Genetic variation

The variation that exists within a species is categorised as continuous and discontinuous. The environment has considerable influence on the expression of features that show continuous (or quantitative) variation.

Discontinuous variation

- qualitative differences

- genetic basis:

• different alleles at a single gene locus have large effects on the phenotype
• different genes have quite different effects on the phenotype

- e.g.: eye colour

Continuous variation

- quantitative differences

- genetic basis:

• different alleles at a single gene locus have small effects on the phenotype
• different genes have the same/additive effect on the phenotype
• polygenes - large number of genes have a combined effect on a particular phenotypic trait

- e.g.: height; weight

Environmental effects on phenotype

e.g.: hair colour of Himalayan rabbits, Siamese and Burmese cats

- development of dark extremeties: tips to ears, nose, paws and tail

- caused by an allele that allows formation of dark pigments only at low temperature

e.g.: cob length of Black Mexican and Tom Thumb maize plants

- difference in light intensity and nutrients will lead to different growth of plants with the same genetic contribution

- use t-test to compare variation of the 2 different populations

Importance of genetic variation in selection - Genetic variation provides the raw material on which natural selection can act. Variation within a population means that some individuals have features that give them an advantage over other members of that population.

17.1 Variation The variation that exists within a species is categorised as continuous and discontinuous. The environment has considerable influence on the expression of features that show continuous (or quantitative) variation. a) describe the differences between continuous and discontinuous variation and explain the genetic basis of continuous (many, additive genes control a characteristic) and discontinuous variation (one or few genes control a characteristic) (examples from 16.2f may be used to illustrate discontinuous variation; height and mass may be used as examples of continuous variation)  b) explain, with examples, how the environment may affect the phenotype of plants and animals  c) use the t-test to compare the variation of two different populations (see Mathematical requirements)  d) explain why genetic variation is important in selection