Alleles which carry genetic information are inherited in pairs for each trait. They can be recessive or dominant. The normal pigs must have been carrying defective genes which are recessive and that is why the phenotypic characteristics did not manifest in them. The 29 normal offspring may have inherited only one recessive defective gene while the 9 which died must have inherited two recessive which were expressed phenotypically i.e the swollen legs.
May 4th, 2015
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