I was taught that both parents contributed equally but an older friend/mentor who was very successful at breeding horses told me the mother contributed more than the father . I asked what made him think that . He told me that some things were only passed on from father to daughter and daughter to her offspring ; making the maternal grandsire the most important horse in the pedigree and thus his daughter was more important than his son . Studying pedigrees of race horses and other four leggers , learning about midachondrial DNA , and now learning about sex linked chickens now makes me think he knew something . Anyone here with a more scientific explanation for that ?