few loci with relatively large effects; environment often has limited effect many loci with small additive effects plus environmental influence

The observed pattern does not prove its cause. Continuous does not mean non-genetic, and discontinuous does not guarantee one gene; explain genetic basis separately from the shape of the data.

Genetic basis of variation

Genetic variation is variation in the DNA base sequence or allele combinations between individuals of a species. It can create different inherited possibilities for phenotype; environmental conditions then affect how those possibilities are expressed.

  • Mutation changes a DNA base sequence and can create a new allele. For the change to be inherited, it must occur in genetic material that contributes to gametes.
  • Crossing over, independent assortment and random fertilisation create new combinations of existing alleles, rather than automatically creating new alleles.
  • A gene is a DNA sequence at a locus; different alleles are alternative versions of that sequence. Their effects can be large at one locus or small and additive across several genes.

Discontinuous phenotypes are commonly associated with a large effect at one gene locus, whereas continuous phenotypes commonly reflect many small genetic effects together with environmental influence. This is a tendency within the syllabus boundary, not a cause that can be diagnosed from the phenotype alone.

Recombination reshuffles existing alleles; mutation can introduce a new allele. Environmental effects can change phenotype without changing the inherited DNA and are not themselves inherited as the same phenotype. Detailed t-test calculations and natural-selection frequency changes are outside this card.

A t-test compares a mean difference with within-sample variation

A t-test judges whether the difference between two sample means is large relative to variation within the samples. Use it for two independent sets of continuous, approximately normally distributed data with similar standard deviations.

t=\frac{|\bar{x}_1-\bar{x}_2|}{\sqrt{\frac{s_1^2}{n_1}+\frac{s_2^2}{n_2}}}

  1. State H0: there is no significant difference between the population means; the observed difference is due to chance.
  2. Calculate each sample mean and standard deviation, then substitute using the absolute mean difference.
  3. Use degrees of freedom = (n1−1)+(n2−1)=n1+n2−2.
  4. Compare calculated t with the critical value at the chosen probability, commonly p=0.05.
  5. If t exceeds the critical value, reject H0; otherwise fail to reject H0.

A significant mean difference does not prove the tested factor caused it or that it is biologically important. Failing to reject H0 does not prove the means equal. The absolute numerator keeps t non-negative regardless of sample order.