Final answer:
The Hox gene responsible for expression in the distal parts of late limb buds in tetrapods but not in fish is most likely Hoxa-11, playing a critical role in limb development during vertebrate embryogenesis.
Step-by-step explanation:
Hox genes are essential in early developmental stages and are responsible for the specific characteristics that distinguish various parts of an organism's body. The Hox gene in question, which is expressed in the distal part of late limb buds in tetrapods but not in fish, is likely the Hoxa-11 gene. This gene is notably absent in fish, which do not have limbs as tetrapods do. During the sixth week of vertebrate embryogenesis, the distal ends of limb buds expand, and the Hoxa-11 gene plays a crucial role in the differentiation process that leads to the formation of the appendicular skeleton, which arises from embryonic mesenchyme. Hoxa-11, as a master control gene, can turn on or off large numbers of other genes, shaping the growth and final structure of the limbs.