Correct option is B
Explanation-
The Dorsal protein is a transcription factor that establishes ventral cell fates in the embryo. In its inactive state, Dorsal is held in the cytoplasm by Cactus, an IκB-like protein. On the ventral side, a Toll receptor is activated, leading to Cactus degradation and Dorsal entering the nucleus.
High nuclear Dorsal = ventral fate, low/no Dorsal in nucleus = dorsal fate.
Option a : In embryos that lack Gurken protein, the Dorsal protein is not translocated to the nucleus of the follicle cells which then causes ventralization of the embryo. - INCORRECT
Gurken is crucial for specifying dorsal follicle cells, not directly for Dorsal nuclear localization. Lack of Gurken leads to loss of dorsal cell fate, but Dorsal still enters nuclei on the ventral side due to Toll signaling. Also, Dorsal does not enter follicle cells; it works within the embryo itself (in the syncytial blastoderm).
Option b : Though Dorsal protein acts as a morphogen, it is found throughout the syncytial blastoderm of the early Drosophila embryo. - CORRECT
Yes, Dorsal is uniformly distributed in the cytoplasm of the syncytial embryo. What differs is nuclear localization, only in ventral nuclei.
Option c : In embryos that lack Cactus protein the Dorsal protein can be found in the nucleus of cells with a ventral fate. - CORRECT
Cactus inhibits Dorsal by sequestering it in the cytoplasm. In absence of Cactus, Dorsal enters all nuclei (not just ventral), leading to ventralization. While slightly simplified, the core idea is valid → Dorsal enters nuclei.
Option d : If the Dorsal protein is blocked from entering the nucleus, the genes responsible for specifying dorsal cell types are not transcribed. - INCORRECT
If Dorsal cannot enter the nucleus, ventral cell fate genes (e.g., twist, snail) are not transcribed. Dorsal-side genes (like decapentaplegic) are transcribed in the absence of nuclear Dorsal. So, the statement is false — Dorsal blocks dorsal fate genes; they are transcribed when Dorsal is absent.
Correct Combination: B and C
Hence, the correct answer is: Option b: B and C
