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When I first encountered homologous genes I thought those genes are identical because the text book said so, and "homologous" meant the same knowledge.

But as I studied along, it comes out that diploid is a two sets of haploid, each from mom and dad.

This means they are not identical, doesn't it? How could they be "identical" since they are from mom and dad seperately?

If they were identical, crossing over during meiosis loses a point(mixing chromatids is meaningless if they are "same").

Then, when the text book says homologous genes are identical, what identical trait is the text book referring to?

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  • $\begingroup$ I wonder which textbook would say that homologous genes are identical. They are not. Homology has to do with common ancestry. $\endgroup$ – Karl Kjer Sep 21 '18 at 23:25
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Can you please directly cite your textbook. Either your textbook is wrong or you massively misinterpret it.

The concept of homology refers to the concept of shared ancestry. From wikipedia

In biology, homology is the existence of shared ancestry between a pair of structures, or genes, in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats, the arms of primates, the front flippers of whales and the forelegs of dogs and horses are all derived from the same ancestral tetrapod structure.

I would add that a common counter example are bats wings and bird wings (which is an analogy).

Two genes are homologous if they both derive from the same ancestral gene. Of course, as ultimately, all genes are related, the concept only make sense by comparison to other structures that are more distantly related or relative to a time point in time.

For more information, just have a look at any good introductory course or textbook to evolutionary biology (for example evo101 > homology)

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The question asks about "homologous genes" but the question body mentions meiosis and chromatids. The latter suggests you are really referring to the concept of "homologous chromosomes". These two terms are used in different contexts in biology to refer to somewhat different concepts (despite the fact that chromosomes contain genes).

Homologous chromosomes are two chromosomes which, although not identical, are highly similar so that they pair up during meiosis and participate in crossing over. If the textbook says they are identical, that is incorrect; they are mostly, but not entirely, identical.

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  • $\begingroup$ I also believe that the OP was referring to homologous chromosomes and not genes since he talks about haploid and diploid. $\endgroup$ – user11230 Sep 22 '18 at 13:31

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