data.json

<!DOCTYPE html>
<head>
  <h1>Gene Mapping</h1>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <script>
		var svg = d3.select("body").append("svg")
    	.attr("width", 1000)
    	.attr("height", 500)
    
    d3.json("data.json", function (error, genedata){
      d3.select("svg").selectAll("rect")
				.data(genedata)
      	.enter()
      	.append("rect")
				.attr("x", function (d) {
        	return d.start})
				.attr("y", function (d){
        	if (d.orientation == "forward") {return 100}
        	else if (d.orientation == "reverse") {return 125}
          })
				.attr("width", function(d) {
        	return d.stop - d.start})
				.attr("height", 50)
    		.attr("fill", function(d) {
        	if (d.orientation == "forward") {return "purple"}
        	else if (d.orientation == "reverse") {return "red"}
      })
      
    })
    
   	
     
  </script>
</body>

https://d3js.org/d3.v4.min.js