Hello guys, can someone tell me what does alleles Y and Z mean? For example this is from a tabular file that shows alleles on chromosome 16:

16  rs2443047   0   56208410    T   C
16  rs2443046   0   56208600    G   A
16  rs77755488  0   56208672    T   C
16  rs1120535   0   56209081    G   T
16  rs1190750   0   56209983    C   T
16  rs4457984   0   56210936    A   C
16  rs112125012 0   56211391    T   A
16  rs1668325   0   56211715    A   G
16  chr16:56212560:I    0   56212560    Z   Y
16  rs1190749   0   56212664    T   G