a chromosome has two alleles

[385835]

Is this statement true or false:

A single chromosome usually carries two alleles of each gene.



The ADA official practice test says it is, but I think it should be false.

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[Demps]

why do you think it's false?

 

[vnicolas]

.

 

[Dew517]

Allele Wiki:
Diploid organisms have one copy of each gene (and therefore one allele) on each [homologous] chromosome.

One allele per gene per chromosome...?

 

[vnicolas]

.

 

[385835]

This is question 28 on the ADA sample test.

I think one allele from mom and one allele from dad and they are on separate, homologous chromosomes. (or at least this is what I thought)

 

[DrewA]

Is this statement true or false:

A single chromosome usually carries two alleles of each gene.



The ADA official practice test says it is, but I think it should be false.

If you look at it this way, I could see how it could be true. Each chromosome is made up of two chromatids if it's after S phase duplication. Each chromatid carries one allele for each gene (both alleles are the same since it's just an exact copy that was made during the S phase). Since each Chromatid carries one allele, then the chromosome as a whole carries two alleles.

 

[LetsGo2DSchool]

Don't trust little wiki dicki.

An organism has one copy of a gene but there are two alleles for that gene on the chromosome.

 

[Dew517]

Don't trust little wiki dicki.

An organism has one copy of a gene but there are two alleles for that gene on the chromosome.

I don't understand. So let's say a genotype is Tt. During meiosis, you will form gametes of T and t (not together), are you telling me that a single chromosome that is holding the T and t alleles splits apart? During Meiosis the homologous chromosomes split, one to each side, one holding T and the other holding t, not the single chromsome itself. 😕

 

[toothhornet88]

I don't understand. So let's say a genotype is Tt. During meiosis, you will form gametes of T and t (not together), are you telling me that a single chromosome that is holding the T and t alleles splits apart? During Meiosis the homologous chromosomes split, one to each side, one holding T and the other holding t, not the single chromsome itself. 😕

Ok let me put it simply as this: 1 chromosome in your body is comprised of 2 chromatids (1 chromatid from your mom and 1 chromatid from your dad) Since you get one allele from each parent you therefore have 2 alleles for each gene on 1 chromosome.

 

[indigenoustw]

personally, i think it's really a bad question. What does "usually" mean?

If two alleles are on the same chromosome, it means that the alleles are linked, which violates the rule of independent assortment. And we know that it CAN happen but it doesn't mean that it's always the case. How are you suppose to know the frequency of linkage when there not no other information to back it up? This question is as vague as it can be. (although if the stage of meiosis is given, then it probably would be more clear.)

 

[toothhornet88]

I don't understand. So let's say a genotype is Tt. During meiosis, you will form gametes of T and t (not together), are you telling me that a single chromosome that is holding the T and t alleles splits apart? During Meiosis the homologous chromosomes split, one to each side, one holding T and the other holding t, not the single chromsome itself. 😕

Don't forget that those chromosomes split again into chromatids= final product of haploid gametes

 

[ROCOCO8]

Ok let me put it simply as this: 1 chromosome in your body is comprised of 2 chromatids (1 chromatid from your mom and 1 chromatid from your dad) Since you get one allele from each parent you therefore have 2 alleles for each gene on 1 chromosome.

I don't think that's accurate... 1 chromosome is made of 2 sister chromatids, but both of these chromatids are identical (hence why crossing over in Prophase I occurs between non-sister chromatids). I think wikipedia is right, we all have 1 copy of a gene and 2 alleles. There should only be one allele per chromosome. In this way, since we all have 2 chromosomes (one from dad, one from mom) per gene we should have 2 alleles per gene. The two alleles should be on separate homologous chromosomes...

 

[vnicolas]

.

 

[JohnnnyD]

I don't think that's accurate... 1 chromosome is made of 2 sister chromatids, but both of these chromatids are identical (hence why crossing over in Prophase I occurs between non-sister chromatids). I think wikipedia is right, we all have 1 copy of a gene and 2 alleles. There should only be one allele per chromosome. In this way, since we all have 2 chromosomes (one from dad, one from mom) per gene we should have 2 alleles per gene. The two alleles should be on separate homologous chromosomes...

Funny that this came up because i just spent some time 2 days ago investigating this same thing. I think Rococo8 is correct and I actually made a flash card of it to help those that are more visual learners.

Notice that on sister chromatids, the allele on each is the same(since they are identical). On homologous chromosomes, the alleles will be in the same place but they can be different. one of the homologs in my picture is slightly smaller, so just disregard that.

 

[vnicolas]

One chromosome has two different alleles...as your picture shows one chromosomes contain allele A and B.

 

[JohnnnyD]

I am confused now... your picture shows that one chromosome has allele A and B... I know they are different alleles but the picture shows two allele per chromosome ( allele A and B).

Sorry, they are for two different genes. so A could be for the eye color gene, and B could be for the gene that determines something else all together.

didnt mean to confuse anyone, it made sense in my head. each chromosome is gonna have info for way more than one gene

 

[JohnnnyD]

One chromosome does contain two different alleles as your picture shows one chromosome contains alleles A and B.

right. so in your question it states "for each gene."

 

[ROCOCO8]

Funny that this came up because i just spent some time 2 days ago investigating this same thing. I think Rococo8 is correct and I actually made a flash card of it to help those that are more visual learners.

Notice that on sister chromatids, the allele on each is the same(since they are identical). On homologous chromosomes, the alleles will be in the same place but they can be different. one of the homologs in my picture is slightly smaller, so just disregard that.

+1 good stuff.

after thinking about it though i started to think that there are technically 2 "alleles" on one chromosome (in johnnnyD's pic there are 2 identical AA "alleles" that reside on the left chromosome)... but the definition of an allele according to wikipedia is "one or two or more forms of a gene"... since both A and A are the same form of one gene that should mean there's only one allele (A) for that left chromosome... the second allele (a) is on its homologous chromosome (on the right).

nonetheless, it's a poorly worded question.

 

[jwnichols21]

So then the original statement would be true?

 

[vnicolas]

From my understanding you get one chromosome from each parent... you have to remember that one chromosome contains varies genes and therefore varies alleles but for each gene that is on that specific chromosome there is only ONE allele for it.... The above statement is false.