Final answer:
The Placozoa is the invertebrate with a single Hox gene and without true tissues, but with four cell types. In a mouse, replacing Hox13 with Hox1 could disrupt normal development, as each Hox gene influences distinct body segments.
Step-by-step explanation:
The invertebrate that has a single Hox gene, no true tissues, but has four different types of cells is Placozoa. The Placozoa are a group of simple and flat invertebrates that exhibit a limited number of cell types and a simple organization, lacking true tissues derived from embryonic germ layers. This group, though lacking the complex structure found in most multicellular animals, still contains the basic building blocks for multicellular life.
The Hox genes play a crucial role in determining the body plan during embryonic development. If, for instance, a Hox13 gene in a mouse was replaced with a Hox1 gene, this could significantly disrupt the normal developmental patterns. Hox13 genes are typically active in the posterior regions of an animal, while Hox1 genes function in the anterior regions. Therefore, replacing a Hox13 gene with a Hox1 gene might result in the anteriorization of posterior body structures, leading to a mismatch of body segments and severe developmental abnormalities.