__DATA__
 child,    Parent,    probability,
 M7,    Q,    P,
 M7,      M28,     E,
 M28,     M6,      E,
 M6,      Q,      Pl,
 & several hundred lines.....

__OUTPUT_1__
 M7:  M7<-Q = P
 M28: M28<-M6<-Q = Pl.E
 M6:  M6<-Q = Pl

__OUTPUT_2__
 M7:  M7<-Q = P
 M7:  M7<-M28<-M6<-Q = Pl.E.E
 M28: M28<-M6<-Q = Pl.E
 M6:  M6<-Q = Pl

#! /usr/bin/perl
 my %DEF = (
 I   => [qw( P Pl P.P P.Pl Pl.P Pl.Pl P.P.P P.P.Pl P.Pl.P P.Pl.Pl Pl.P.P Pl.P.Pl Pl.Pl.P Pl.Pl.Pl )],
 II  => [qw( E P.E Pl.E P.P.E P.Pl.E Pl.P.E Pl.Pl.E )],
 III => [qw( E.P E.Pl P.E.P P.E.Pl Pl.E.P Pl.E.Pl E.P.P E.P.Pl E.Pl.P E.Pl.Pl )],
 IV  => [qw( E.E P.E.E Pl.E.E E.P.E E.Pl.E E.E.P E.E.Pl E.E.E )] );
 my @rank = map @$_, @DEF{qw(I II III IV)};
 my %rank = map {$rank[$_-1] => $_} 1..@rank;
 my @group = map {($_) x @{$DEF{$_}}} qw(I II III IV);
 my %group = map {$rank[$_-1] => $group[$_-1]."_".$_} 1..@group;
 sub rank { $rank{$a->[2]} <=> $rank{$b->[2]} }
 my %T;

 sub oh { map values %$_, @_ }
 sub ab {   my ($b, $a) = @_;   [$b->[0], $a->[1], qq($a->[2].$b->[2]), qq($b->[3]<-$a->[3])] 
}
 sub xtend {
 my $a = shift;
 map {ab $_, $a} oh @{$T{$a->[0]}}{@_} }
 sub ins { $T{$_[3] //= $_[1]}{$_[2]}{$_[0]} = \@_ }

 ins split /,\s*/ for <DATA>;
 ins @$_ for map {xtend $_, qw(P E Pl)} (oh oh oh \%T);
 ins @$_ for map {xtend $_, qw(P E Pl)} (oh oh oh \%T);

 for (sort {rank} grep {$_->[1] eq 'Q'} (oh oh oh \%T)) {
 printf "%-4s: %20s,  %-8s %6s\n",
     $_->[0], qq($_->[0]<-$_->[3]), $_->[2], $group{$_->[2]};
 }  

 __DATA__
 M7    Q    P
 M54    M7    Pl
 M213    M54    E
 M206    M54    E
 M194    M54    E
 M53    M7    Pl
 M186    M53    Pl
 M194    M53    Pl
 M187    M53    E
 M204    M53    E
 M201    M53    E
 M202    M53    E
 M179    M53    E
 M173    M53    E
 M205    M53    E
 M195    M53    E
 M196    M53    E
 M197    M53    E
 M198    M53    E
 M57    M7    E
 M44    M7    E
 M61    M7    E
 M13    M7    E
 M50    M7    E
 M158    M50    P
 M157    M50    P
 M153    M50    Pl
 M162    M50    E
 M164    M50    E
 M165    M50    E
 M147    M50    E
 M159    M50    E