Question 28 from ADA practice

[haagendazs]

In the ADA sample items given here http://www.ada.org/prof/ed/testing/dat/dat_test_sampleitems.pdf , I was wondering if #28 is correct. I'm specifically talking about (4) A single chromosome usually carries two alleles of each gene.

I thought diploid organisms have 23 pairs of chromosomes, which means 46 chromosomes. Within each pair, one *chromosome* carries one allele of a gene, and its homologous chromosome carries the other. So how is it true that a single chromosome can carry 2 alleles of the same gene?

Someone please explain it to me, I'm confused

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[Svart Aske]

Remember that a chromosome is made of a pair of sister chromatids (each of which contains a copy of the allele). During meiosis, the chromatids separate, and during S phase DNA replication restores the chromatid pair (resulting in 2 alleles).

 

[haagendazs]

Ah I see what you mean. So then is it true that during anaphase, telophase, cytokinesis, G1 phase, and the first half of S phase, there is 1 allele per chromosome. And during the second half of S phase, G2 phase, prophase, and metaphase, there are 2 alleles per chromosomes due to replication?

If that's the case, then doesn't the chromosome have 1 allele half the time and 2 alleles the other half? The above 2 options are approximately halfway in terms of time. And then there are the cells that are in G0 phase, and they clearly have 1 allele per chromosome.

Thanks for the clarification Svart Aske, but I'm still a bit confused...

 

[Svart Aske]

Hmm, you're right that the chromosome will not always have two identical alleles (the question did say "usually"). But I guess because the standard representation of a chromosome consists of two chromatids, that choice would be correct.

 

[haagendazs]

Ah that's probably it. Thanks Svart Aske!