A scientist wishes to generate transgenic mice, which express a particular exogenous gene only in their muscles. It is expected that the mice will transmit the exogenous gene to their offspring. Which of the following approaches to generate the mice would be most effective?  The gene could be injected directly into the muscle of adult mice. mRNA from the gene, made by transcription in vitro could be injected into muscle of an adult mouse. CRISPR could be used to disrupt a muscle-specific myosin gene, followed by repair by non-homologous recombination. The gene could be fused to the promoter from a muscle-specific myosin gene and injected into newly fertilised mouse eggs. Newly fertilised eggs could be infected with Agrobacterium tumefaciens cells that carry the gene on the Ti plasmid.

To determine the most effective approach for generating transgenic mice that express a particular exogenous gene specifically in their muscles and can transmit this gene to their offspring, let's analyze each option step by step:

1. The gene could be injected directly into the muscle of adult mice.

   • Injecting the gene directly into the muscle of adult mice would result in the gene being expressed only in the injected muscle cells. However, this method would not ensure that the gene is incorporated into the germline, meaning it would not be passed on to offspring.

2. mRNA from the gene, made by transcription in vitro could be injected into muscle of an adult mouse.

   • Injecting mRNA into the muscle of an adult mouse would lead to temporary expression of the gene in the muscle cells. Like the first option, this method would not result in the gene being incorporated into the germline, so it would not be transmitted to offspring.

3. CRISPR could be used to disrupt a muscle-specific myosin gene, followed by repair by non-homologous recombination.

   • Using CRISPR to disrupt a muscle-specific myosin gene and then repairing it by non-homologous recombination would likely result in a loss-of-function mutation in the myosin gene. This approach does not involve inserting the exogenous gene, and it would not ensure muscle-specific expression of the exogenous gene.

4. The gene could be fused to the promoter from a muscle-specific myosin gene and injected into newly fertilised mouse eggs.

   • Fusing the gene to a muscle-specific myosin gene promoter and injecting it into newly fertilized mouse eggs would allow the gene to be incorporated into the genome of the developing embryo. This method ensures that the gene is expressed specifically in muscle cells due to the muscle-specific promoter and that it is present in the germline, allowing it to be transmitted to offspring.

5. Newly fertilised eggs could be infected with Agrobacterium tumefaciens cells that carry the gene on the Ti plasmid.

   • Agrobacterium tumefaciens is a bacterium used to transfer genes into plant cells, not animal cells. This method is not suitable for generating transgenic mice.

Based on the analysis, the most effective approach to generate transgenic mice that express the exogenous gene specifically in their muscles and can transmit the gene to their offspring is:

The gene could be fused to the promoter from a muscle-specific myosin gene and injected into newly fertilised mouse eggs.