During meiosis, homologous chromosomes synapse and form tetrads. In a homologous pair, one of the chromosomes comes from the mother (maternal homologue) and one comes from the father (paternal homologue). Since replication occurs before meiosis, each tetrad consists of four chromatids (tetra means four). The chromatids of the same homologue are called sister chromatids, and the chromatids of different homologues are called non-sister chromatids. If the maternal homologue carries allele A and the paternal homologue carries allele a for a certain gene, which of the following will have different alleles for this gene?

Select one:

Sister chromatids

Non-sister chromatids

 None of the chromatids, since each chromatid in a tetrad contains the same allele

 All four chromatids, since each chromatid in a tetrad has a different allele

4 years ago

Answered By Micah Y

Let's knock off the obvious incorrect answers right away. It's not "None of the chromatids" because the maternal and paternal homologues are different. Likewise it's not "All four chromatids" since two carry the maternal allele and two carry the paternal allele.

What we have left are the sister chromatids and the non-sister chromatids. You can probably guess already that it's going to be the non-sister chromatids just because of the name, but here's why - sister chromatids come from the same original strand of DNA. Non-sister chromatids have the same genes (like they both encode for eye colour for instance), but different alleles (one encodes for green eyes and the other for brown). 

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