Genetics: Mutation and Complementation

[Beth N]

Homework Statement

Question 2[/B]
You mutagenize male flies with EMS. Next you mate
an EMS-treated male fly to a wild-type female fly.

(i) If you find an F1 offspring with a sleep-too-little
mutant phenotype, what does that suggest to you about this mutation?
(ii) You expect that majority of the F1 offspring will not have the sleep-too-little mutant
phenotype. Explain why.

Homework Equations

Mendelian Genetics
Complementation test cross
Logic

The Attempt at a Solution

I don't understand the answer key given for these two questions The part I'm confused with is highlighted in yellow.

1. I'm not sure whether the mutation on the Y chromosome has to be recessive or dominant. If the mutation is Y-linked recessive, then we can't be sure that F1 progeny would have the phenotype right? If so, why is dominance/recessive not specified here.
2. How do we know that the majority of mutations from EMS will be recessive? What actually causes a gene to be dominant or recessive?

Thank you very much for your help!

 

[epenguin]

1. I'm not sure whether the mutation on the Y chromosome has to be recessive or dominant. If the mutation is Y-linked recessive, then we can't be sure that F1 progeny would have the phenotype right? If so, why is dominance/recessive not specified here.

For your first question I would say the question dominance/ recessiveness does not arise, or rather it arises only for females, not males.

Dominance means that when two chromosomes are present together it suffices for one so-called dominant character to be present on one of the two chromosomes for that character to be expressed as phenotype.

In the case of sex linked linked characters in the male (in mammals and drosophila) only one chromosome is ever present, so the character that this chromosome carries is the one that is expressed.

 

[epenguin]

For your second question, it doesn't cover everything but take the case that the gene corresponds to a stretch of DNA that codes for a protein, e.g. an enzyme of a metabolic pathway or something like that. (I am taking it that you have some familiarity with basic functioning of genes). Then as long as one of these genes is 'normal', cells will make the enzyme and that will be enough, it will be compatible with life and in fact cause no obvious difference from normal in the phenotype of the organism bearing the mutation. On the other hand when through back crossing or breeding the F1 generation you get some progeny with the mutation in both chromosomes, then that will not be able to make the proper protein, and you have a phenotypcally defective organism. Take these two things together and you have the characteristics of what is called recessiveness.

Having said that, you can see that in many cases the double dose of the mutation would be lethal. So for many characters the mutations that we will find will have to be not too, too damaging, e.g. a poorly functioning protein rather than totally unfunctional one – would have to be compatible with life. Thus in the case of human mutations we find (the major theme of medical genetics) as a result of double dose (homozygousity for the traat) poor functioning, a diseased state and short life unless a remedy or cure can be found. In the case of microorganisms we can pick a lot of interesting mutations up because although they would be lethal in the absence of a 'dietary supplement', we can supply a downstream metabolite supplement. So these mutants will not grow in the absence of this supplement but they will in its presence.

Hope this helps.

 

[jim mcnamara]

@epenguin
AFAIK
Drosophila sex is determined by the ration of X:A where A represents autosomes (as in Drosophila melanogaster). So I'm not sure what or where this question came from. But I will admit to be being completely confused. Some normal females have a Y chromosome or even two of them.

See the chart here to get an idea about allosomes (sex chromosomes) and autosomes in fruit flies:

From wikipedia, see:
https://en.wikipedia.org/wiki/Drosophila_melanogaster#Sex_determination

 

[Beth N]

For your first question I would say the question dominance/ recessiveness does not arise, or rather it arises only for females, not males.

Dominance means that when two chromosomes are present together it suffices for one so-called dominant character to be present on one of the two chromosomes for that character to be expressed as phenotype.

In the case of sex linked linked characters in the male (in mammals and drosophila) only one chromosome is ever present, so the character that this chromosome carries is the one that is expressed.

Thank you for answering! I wonder what if the Y chromosome carries a recessive gene, and the X chromosome of that same male individual carries a dominant gene, then wouldn't the trait on X chromosome be expressed instead of Y? What about Xdom.Ydom.? Xrec.Ydom.? In the context of my class we usually talk about X linked recessive traits, but I don't recall talking about Y having any trait at all.

 

[Beth N]

@epenguin
AFAIK
Drosophila sex is determined by the ration of X:A where A represents autosomes (as in Drosophila melanogaster). So I'm not sure what or where this question came from. But I will admit to be being completely confused. Some normal females have a Y chromosome or even two of them.

See the chart here to get an idea about allosomes (sex chromosomes) and autosomes in fruit flies:

View attachment 238295

From wikipedia, see:
https://en.wikipedia.org/wiki/Drosophila_melanogaster#Sex_determination

Thank you for answering! That is good to know. I believe we are just making this assumption in the context of my class.

 

[epenguin]

Thank you for answering! I wonder what if the Y chromosome carries a recessive gene, and the X chromosome of that same male individual carries a dominant gene, then wouldn't the trait on X chromosome be expressed instead of Y? What about Xdom.Ydom.? Xrec.Ydom.? In the context of my class we usually talk about X linked recessive traits, but I don't recall talking about Y having any trait at all.

Your last sentence partly answers your question. The Y chromosome, at least in mammals, carries very few functional genes (that's why are you don't recall any traits) it is essentially not homologous to the X chromosome. So in the male the question dominance/recessivity does not arise.Inheritance is without the complication of diploidy. It is simply similarly simple in haploid organisms such as the fission yeast Schizoaccharomyces pombe. (You have to say haploid most of the time or during most of their lifecycles.) With S. pombe you will get colonies of essentially identical cells derived from a single ancestor. And they either all do or all do not have a functioning enzyme in e.g. a given amino acid synthetic pathway pathway, and as a result they are all able or all not able to grow on the medium not containing that amino acid. So studying them has one complication less than diploid organisms. Coming back to diploids like humans, yes in the female you have the dominant/recessive issue. The harmful mutations are mostly recessive for reasons we already mentioned, hence probably why you have heard mention X-linked recessive traits. In humans a number of well known disease syndromes occur almost only males for this reason. (Daltonism is a mild example).

 

[epenguin]

https://www.physicsforums.com/members/jim-mcnamara.35824/

Re Jim Macnamara. Yes I remembered that drosophila was a bit different from cases I remembered better like humans! I didn't really remember, was it that the Y had the function of the X or something? I did check it out before answering, and the answers I found were that the system is formally much like humans. https://biocyclopedia.com/index/gen..._limited_traits/sex_linkage_in_drosophila.php https://www.ncbi.nlm.nih.gov/books/NBK22079/

So maybe this is just an approximation which is only true against a constant background of the other factors you mention (and maybe these were constant when Morgan first worked it out).

Anyway I found what was bugging me. It was that although the formal rules were the same in man and drosophila, this is almost an accident - the mechanisms that explain them are quite different.

The thing is that these rules seem to be taught as though they were universal. At least I was under the impression for a long time tht they were. Whereas the mechanisms of sex determination are quite different across organisms. They are already different in birds from in mammals! And if I am not mistaken there are differences between placental mammals and marsupials.

Also I might say that although the XX female with one of the genes defective ma ybe healthy because she expresses the healthy gene, not all of her cells are necessarily doing so, maybe only half of them, but that is enough. One of the X chromosomes is inactivated, but not the same one in all cells. A well-known and striking manifestation of this "mosaicism" is in tortoiseshell cats.

 

[Beth N]

Your last sentence partly answers your question. The Y chromosome, at least in mammals, carries very few functional genes (that's why are you don't recall any traits) it is essentially not homologous to the X chromosome. So in the male the question dominance/recessivity does not arise.Inheritance is without the complication of diploidy. It is simply similarly simple in haploid organisms such as the fission yeast Schizoaccharomyces pombe. (You have to say haploid most of the time or during most of their lifecycles.) With S. pombe you will get colonies of essentially identical cells derived from a single ancestor. And they either all do or all do not have a functioning enzyme in e.g. a given amino acid synthetic pathway pathway, and as a result they are all able or all not able to grow on the medium not containing that amino acid. So studying them has one complication less than diploid organisms. Coming back to diploids like humans, yes in the female you have the dominant/recessive issue. The harmful mutations are mostly recessive for reasons we already mentioned, hence probably why you have heard mention X-linked recessive traits. In humans a number of well known disease syndromes occur almost only males for this reason. (Daltonism is a mild example).

Thank you!