The body of a child is formed from the cells of the father and mother.
The soul of the child, however, is formed by neither, but rather is created by a "copulation" of the Sefirot.
When the Sefirot mix with one another, they give birth to souls and these souls are combinations of Sefirot in an infinite variety of relations.
Every time and in every place where there is a new combination (usually the combinations are old, fairly fixed), a new soul is born.
–Rabbi Adin Steinsaltz
From In the Beginning by Rabbi Adin Steinsaltz