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The small size and the lack of recombination set the fourth chromosome of Drosophila melanogaster apart from the other chromosomes. I have shown that the Minute gene on chromosome 4, earlier named Minute-4, encodes the ribosomal protein RpS3A. Two Pax6 genes, eyeless (ey) and twin of eyeless (toy) are also located on chromosome 4.

Pax6 genes are important in head and eye development in both mammals and Drosophila. I have focused much of the study on ey and toy. The first mutant of toy that was characterized showed a headless phenotype. This indicates that Toy is important for the development of both the eye and antennal discs. The phenotype of the null mutation in toy is temperature sensitive due to that transcription of ey is temperature dependent in the eye-antennal primordium in absence of Toy. This temperature dependence was used to find out that the phenocritical period for ey in the adult head development is during embryonic stage 12-16 when ey first is expressed in the eye-antennal primordium. I also conclude that ey is activated by Toy in the eye-antennal primordium.

The strong eyD mutation was molecularly characterized and it was finally settled that it is an allele in the ey locus. I also show that eyD homozygotes have a headless phenotype, much stronger than the earlier ey mutations.