Genetic Approaches

FORWARD GENETICS: PHENOTYPE -> GENOTYPE

Forward genetics involves the use of a naturally occuring abnormal phenotype or random mutogenesis of an organism in order to produce an abnormal phenotype. The genetics of the mutant animals with the abnormal phenotype are then compared with that of a wild type animal and any discrepancies are noted. The differences in the genes are then tested using knock out and knock down techniques to reveal which gene is responsible for the abnormal phenotype observed.

Drosophila Melanogaster - Eye Colour
Ernst Caspari was the first person to study developmental genetics. He noticed that Drosophila Melanogaster have a mutation causing the normal red colour of the eye to be lost and instead, mutants have white eyes.
He used forward genetic screens to identify the white gene - a gene in which a recessive mutation causes an orange eye phenotype in heterozygous mutants and a white eye colour in homozygous mutants. 

Image courtesy of wiki commons under creative commons liscence.

REVERSE GENETICS: GENOTYPE -> PHENOTYPE

In reverse genetics scientists have a known sequence of DNA which they wish to know the function of. In this case mutations are created in this specific sequence and changes in phenotype are observed to reveal the function of the gene.