The mouse homozygous null mutant for a gene bfg always dies mid-gestation. Chimeric mice made of bfg null cells and wild-type cells have healthy pups. When the developing brain of these pups is examined, they have more than two hippocampi. Closer examination reveals bfg null cells at the center of these supernumerary hippocampi.

Which of the following inferences can correctly be drawn from this experiment?

Correct option is B

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The experiment involves bfg null mutant mice and chimeric mice (mice composed of both bfg null cells and wild-type cells). The key findings are:

1. Homozygous bfg null mutants die mid-gestation, indicating bfg is essential for survival.
2. Chimeric mice (bfg null + wild-type cells) survive, meaning that wild-type cells compensate for the loss of bfg in some way.
3. Chimeric mice have more than two hippocampi, meaning that additional hippocampi are formed due to the presence of bfg null cells at their centers.

Step-by-step Analysis of the Given Options:

1. "Wild-type bfg function in the developing brain induces hippocampus specification in neighboring cells." (Incorrect)

   • If wild-type bfg induced hippocampus formation, we would expect the hippocampus to form only where wild-type cells are present.
   • However, the experiment shows that bfg null cells are at the center of extra hippocampi.
   • This suggests that hippocampus formation is occurring in the absence of bfg function, meaning that bfg normally suppresses hippocampus formation rather than inducing it.

2. "Wild-type bfg function in the developing brain suppresses hippocampus specification in neighboring cells." (Correct )

   • The presence of bfg null cells at the center of the supernumerary hippocampi suggests that in wild-type mice, bfg normally inhibits the formation of additional hippocampi.
   • The loss of bfg function in some cells (bfg null cells) leads to additional hippocampus structures, meaning that bfg normally acts as a suppressor of hippocampus specification.
   • This matches the observations in the chimeric mice, making this the correct answer.

3. "bfg function is completely cell autonomous." (Incorrect)

   • Cell autonomous genes act only within the cells that express them.
   • However, in this case, bfg null cells are influencing neighboring cells to form hippocampi, suggesting a non-cell-autonomous function.
   • This means that bfg does not act solely within its own cells but has broader regulatory effects in the brain.

4. "bfg function is hippocampus-specific." (Incorrect)

   • The experiment only examines the hippocampus, but it does not provide evidence that bfg is exclusively involved in hippocampus development.
   • The lethality of homozygous bfg mutants at mid-gestation suggests that bfg is also essential for other developmental processes.
   • Thus, it is incorrect to conclude that bfg function is only specific to the hippocampus.