Multi condition filtering with checkboxes JavaScript

I'm trying to create a filter based that is triggered by checkboxes, which shows and hides options from a list. I have tried to use resources online to put the script together but nothing seems to work.

In order to hide or show list items
I am editing a - class= "marker" parent element
based on wether - class= "condtx" of its child element
is matching the array of checked boxes "value" or not.

I have included comments on how I am proceeding.

Can anyone point me in the right direction please?

function filterCondtn1() {
  var i, z;

  //"chekedVal" array Keeps returning "undefined" I have no idea why

  var chekedVal = document.getElementById("InputOpts").getElementsByTagName("input").checked.value;

  //Gathered all <h3> tag options into array to be tested against "chekedVal" for match

  var condt1 = document.getElementsByClassName("condtn1").innerHTML;

  //Parrent class that will be edited to show <div> or not if True or false.

  var parrentClss = document.getElementsByClassName("marker");

  //From here it's a blur. I've trying everything I can. I think I should be something of that sort.

  for {
    (var i = 0; i < condt1.length; i++) &&
    (var z = 0; z < parrentClss.length; z++)
    RemoveClass(parrentClss[z], "show");
    if (condt1[i].indexOf(chekedVal) > -1) AddClass(parrentClss[z], "show");
  }
}

//Currently using these two functions in my script and they work

function RemoveClass(element, name) {
  var i, arr1, arr2;
  arr1 = element.className.split(" ");
  arr2 = name.split(" ");
  for (i = 0; i < arr2.length; i++) {
    while (arr1.indexOf(arr2[i]) > -1) {
      arr1.splice(arr1.indexOf(arr2[i]), 1);
    }
  }
  element.className = arr1.join(" ");
}

function AddClass(element, name) {
  var i, arr1, arr2;
  arr1 = element.className.split(" ");
  arr2 = name.split(" ");
  for (i = 0; i < arr2.length; i++) {
    if (arr1.indexOf(arr2[i]) == -1) {
      element.className += " " + arr2[i];
    }
  }
}

.markerA {
  display: none;
}

.show {
  display: block;
}

<div id="InputOpts">
  <input type="checkbox" value="option 1" oninput="filterCondtn1();">Option 1
  <input type="checkbox" value="option 2" oninput="filterCondtn1();">Option 2
  <input type="checkbox" value="option 3" oninput="filterCondtn1();">Option 3
  <input type="checkbox" value="option 4" oninput="filterCondtn1();">Option 4
</div>

<div class="marker">
  <div class="necessary">
    <h3 class="condtn1">Option 1</h3>
  </div>
</div>

<div class="marker">
  <div class="necessary">
    <h3 class="condtn2">Option 3</h3>
  </div>
</div>
<div class="marker">
  <div class="necessary">
    <h3 class="condtn1">Option 2</h3>
  </div>
</div>
<div class="marker">
  <div class="necessary">
    <h3 class="condtn2">Option 4</h3>
  </div>
</div>