- Homeotic genes are master regulator genes that direct the development of particular body segments or structures.
- When homeotic genes are overactivated or inactivated by mutations, body structures may develop in the wrong place—sometimes dramatically so!
- Most animal homeotic genes encode transcription factor proteins that contain a region called the homeodomain and are called Hox genes.
- Hox genes are turned on by a cascade of regulatory genes; the proteins encoded by early genes regulate the expression of later genes.
- Hox genes are found in many animals, including fruit flies, mice, and humans. Mutations in human Hox genes can cause genetic disorders.
Homeotic mutations in fruit flies
Overview of fruit fly Hox genes
How are fly Hox genes turned on?
- Maternal effects genes, which are genes whose ms are placed in the egg cell by the mother fly before fertilization. Some of the ms are “tied” to the head or tail end of the embryo and are responsible for setting up the head-tail polarity. The maternal effects genes encode regulators of transcription or translation that control each other as well as other genes.
- Gap genes are named appropriately. If gap genes are missing due to a mutation, there is a big gap in the fly larva—it is missing a large chunk of its normal segments. Gap genes are activated through interactions between the protein products of the maternal effects genes, and they also regulate each other. They're responsible for defining large, multi-segment regions of the fly, the ones that are missing when the gene is mutated.
- Pair-rule genes are turned on by interactions between gap genes, and their expression patterns are refined by interactions with one another. They appear in multiple “stripes” along the embryo, similar in pattern to the segments of the mature fly but slightly offset. When a pair-rule gene is missing due to mutation, there is a loss of structures in the segment regions where the gene is normally expressed.