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became known that he has a son, his gift to the others is
nullified. Will this also apply with a partial ‘son’?

It may be possible to resolve this doubt based on the gemara in Chul-
lin (79b) which states,“A partial lamb is a lamb.” [The gemara there is
discussing the Torah prohibition of slaughtering“it and its son on the
same day” (Vayikra 22:28) which only applies only to domesticated
animals but not to wild animals. If a ram mated with a doe (a female
deer) and it gave birth to a female, and that female gave birth to a
male and the female and its offspring were slaughtered on the same
day, the Rabannan (ibid.) are of the opinion that the prohibition
has been violated since we reckon with the input of the ‘grandfather’
ram, for even “a partial lamb is a lamb.” The slaughtered mother and
offspring are therefore considered domesticated animals.] The Shach
(Yoreh De’ah 16:16) implies that this principle applies in all areas of
Torah. Accordingly, in our case too, a partial son would be considered
a complete son.

The opinion in the Yerushalmi that a woman can become pregnant
from two men at the same time requires further clarification: Does
this means that both are equal partners in the fetus’ formation or is
only one of them the principle father, while the other one plays an an-
cillary role? There will be a difference between these two possibilities
nowadays should we decide that tissue-typing is reliable,1 if genetic
testing shows that the child strongly resembles one of the fornicators
but bears no resemblance whatsoever to the second one. If we explain
the Yerushalmi as meaning that they are equal partners in the cre-
ation of the child, we know that in such a case, fertilization only took
place from one of them, not both and the child is the offspring of only
one. However, if we explain the Yerushalmi’s intention as being that
one is the main progenitor while the other one only makes a minor
contribution, we will no have way of finding out whether or not the
other one made some contribution.

1. See earlier, siman 260, for a lengthy discussion of the reliability of tissue typing,
at the end of which we cite my late father-in-law’s opinion.

126 1 Medical-Halachic Responsa of Rav Zilberstein