Define multiple expression to single array?

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1

This is fine and working

a [1] = "A/A"

How to define some thing like this ???

a [1] = "A/A" or "T/T or some thing else
2

I couldn't understand fully what you're expecting.

Are you expecting like this?

a[1]="A/A" || "B/B" || "C/C"
3 
awk '
BEGIN {
  a[1] = "\"A/A\""
  a[2] = "\"A/G\""
  a[3] = "\"G/G\""
  OFS = "\t"
}
NR == FNR {
  if (NR == 1) {
    for (i = 2; i <= NF; ++i)
      s = $i
    next
  }
  for (i = 2; i <= NF; ++i)
    ++b[s, $i]
  next
}
{
  $6 = $3
  if (FNR == 1) {
    $3 = "\"StYPE\""
    $4 = "\"Ntype\""
    $5 = "\"Stype_No\""
  } else {
    if ($1 == last) ++i
    else { last = $1; i = 1 }
    if (i in a) {
      $3 = a
      $4 = i
      $5 = b[$1, a]
    } else {
      $3 = "\"NULL\""
      $5 = $4 = "\"null\""
    }
  }
}
1
' input1.csv input2.csv

Take a look at bold letters
I want to define them as

  a[1] = "\"A/A\"" || "\"T/T\""|| or something else

but they are not working