ADA Sample Test-bio #28

[mocha13]

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Hi! The question 28 on ADA sample test asks, "Which statement concerning alleles is true for diploid organisms?" The answers include this statement, "A single chromosome usually carries two alleles of each gene." Isn't this false? I thought a single chromosome carries one allele of each gene, and a pair of homologous chromosomes carries two alleles of each gene. My guess is that this "single chromosome" refers to sister chromatids with one allele on each, but I am not sure. Can anyone explain to me please? 😀DD Thank you very much!

 

[UndergradGuy7]

I agree with the part that you said two chromatids = 1 chromosome. So 1 allele on each chromatid.

It is 1 chromosome because it is still connected. It is easier to count the number of chromosomes by the number of centromeres.

 

[KillaGorilla]

I was glancing through this test and got stuck at the same question . Wouldn't sister chromatids only be an acceptable answer after the S sub-phase of Interphase. This is not the standard configuration of a chromosome so "usually" should not apply.
I hope the question quality has improved since 2007.

 

[amalgamgrillz]

I think you might be thinking a little too deep. When a zygote forms, 23 chromosomes come from the egg, and 23 chromosomes come from the sper
. From that, both will carry 1 of the alleles..so you'll have 2 copies of each allele. I.e) mom gives you 'A', dad gives you 'a'..for a combination of Aa allele pairs for the A chromosome. I could be way off but that's how I understand it.

 

[KillaGorilla]

I think you might be thinking a little too deep. When a zygote forms, 23 chromosomes come from the egg, and 23 chromosomes come from the sper
. From that, both will carry 1 of the alleles..so you'll have 2 copies of each allele. I.e) mom gives you 'A', dad gives you 'a'..for a combination of Aa allele pairs for the A chromosome. I could be way off but that's how I understand it.

What you are saying is accurate, but the question is referring to having two alleles on one chromosome. It's true I may have been thinking to deep, but it was ambiguity in the answer choice that bothered me. I was mostly blowing off steam after getting stuck on a question at 4am.