how do I make my character jump while running without losing all your speed

I made a script with character controller , but when the User button to jump button it jumps but misses full speed (do a parable ) he jumps straight and up only, anyone know what I can do for the player while running , jump and receive speed. This is my script:

using UnityEngine; using System.Collections;

public class PlayerBehaviour : MonoBehaviour {

 //Speeds and Movimentation
 public float speed;
 public float walkSpeed = 6.0f;
 public float turningSpeed = 100.0f;
 public float runSpeed = 12.0f;
 public float jumpSpeed = 8.0f;
 //Phisics
 public float gravity = 20.0f;
 private Vector3 moveDirection = Vector3.zero;
 //Booleans

 //Animation
 public Animator anim;

 //Start
 void Awake () {
 
     anim = GetComponent<Animator> ();
 }

 void FixedUpdate () {
     speed = walkSpeed;
     CharacterController controller = GetComponent<CharacterController>();
     if (controller.isGrounded) {
         float horizontal = Input.GetAxis ("Horizontal") * turningSpeed * Time.deltaTime;
         transform.Rotate (0, horizontal, 0);
         
         float vertical = Input.GetAxis ("Vertical") * speed * Time.deltaTime;
         transform.Translate (0, 0, vertical);
         //Run method
         if(Input.GetKey("left shift")&& controller.isGrounded){
             speed=runSpeed;
             float v=Input.GetAxis ("Vertical") * speed * Time.deltaTime;
             transform.Translate (0, 0, vertical);
         }
     }
         if (Input.GetButton("Jump")&& controller.isGrounded){
             moveDirection.y = jumpSpeed;
     }
     moveDirection.y -= gravity * Time.deltaTime;
     controller.Move(moveDirection * Time.deltaTime);
 
 }

}