Reference for Chullin 159:11

בצבי הבא על התיישה ולמלקות רבנן סברי

There is only a prohibition', perhaps we do not take into consideration the seed of the male 'parent and therefore this is a proper sheep; 'but stripes cannot be inflicted', for it may be that we ought to take into consideration the seed of the male parent [so that it is only a part sheep], and we do not say that the term 'sheep' includes that which is a sheep in par only.

Jastrow