Reference for Bekhorot 33:21

ר' מאיר מיחייב למאי

For R'Meir holds that we take into consideration the nature of the sire, whereas the Rabbis hold that we do not take into consideration the nature of the sire.<span class="x" onmousemove="('comment',' And he is consequently exempt since we follow the mother and here it bears no resemblance to the mother.');">¹³</span>

Jastrow