ADA free sample question 28

[JNew]

---

Members don't see this ad.

Which of the following statements
concerning alleles is true for diploid
organisms?
1. At most only two alleles occur at a
given locus in an organisms
genome.
2. Alleles occupy an identical locus in
homologous chromosomes.
3. Alleles of a given gene usually
occur on non-homologous
chromosomes.
4. A single chromosome usually
carries two alleles of each gene.

The answer given is 1,2, and 4 are all correct. But I don't understand why 4 is correct. Doesn't a single chromosome usually carry one allele while the homologous chromosome carries the other?

 

[JNew]

If anyone knows the explanation to this one also, it would be great:
Of the following, which group of
invertebrates is apparently most closely
related to primitive vertebrates?
A. Annelida
B. Mollusca
C. Cnidaria
D. Arthropoda
E. Echinodermata

The answer is E.
(I chose D because it's the only one with bilateral symmetry, characteristic of most vertebrates?)

 

[herkulease]

If anyone knows the explanation to this one also, it would be great:
Of the following, which group of
invertebrates is apparently most closely
related to primitive vertebrates?
A. Annelida
B. Mollusca
C. Cnidaria
D. Arthropoda
E. Echinodermata

The answer is E.
(I chose D because it's the only one with bilateral symmetry, characteristic of most vertebrates?)

A-D can all be classified as protosomes. They form a mouth first. Then an anus later.

Echinodermtata, primitive vertebrates and even advance vertebrates are Deuterosomes. radial cleavage, indeterminate growth. anus 1st mouth later. Gotta poop before you eat.

Oh and your bilateral symmetry train of thought doesn't work. Annelids also have bilateral symmetry.

 

[JNew]

Thanks. Any explanation for the first one?

 

[Baylor2011]

Which of the following statements
concerning alleles is true for diploid
organisms?
1. At most only two alleles occur at a
given locus in an organisms
genome.
2. Alleles occupy an identical locus in
homologous chromosomes.
3. Alleles of a given gene usually
occur on non-homologous
chromosomes.
4. A single chromosome usually
carries two alleles of each gene.

The answer given is 1,2, and 4 are all correct. But I don't understand why 4 is correct. Doesn't a single chromosome usually carry one allele while the homologous chromosome carries the other?

Not in diploid organisms, you're thinking haploid. It's like a backup, if one's screwed up...your body will use the other. Think of it this way...look at the sex chromosomes. Females have two X chromosomes which is basically 2 homologs, similar to autosomal chromosomes having 2 homologs per chromosome. If one of the females X chromosomes (1 homolog) has a problem, the body will select to use the other one, whereas males only have 1 X (1 X homolog) chromosome, so if there's an issue with that one, the individual will display that problem.

So to wrap up: autosomal chromosomes have 2 alleles per homolog..for a total of 4 per chromosome.

Maybe looking at a karotype will help: http://images1.clinicaltools.com/images/gene/karyotype_female.jpg

 

[JNew]

Not in diploid organisms, you're thinking haploid. It's like a backup, if one's screwed up...your body will use the other. Think of it this way...look at the sex chromosomes. Females have two X chromosomes which is basically 2 homologs, similar to autosomal chromosomes having 2 homologs per chromosome. If one of the females X chromosomes (1 homolog) has a problem, the body will select to use the other one, whereas males only have 1 X (1 X homolog) chromosome, so if there's an issue with that one, the individual will display that problem.

So to wrap up: autosomal chromosomes have 2 alleles per homolog..for a total of 4 per chromosome.

Maybe looking at a karotype will help: http://images1.clinicaltools.com/images/gene/karyotype_female.jpg

I still don't understand. You have gene x, with a genotype of Pp. Your father gave you P, and your mother p. Now in everyone of your cells you have 23 pairs of homologous chromosomes. Now with regard to your genotypic makeup you would have P on one of the chromosome pairs and p on the other.
If what you are trying to say it that when the cell copies the DNA to get ready for replication, in which you have 46 chromosomes with 92 chromatids, I understand why then you would have 2 alleles on each chromosome. If that's what the question is asking, how are you supposed to know that we are talking about a time before cell division.?
Otherwise, when each chromosome contains one chromatid, it would seem to me that you should only have one allele per chromosome?

 

[Baylor2011]

I still don't understand. You have gene x, with a genotype of Pp. Your father gave you P, and your mother p. Now in everyone of your cells you have 23 pairs of homologous chromosomes. Now with regard to your genotypic makeup you would have P on one of the chromosome pairs and p on the other.
If what you are trying to say it that when the cell copies the DNA to get ready for replication, in which you have 46 chromosomes with 92 chromatids, I understand why then you would have 2 alleles on each chromosome. If that's what the question is asking, how are you supposed to know that we are talking about a time before cell division.?
Otherwise, when each chromosome contains one chromatid, it would seem to me that you should only have one allele per chromosome?

AH! I think I see how you're thinking. Reread the question. It's talking about diploid (2n) organisms. We as humans are diploid organisms...as well are our somatic cells (body cells), therefore our DIPLOID number (2n) of chromosomes in each cell is 46. Therefore, based on the question there is 2 alleles per chromosome, and you have 2 sets of each chromosome... giving you 4 per cell.

Now if the question were phrased to include GAMETES or HAPLOID...then you know it's talking about 23 chromosomes, giving 2 per chromosome but you only have 1 set, since it's talking about our haploid # of chromosomes .

 

[JNew]

AH! I think I see how you're thinking. Reread the question. It's talking about diploid (2n) organisms. We as humans are diploid organisms...as well are our somatic cells (body cells), therefore our DIPLOID number (2n) of chromosomes in each cell is 46. Therefore, based on the question there is 2 alleles per chromosome, and you have 2 sets of each chromosome... giving you 4 per cell.

Now if the question were phrased to include GAMETES or HAPLOID...then you know it's talking about 23 chromosomes, giving 2 per chromosome but you only have 1 set, since it's talking about our haploid # of chromosomes .

I'm sorry. I still don't think you are getting what i'm asking. Take your Karyotype for example. On it there are 23 pairs (2n) of non-replicated (46 chromosomes and 46 chromatids). Now all I want to know is if on every one of those single chromosomes (not the pairs of chromosomes just on every single one of the 46 chromosomes) are there 2 alleles for a gene. That is as I said before, with a trait that's Pp, are you telling me that on chromosome (let's call the homologous pair 1, and the chromosomes 1a and 1b) 1a there are two alleles for P, and on 1b there are two alleles for p. As far as I understood on a non-replicated individual chromosome there should be one allele and another on it's homologous pair.
I hope I am being clear what I am asking. Thanks for taking the time to respond.

 

[Baylor2011]

Based on that karyotype, which is NOT diploid...sorry, per each chromosome, as you labeled them 1a and 1b, there is one allele per each one...which gives you 2 per chromosome. However, since that is only a haploid karyotype , you have to multiply it by 2n to get 4.

Here, this is a correct diploid karyotype for a human.

http://www.colorado.edu/intphys/Class/IPHY3730/image/figure19n.jpg

So based on how you described it before, if you break them apart.. like look at chromosome 1, there's 2 pairs...so you can break it up as you did before, 1a 1b 1c 1d, each one having ONE allele, for a total of four.

Sorry for the confusion

In the other one, there were no homologous pairs of chromosomes, just one chromosome and two chromatids...here I'll do some MS Paint work.

OK, the picture which says "Human Karyotype" is a diploid karyotype, and I've put arrows indicating each "chromosome", and since there are two of them, two arrows...that is a homologous pair. In the other picture, that is a haploid karyotype, and i put a bracket around ONE chromosome. Notice, there is no homolog to it, therefore it's showing haploid...or what our gametes would look like. All our somatic cells are diploid, as the karyotype which says "Human Karyotype" shows. Since there is a pair of chromosomes (homologous pair), each chromatid has one allele, and you have 4 chromatids since this is a diploid organism, giving you a total of four alleles per locus.

Hope that clears everything up...should have noticed the first karyotype wasnt diploid, sorry!

 

[JNew]

Based on that karyotype, which is NOT diploid...sorry, per each chromosome, as you labeled them 1a and 1b, there is one allele per each one...which gives you 2 per chromosome. However, since that is only a haploid karyotype , you have to multiply it by 2n to get 4.

Here, this is a correct diploid karyotype for a human.

http://www.colorado.edu/intphys/Class/IPHY3730/image/figure19n.jpg

So based on how you described it before, if you break them apart.. like look at chromosome 1, there's 2 pairs...so you can break it up as you did before, 1a 1b 1c 1d, each one having ONE allele, for a total of four.

Sorry for the confusion

In the other one, there were no homologous pairs of chromosomes, just one chromosome and two chromatids...here I'll do some MS Paint work.

OK, the picture which says "Human Karyotype" is a diploid karyotype, and I've put arrows indicating each "chromosome", and since there are two of them, two arrows...that is a homologous pair. In the other picture, that is a haploid karyotype, and i put a bracket around ONE chromosome. Notice, there is no homolog to it, therefore it's showing haploid...or what our gametes would look like. All our somatic cells are diploid, as the karyotype which says "Human Karyotype" shows. Since there is a pair of chromosomes (homologous pair), each chromatid has one allele, and you have 4 chromatids since this is a diploid organism, giving you a total of four alleles per locus.

Hope that clears everything up...should have noticed the first karyotype wasnt diploid, sorry!

Appreciate the explanation. Just one last question. When human cells are not undergoing mitosis or meiosis, let's call it a resting stage, do the cells contain 23 homologous pairs, with 46 chromatids, or 92 chromatids. If we are going to say that they contain 46 chromatids, than as we were saying before, there is only a possibility of 1a, or 1b.
To sum up what i'm asking, when the question states:
"A single chromosome usually
carries two alleles of each gene."
We are just supposed to know that "usually" is referring to when the chromosomes are getting ready for cell division (46 chromosomes with 92 chromatids) and therefore at that point they contain 2 copies of allele/chromosome, with 4 alleles per homologous pair?

 

[Baylor2011]

Here...I'm going to paste the question and the answer choice in question.

Which of the following statements
concerning alleles is true for diploid
organisms?

D. A single chromosome usually
carries two alleles of each gene.

Since it's talking about diploid organisms, I've already got in my mind 46 chromosomes, 92 chromatids. Then I go on to read the answer choice D

A single chromosome (2 chromatids) usually carries two alleles of each gene, which is correct. Each chromatid has one allele, for a total of two. Now if it asks how many alleles per PAIR or per homologous chromosomes, it means to include the second diploid pair which is a total of 2 chromosomes, 4 chromatids...and one allele per chromatid so there'd be 4.

So to answer the question regarding meiosis (sexual reproduction, in which gametes are formed)...the cell goes from 46 chromosomes (92 chromatids) to 23 chromosomes (46 chromatids) in the haploid gametes.

To recap..since the question is regarding a diploid organism it's talking about 46 chromosomes and 92 chromatids. Then it goes on even further talking about one single chromosome (two chromatids) which have one allele per chromatid, for a total of 2 per chromosome.

Hope I've helped!

 

[UCB05]

We do NOT have 2 alleles per chromosome. That defies all of what we know of diploid genetics (well, maybe not all). We have one allele from one chromosome that we get from our father, one allele from one chromosome that we get from our mother. Statement 4 contradicts statement 1 for a cell in G0. If a diploid organism carries 2 alleles at a given locus, there must be 1 allele in each chromosome. The only time you can say there are 2 alleles per chromosome is during mitosis/meiosis, from S phase up to Metaphase, when the chromosome has replicated into 4N, thus having sister chromatids joined together. Once anaphase occurs and sister chromatids separate, they are distinct chromosomes and once again have one allele per chromosome. This persists until they are once again replicated.

Given that, if "usually" means a cell preparing to replicate, then statement 4 is true (which i would still debate because they're the same allele, not two different alleles on the one chromosome). If "usually" means a cell in G1 or G0, statement 4 is false. There have been errors on the ada sample test caught in the past. This could be one of them.

 

[Baylor2011]

Well crap. I stand corrected.