Before us is Mishnah 7 in chapter 3 of Maseches Challah, which deals with a dough made from two kinds of flour: wheat flour and rice flour.
"Ha'oseh isah min hachitim umin ha'orez":
A person makes a dough from wheat flour and adds rice flour to it as well. The wheat flour is of course obligated in the mitzvah of challah, while the rice flour is not, since rice is not among the species obligated in challah. Even so, the Mishnah rules: "Im yesh bah taam dagan - chayeves bechallah" - "dagan" here refers to the wheat that is obligated in challah, and as long as the taste of wheat is discernible in the dough, the obligation to separate challah applies to it.
And not only that, but "veyotzei bah adam yedei chovaso bePesach" - a person fulfills his obligation to eat matzah on Pesach with such a dough, even though only the wheat is fit for the mitzvah of matzah while rice is not fit for it.
Two ways of explaining the case:
Some hold that this ruling was stated specifically regarding the relationship between wheat and rice, because rice tends to be drawn after wheat. Under these circumstances, once the taste of the wheat is discernible in the dough, we regard the entire dough as though it were all wheat.
Others explain that the case is one in which the wheat flour by itself contains a full measure for challah, and the rice was added to it. Even if most of the dough is now made of rice flour, since the taste of the wheat is discernible in it and the wheat contains the measure for challah, it is obligated in challah.
"Ve'im ein bah taam dagan":
The Mishnah continues: "Ve'im ein bah taam dagan - einah chayeves bechallah, ve'ein adam yotzei bah yedei chovaso bePesach". When the taste of the grain, that is, the taste of the wheat, is not discernible in the dough, the mixture is not obligated in the separation of challah, and a person also does not fulfill his obligation with it on Pesach.
In summary: in a dough made from wheat flour mixed with rice flour, everything depends on the taste of the grain: if the taste of the wheat is discernible in it, it is obligated in challah and one may also fulfill the obligation of matzah with it on Pesach; and if it has no taste of grain, it is not obligated in challah and one does not fulfill the obligation with it. Two ways of explaining this were brought: that the rice is drawn after the wheat and the whole is considered wheat, or that the case is a dough in which the wheat flour itself contains a full measure for challah.