Help with chromosome question.

[Nirvrex]

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So I was going over the test that ada gives out for free on their website. And number 28, says answer D which basically includes the statement that: for a diploid organism

A single chromosome carries two alleles of each gene.


Now I might be mistaken but I thought each individual chromosome has one allele from each parent, and that its the fact that you have 2 homologous chromosomes (one from each) that gives you your two alleles?

If anyone understands this please help me out, its really bothering me.
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[delsol7]

Please correct me if I'm wrong.

You get 1 chromosome from each parents.
Let's say dad gave a child A allele and mom gave a allele. Then child would have Aa allele. This case will be heterozygous allele. But if child got AA or aa then yes, it will be homozygous allele.
Therefore once egg and sperm fertilized (became zygote), a single chormomosome will have 2 alleles which could be homozygous or heterozygous.

 

[Nirvrex]

Im confused.. then are homologous chromosomes in diploid organisms just 2 of the exact same chromosome??

 

[delsol7]

Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same genes at the same loci but possibly different genetic information, called alleles, at those genes. For example, two chromosomes may have genes encoding eye color, but one may code for brown eyes, the other for blue.
Non-homologous chromosomes representing all the biological features of an organism form a set, and the number of sets in a cell is called ploidy. In diploid organisms (most plants and animals), each homologous chromosome is inherited from a different parent. But polyploid organisms have more than two homologous chromosomes.
Homologous chromosomes are similar in length, except for sex chromosomes in several taxa, where the X chromosome is considerably larger than the Y chromosome. These chromosomes share only small regions of homology.

 

[151AND8TH]

please correct me if I am wrong....single chromosome X or [(/) before S phase]..homologous chromosomes XX ..In the S phase chromosome doubles its DNA molecules .. a chromosome with 1 DNA molecule (/) to a chromosomes with 2 DNA molecules (X) .. so the gene that was on (/) is now on (\) forming a new chromosomes (X) with double copies hence the same genetic information. This is in Mitosis.. independant assorment and crossing over would be in Meiosis

 

[TheWiredNerv]

Diploid organisms contain a pair of homologous chromosomes that each contain a different allele. This is how you get your genotype Aa or AA or whatever it happens to be. Each chromosome has 1 allele unless there is some genetic defect. The question ADA gave out seems to be incorrect as only one chromosome carries 1 allele. Unless they are being very ambigious and have considered that a homologous chromosome pair counts as 1 chromosome then I don't see how they came about their answer.

 

[Nirvrex]

yea I totally thought what wired nerv said. Wow that stupid f'in question, I have been trying to figure out how I could be wrong for sooo long. Man thats such a relief, so f'in stressful tho, because I have read soo much on chromosomes to try and figure out what I am missing. Oh well, better late than never. Thanks so much guys

 

[151AND8TH]

After G1 of the cell cycle a single chromosome doubles its DNA molecule or chromatid number.. which means every genes and alleles that was found on that single chromosome is now double-- sister chromatids.. so those sister chromatids on that single chromosome have the same allele and genes at every loci until they get to the end of mitosis wher they again become single chromosomes.. I think that they are reffering to a "carbon copy" of a gene/allele on a chromosome.. so the question is correct and true in all phases of mitosis except for G1.. please correct me if i am wrong

 

[toothhead]

Wait, I'm confused. Isn't the question right? I mean, on a karyotype, for each chromosome, there is one maternal strand and one paternal strand, and an allele on each, so that makes 2 alleles per chromosome. Like this, right?

 

[TheWiredNerv]

I see. Problem with being a biology major: you look way to into a question. lol

 

[Nirvrex]

actually I dont think the karyotype explanation works. Each one of those strands is a chromosome each, thats why their are 46 strands.

When they double they still have only one allele each dont they (I mean two copies of the same allele, but from the question asked, it seems like they are talking about different alleles).