Question about explanation of 2007 ADEA Test from Bootcamp

[patricklin27]

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For Question 28, the question is:

Which statement 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.

A. 4
B. 1 and 2
C. 3
D. 1, 2 and 4
E. 3 and 4


Bootcamp gives an explanation: For a given locus or location on a
gene, there are at most two types of alleles, which could either be recessive and
dominant. One chromosome often carries two alleles, either two recessive alleles
(homozygous recessive), two dominant alleles (homozygous dominant), or one
dominant and one recessive allele (heterozygous). This is “usually” true as some
traits may involve multiple alleles, more than just a dominant and a recessive, as is
the case with blood groups. Likewise, the alleles that code for the same gene are
located in identical areas on homologous (related chromosomes). A single
chromosome only contains one copy of an allele.


not sure what the explanation means.. seems a little contradictory but im probably interpreting it wrong. A single chromosme i thought can contain two different alleles (heterozygous.." A" on one sister chromatid and an "a" on the other sister chromatid)?

 

[Ari Rezaei]

It's an important distinction to make. A single chromosome can only contain one allele in a diploid organism. A single chromosome can mean two things:

A single chromatid which is called a chromosome, or one strand of DNA. This only has one copy of 1 allele.

Two sister chromatids together, collectively called a chromosome as well (it can be a little confusing). This will have two copies of the SAME allele.

Think about the single chromatid. When it replicates in the S phase, it'll create an identical copy of itself, including the same exact allele, and become a pair of sister chromatids.

These sister chromatids will line up with their homologous sister chromatid pair during meiosis, and you get a tetrad. These are homologous chromosomes now. They are not the exact same chromosomes anymore, they have different alleles.

I think the explanation should read "One homologous chromosome carries two different alleles..." to be more clear, and specify that a single chromosome refers to a single chromatid or sister chromatids.

 

[sakthivs]

so is the answer for this question B? so "At most only two alleles occur at a given locus in an organisms genome." this statement is true? I thought the explanation said that a given locus can involve more than two alleles such as blood type.

 

[sakthivs]

Oh nvm i think i understand it. So the statement is true because:

while it is true that there may be more than 2 alleles for a trait such as blood, because an organism is diploid only two alleles can occur at a given locus (one allele from the paternal chromosome and the other allele from the maternal homologous chromosome). Is this right?

 

[Ari Rezaei]

It's still true that a diploid organism will only have two alleles in its genome at any one time. But blood groups in humans starts to get tricky. There are multiple alleles, but an individual will only carry two at most, like statement 1 says. There's more information about genetics of blood groups here: http://www.cod.edu/people/faculty/fancher/Bio1100/ABO.htm

 

[Ari Rezaei]

Oh nvm i think i understand it. So the statement is true because:

while it is true that there may be more than 2 alleles for a trait such as blood, because an organism is diploid only two alleles can occur at a given locus (one allele from the paternal chromosome and the other allele from the maternal homologous chromosome). Is this right?

👍 You got it!