It is not required that B have trait M; it may have disappeared in the lineage leading to B. Example: the tetrapods consist of the first ancestor of humans (A) from which humans inherited limbs with fingers or toes (M) and all descendants of that ancestor.
Non é necesario que B teña o trazo M; puido ter desaparecido na liñaxe que leva a B. Exemplo: os tetrápodos consisten no primeiro antepasado dos humanos (A) do cal os humanos herdaron as extremidades con dedos ou dedas (M) e todos os descendentes dese entepasado.