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I'm having some trouble determining what exactly is the difference between Mendelian inheritance and non-Mendelian inheritance. For instance, I understand that chromosomal abnormalities such as Down's Syndrome fall under non-Mendelian inheritance because they concern chromosomes, not single genes. And I also understand that Mendelian inheritance concerns single genes, as in Sickle-cell anemia (which is an autosomal recessive disorder).

What confuses me is the fact that our textbook discusses dihybrid and trihybrid (concerning 2 genes and 3 genes, respectively) crosses under the Mendelian inheritance chapter, when to me it seems like these crosses are non-Mendelian because they deal with multiple genes. However, Gregor Mendel did in fact use the dihybrid cross to deduce the law of independent assortment, so I'm completely confused. Could someone please clarify this for me? I'm afraid that I'm maybe misinterpreting something.

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You can discuss multiple genes within the framework of Mendelian inheritance; what you're probably thinking of, though, is the fact that Mendelian inheritance doesn't recognize the idea of multiple genes that contribute to a single trait.

For example, if there is a gene that controls petal color (blue vs. white, with blue = dominant) and a gene that controls height (short vs. tall, with tall = dominant), then Mendelian inheritance predicts that two short plants with white flowers will only produce short plants with white flowers.

But if there are multiple genes that interact to determine height in a complex way, that's outside the scope of Mendelian inheritance.

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  • $\begingroup$ Let me know if I understand this correctly: so dihybrid crosses in which the genes contribute to different traits are still within the scope of Mendelian inheritance, correct? And so things like epistasis fall outside of the scope? $\endgroup$
    – Aleksandr Hovhannisyan
    Commented Jan 13, 2016 at 1:47
  • $\begingroup$ @AleksandrH: Correct. $\endgroup$
    – ruakh
    Commented Jan 13, 2016 at 1:49
  • $\begingroup$ Okay, and so what about things like Incomplete Dominance and Codominance--are those still a part of Mendelian Inheritance or are they within the separate branch of non-Mendelian inheritance? That was also one of the things that was confusing me $\endgroup$
    – Aleksandr Hovhannisyan
    Commented Jan 13, 2016 at 12:09
  • $\begingroup$ @AleksandrH: Those are non-Mendelian. ("Mendelian inheritance" specifically describes traits that are controlled by a single autosomal gene with exactly two alleles, one of which is dominant and one of which is recessive.) $\endgroup$
    – ruakh
    Commented Jan 13, 2016 at 17:24
  • $\begingroup$ Thanks again for responding. So if I understand this correctly, then non-Mendelian genetics is everything else, like polygenic inheritance, epistasis, incomplete and co-dominance, poly allelic traits, sex-linked traits, mitochondrial inheritance, linked genes, etc? $\endgroup$
    – Aleksandr Hovhannisyan
    Commented Jan 13, 2016 at 19:18

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