Linked Genes- Cliffs Bio Question

[Incis0r]

Hey there!

So, in Cliffs AP bio, the following is written:

"In the fruit fly Drosophila melanogaster, flies reared in the laboratory occasionally exhibit mutations in their genes. Two such mutations, affecting body color and wing structure, are linked. The normal, or wild, body color is gray (B), while the mutant allele is expressed as black (b). The second mutation, for wing structure, results in vestigial wings (v) (small, underdeveloped, and nonfunctional). (Note that for Drosophila mutations, the gene notation uses letters that denote the name for the mutation.) Since these two genes are linked, a fly heterozygous for a gray body and normal wings (called

gray-normal), indicated by BbVv, would have the BV on one chromosome and the bv on the homologous chromosome...the gray- normal fly produces only two kinds of gametes, BV and bv. Bv and bV gametes are not produced. "

Here is my source of confusion. Why is it necessary that the BbVv fly have one chromosome with BV and the other with bv. Why can't it be Bv and bV? Let me be clear- I understand that if the Parent is BV and bv, then it can only produce gametes that are BV and bv. What I don't understand is why in the first place must the parent be BV and bv.

The way the text is written....it implies that if you have a mutation on one gene, then you MUST have a mutation on the other gene right next to it. Isn't it possible to have a gray (B) fly that had a mutation in its wing structure (v)?

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[DrNutmeg]

I think it's just the definition of a linked gene. It is saying there are only two possible phenotypes: gray + normal, and black + bad wings. How can this be explained?

If a mother is BbVv, she can either pass BV or bv to her offspring. This is also true of the father. Thus, the only possible genotypes are BBVV, BbVv, and bbvv (the mutation). This is the only way that having 1 mutation and not the other is impossible.

The scientists know that the gene is passed this way because no flies are observed with a black body + good wings, or bad wings + gray body; this is how they explain it.

 

[DAT Destroyer]

Hey there!

So, in Cliffs AP bio, the following is written:

"In the fruit fly Drosophila melanogaster, flies reared in the laboratory occasionally exhibit mutations in their genes. Two such mutations, affecting body color and wing structure, are linked. The normal, or wild, body color is gray (B), while the mutant allele is expressed as black (b). The second mutation, for wing structure, results in vestigial wings (v) (small, underdeveloped, and nonfunctional). (Note that for Drosophila mutations, the gene notation uses letters that denote the name for the mutation.) Since these two genes are linked, a fly heterozygous for a gray body and normal wings (called

gray-normal), indicated by BbVv, would have the BV on one chromosome and the bv on the homologous chromosome...the gray- normal fly produces only two kinds of gametes, BV and bv. Bv and bV gametes are not produced. "

Here is my source of confusion. Why is it necessary that the BbVv fly have one chromosome with BV and the other with bv. Why can't it be Bv and bV? Let me be clear- I understand that if the Parent is BV and bv, then it can only produce gametes that are BV and bv. What I don't understand is why in the first place must the parent be BV and bv.

The way the text is written....it implies that if you have a mutation on one gene, then you MUST have a mutation on the other gene right next to it. Isn't it possible to have a gray (B) fly that had a mutation in its wing structure (v)?

The most important thing here is that genes are linked. That means, no crossover between them.

The book says: "Two such mutations, affecting body color and wing structure, are linked"

So, if b and v mutations are present, since they are linked, they will only be inherited together, since the distance between them on the chromosome is short.

It is possible to have a mutation only in wing structure and have a Bv genotype, but only as a result of mutation, fly cannot produce these kind of gametes during the crossover event.

Hope it helps.

 

[Incis0r]

The scientists know that the gene is passed this way because no flies are observed with a black body + good wings, or bad wings + gray body; this is how they explain it.

See now if the authors of Cliffs included this sentence in the paragraph, things would be so much better.

It is possible to have a mutation only in wing structure and have a Bv genotype, but only as a result of mutation, fly cannot produce these kind of gametes during the crossover event.

This. Exactly- so you CAN technically have a Bv just from mutation and not from crossover. So you could theoretically have a fly that is BBVv with a BV on one chromosome and a Bv on the other if one of its parents (the Bv) had a mutation in the gene for v.

That's what was confusing me...was how they implied that pretty much every BbVv must be BV and bv. Thank you for helping!!

 

[510586]

See now if the authors of Cliffs included this sentence in the paragraph, things would be so much better.



This. Exactly- so you CAN technically have a Bv just from mutation and not from crossover. So you could theoretically have a fly that is BBVv with a BV on one chromosome and a Bv on the other if one of its parents (the Bv) had a mutation in the gene for v.

That's what was confusing me...was how they implied that pretty much every BbVv must be BV and bv. Thank you for helping!!

I always thought that linked genes that have a crossover frequency of 18% meant that 18% of the time they have the alleles that are not linked end up together as a result of recombination. Is this not the case?

 

[Incis0r]

I always thought that linked genes that have a crossover frequency of 18% meant that 18% of the time they have the alleles that are not linked end up together as a result of recombination. Is this not the case?

That is the case you are correct.
Here we were just talking about the basic premise of linkage and how it compares to independent assortment.

 

[510586]

That is the case you are correct.
Here we were just talking about the basic premise of linkage and how it compares to independent assortment.

Oh I think I see it now. Yall were saying that a mutation could cause a Bv not only recombination right? Is that the jist of it?

 

[Incis0r]

Oh I think I see it now. Yall were saying that a mutation could cause a Bv not only recombination right? Is that the jist of it?

Yep- Bv by recombination would be highly unlikely since the genes are linked so close to each other (so if you tried to replace V with v you'd also replace B with b). I was asking if it would be possible to create Bv just by mutation. No recombination- and the answer to that was yes.

The reason I asked was that the Cliffs book stated that heterozygous BbVv MUST come from BV and bv gametes. This made no sense to me since you could also have a Bv and bV set of gametes mix to make the exact same genotype.