Homeotic genes, known as Hox genes, play a crucial role in determining the identity of body segments along the anterior-posterior axis during Drosophila embryogenesis. Unlike genes involved in segment formation or polarity, homeotic genes specify which structures each segment should develop.

Function of Homeotic Genes

Homeotic genes encode transcription factors that regulate the expression of various downstream genes controlling morphological features specific to each body segment. The spatial order of these genes in the genome corresponds roughly to the order in which they are expressed along the body axis, a phenomenon called colinearity.

Effects of Homeotic Gene Mutations

When homeotic genes mutate, they disrupt the normal identity of body segments without affecting the overall number or polarity of segments. Instead, such mutations lead to homeotic transformations, where one segment develops the features typical of another. For example, a mutation in the Antennapedia gene can cause antennae to transform into legs. Such phenotypic changes reflect the reprogramming of segment fate rather than segment deletion or polarity alterations.

Distinguishing From Other Mutations

• Mutations affecting segment polarity genes influence the internal structure and polarity within segments, not segment identity.

• Mutations disrupting dorsal-ventral polarity affect tissue organization along the dorsal-ventral axis.

• Segment deletions are caused by mutations in segmentation genes but not homeotic genes.

Conclusion

Therefore, mutations in homeotic genes during Drosophila embryogenesis primarily cause the replacement of one body part with another corresponding to altered segment identity.

The correct answer is:

(4) replacement of one body part by another body part