I don’t think anyone knows why most pregnancies, especially very early pregnancies, end. There is very little data available. The whole “50% of fertilizations don’t result in a pregnancy” is mostly just an estimate. No one has a chance to study all those unused fertilized eggs to see what, if anything, is wrong with them.
Right. We can’t study these zygotes and we’ll never know what percentage of pregnancies end in miscarriage. I don’t think its too far of a stretch to say that a bad combination of genes is preventing development in many or most cases.
Well, except that I don’t think there are deleterious recessives that are so commonplace that they would cause that many dead zygotes. And if there were, they would quickly be thinned out of the population and reduced to a very small number.
I mean rare deleterious recessives can hang around for quite awhile, because they rarely get paired up with another recessive. They are usually just silently passed along. But common ones should disappear very quickly after they appear.
And I have pointed out why it is invalid.
Cite! Please provide evidence that more recessive genes are mutated than dominant genes. I have already pointed out that you have no reason to make such a claim.
Cite!. Please provide evdience that recessive genes are more likely to be detrimental than dominant genes.I have already pointed out that you have no reason to make such a claim.
Total non sequtiur.
The problem is that you have no reason to believe that most recessive alleles are egative. This isn’t logical. It is argument by assertion, which is provably illogical.
That’s right. Once again, you have absolutely no reaosn to suspetc that genetics plays any role at all.
:eek: You teach this? For a living?
Seriusly solkoe, at this stage you either need to presnt your evdience ot take it out of GQ. This is the forum for factual answers, not baseless speculation. Show us some facts.
WTF?
This condition is inherited in an autosomal recessive pattern
Autosomal Recessive: Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs Disease
Sickle cell diseases are inherited in an autosomal recessive pattern.
At this stage Dr. Lao I am going to ask you to provide even one reputable reference that suggests that sickle cell is not a recessive trait. I suspect you have failed to understand that recessive does not have to mean simple recessive.
I think that Dr Laos point is that one sickle cell gene and one normal gene leads to malaria resistance. This is different from two normal genes that will not be resistant to malaria. So in this case the sickle cell gene is not completely masked by the normal gene, so it is not entirely correct to call it recesive. It is correct that sickle cell will not develop without two of the sickle cell genes, and in that respect it is recesive.
solkoe - Do you consider blonde hair and blue eyes to be detrimental?
I would go with Dr. Lao on this one. The sickle-cell allele is recessive as far as sickle cell trait goes, but it is not a true recessive in that heterozygotes have a different phenotype with respect to malaria resistance than homozygotes for the non-sickle cell allele. The fact that your cites refer to it as a recessive is due to the simplistic way in which such traits are typically categorized rather than something inherent in the trait.
In actual fact, relatively few alleles are simple recessives in the sense that the phenotype of the “dominant” homozygote is identical to the heterozygote. Many more alleles show “co-dominance” to one degree or another, in that the heterozygote is intermediate between the two homozygotes.
Basic courses in genetics, even at the college level, tend to place far too much emphasis on the kind of clear-cut dominance-recessive relationships studied by Mendel, and give people the impression that most genes work this way. This is in part the source of solkoe’s confusion on the issue. (By the way, solkoe, your understanding of genetics is so rudimentary that your speculations have little justification. I would suggest you do a lot more reading in the subject before you attempt to debate Blake.)
To back up Blake, I quote from Topics in Population Genetics (1968), by Bruce Wallace (with whom I studied population genetics), p. 324:
Wallace goes on to discuss the idea that advantageous alleles will tend to evolve dominance as they become more common.
With regard to the prevalence of deleterious recessives, H. J. Muller, in his classic paper Our Load of Mutations (1950, American Journal of Human Genetics 2:111-176), calculated that on average humans carry a least 8 deleterious recessives in heterozygous condition, and perhaps many more. Note that deleterious is not synonymous with lethal. If this is so, then the number of deleterious recessives is small compared to all recessives. However, I am not up to date on what the current estimates are on these figures.
Perhaps I should also follow up with the point that because most common alleles are advantageous (or else they wouldn’t have become common through selection), most mutations will be either deleterious or neutral (in the case of non-coding and “silent” mutations). However, there is no necessary correlation between mutation and dominance or recessiveness and thus between the latter and advantageousness or disadvantageousness.
I never said that sickle cell was simple recessive or “true recessive” (whatever that may mean). I said it was recessive, which it undoubtedly is. If Dr. Lao made an erroneous assumption that I meant “simple recessive” when I clearly typed “recessive” that is hardly my fault.
Moreover Dr. Lao’s comment that sickle cell is not recessive is factually in error. Sickle cell is recessive as all the references show.
The undisputable facts are as follows:
Sickle cell is a recessive trait.
Sicle cell is is evolutionarily benefical under some circumstances.
The whole reason I brought this up was to illustrate Solkoe’s error in assuming that all recessive traits had to be deleterious. Sickle cell remains a perfect example of a recessive trait that is beneficial. If for some reason someone needs an exmaple of a simple recessive trait that is beneficial i may have to look further afield, but until then it remains as a perfect exmaple.
I think the confusion here is in part in not distinguishing clearly between the “recessive gene (or allele)” and the “recessive trait.” Properly speakly it is traits that are recessive (or dominant), not alleles. Previously you sometimes have not been clear in whether you were speaking of the trait or the allele, as for example here:
I think here in the second sentence you accidentally wrote “gene” when you meant to refer to the form of hemoglobin.
In a number of other cases you have just referred to “sickle cell” without specifying whether you were referring to the allele or to the trait, although in context it seems you have mostly been referring to the sickle cell disease* trait.* On the other hand **Dr. Lao ** has referred to the sickle cell gene, meaning the allele.
You are quite correct that sickle cell disease is a recessive trait. However, malaria resistance is not a recessive trait, being found in the heterozygote but not in the non-sickle-cell allele homozygote. Both these traits are governed by the sickle cell allele, which therefore cannot be said in itself to be recessive.
While I agree with you completely in your main point that recessives are not necessarily deleterious, the way you have phrased the example here is potentially confusing. Whether the sickle cell allele is beneficial or not is entirely due to its genetic environment. In heterozygous condition the allele is beneficial in conferring the trait of malaria resistance; in homozygous condition it is quite deleterious in causing the trait of sickle cell disease. It is notable that the trait that is beneficial is not recessive while the trait that is deleterious is recessive. But the sickle cell allele can’t be considered to be either recessive vs dominant or beneficial vs deleterious in and of itself.
Again, I am not disagreeing with you in general regarding your remarks to solkoe. But clearly distinguishing between alleles and traits would help avoid confusion.
Thanks for enlightening me gentlemen.
I have doubled my understanding of genetics. Clearly, my thoughts about dominant and recessive genes were incorrect.
In my defence, I refer you to this link (from Wiki, it is not allowing me to insert a hyperlink) from which comes this quote.
That being said, I have learned that this type of inheritance is in fact rare and that few of these types of genes exist in a population. The article goes on to say that this mutation is over 50 000 years old and the reason for its persistance is unknown.
In terms of teaching genetics to grade 9 students, I believe that a disease like CF or any other autosomal recessive disease is the best way to express the nature of genes and mutation and the role of meiosis in segragating alleles and the recombination of alleles as part of sexual reproduction. Is there a better way?
Of course, the real story is much more complex and I appreciate all the comments regarding codominance etc.
From Christopher
They can be in the right environment. Why are there no blonde people in tropical areas?
With regard to miscarriages, again from wiki
It does not say what percentage of miscarriages arise from genetics and goes on to list other reasons for miscarriages. Clearly genetics is a major factor.
Recent research suggests that the allele for cystic fibrosis, like that for sickle-cell anemia, may have persisted because heterozygotes are protected from a serious disease, in this case tuberculosis:
Solkoe you seem to be ignoring my quetsions and requests for evidence.
Well thanks for that at least. However you seem to missing a very important point. This thread is about recessive genes, and you introduced failed pregnancies on the assumption that “Two bad genes… is the reason that over 50% of all pregnancies end in miscarriage.” Note that the article never says or even implies anything like that. It says that chromosomal abnormalities are found in a majority of failed pregnancies. Chromosomal abnormalities are things like Downs Syndrome, where an embryo has excess or shortage of chromosomes. Simply having. “Two bad genes” is not a chromosomal abnormality, it is a perfectly normal condition in mammals.
So while we now have evidence that there are genetic factors in a majority of failed pregnancies we still have nor reason to believe your claim that having dual recessive genes plays any role at all.
IMO , yes.
Teaching children genetics using detrimental genetic traits in my experience produces a eugnecist view of human genetics. 14 year olds are very prone to simplistic reasoning leading to them developing the same misunderstanding that you had: That some genes are “bad”. That some people carry these “bad” genes That there once existed an ‘ideal’ human genetic structure that has become corrupted over time to produce people who carry these ‘bad’ genes yet who look perfectly normal.
Far better IMO to concentrate on traits that simply represent diversity rather than on genetic diseases. The nature of genes and the role of meiosis is just as easily expressed by looking at traits like eye colour or earlobes as by looking at genetics disease.
You would be remiss if you didn’t discuss genetic diseases at all, but to use such traits as though they are somehow typical of the nature of genetic assortment or an example of how meiosis works is hardly likely to produce a correct impression the minds of young teenagers. Most genetic diversity is just that : diversity. No traits is either universally detrimental or universally beneficial.
This a point that you yourself have apparently failed to grasp. Yes blonde hair may be detrimental in the tropics, but all traits are like that. No trait in any organism is universally beneficial under all potential environmental conditions. Merely having hair is detrimental under many conditions. By concentrating on genetic diseases you risk transferring this misconception to your students.
That risk is further compounded because you seem to be struggling with the notion that recessive genetic diseases such as Tay-Sachs, Cystic Fibrosis and Sickle Cell exist precisely because they re beneficial under some environmental conditions. If you are educating students about the basics of genetics and chromosomal assortment you will eb throwing them into the deep end by using complex examples of co-dominance and variably beneficial genetic traits like these. You and your students will just be learning that even no trait is universally beneficial. This is not the time to introduce the doubly complex concept that some traits are beneficial when heterozygous but detrimental when heterozygous.
You avoid all these problems you are better off IMO sticking with simple dominant-recessive traits with no obvious beneficial or detrimental roles, with a brief foray into simple genetic diseases such as haemophilia or basic colour blindness. Starting out teaching about diseases like cystic fibrosis is like trying to teach a grade 9 student about electricity by discussing spin pairing in electron shells. It is far more likely to be confusing than instructive.
from Blake
I sent a cite from wiki concerning CF regarding the functionality of proteins. I learned from Colibri that the heterozygous condition could reduce the effect of TB. Again, my original assumption that there are bad genes is obviously not correct. I could list a hundred different autosomal recessive conditions that occurred becaues of altered functionality of proteins (notice I am not writing non functionality). Can I assume in every case there is some benefit to the heterozygous condition (even if that benefit has not been determined)?
Fair enough. When I teach my class about mutations, I usually mention the lead skirt worn during Xrays. Is this more too protect from chromosomal damage in gamates or pont mutations or both?
Also, (nitpick), I have learned (from you and others) that there is no such thing as a recessive gene. The term recessive should apply to phenotypes and not genotypes.
Right. For many conditions this is true. Like blonde hair for example. No argument.
No evidence is correct. It is not possible to know this. No one is able to do genetic tests on miscarriages to find out. I will be reseaching this further.
A key point of our curriculum is the role of the genetic code and the nature of mutations. I have overstated the role of mutation in my lessons but to take the path you suggest does not instill in them the need to avoid exposure to sunlight or cigarette smoke. I emphasize that only mutations in zygotes would carry to the next generation and that mutations in somatic cells could result in cancer.
Correct. I have definitely oversimplified the role of genes and will try to correct myself in the future but, for this age group, I have to keep it simple. The idea of a mutation knocking out the functionality of a gene is something they get. A lot of this thread has dealt with genes in populations not individuals.
I hope I have shown that I have grasped these points.
Both of these are sex linked diseases and are not within the scope of what I can teach. I only have a few days to get my point across. My usual example is diabetes because they all know someone with diabetes and I can explain the role of insulin easily. It might be better to use sickle cell anemia as an example because then I then I can discuss the benefit of the heterozygous condition. The problem is that SCA will be a tough sell as none of them know what it is and I can’t explain how the change in the protein is exhibited.
Your teaching grade 9 students. I think your doing alright. Too much detail would bore them to the point of making anything you teach go in one ear and out the other. It’s good to know the detail though so when you get an exceptional student that asks the correct question you don’t end up BSing yourself into a corner.
No, that wouldn’t be avalid assumption. In the vast majority o cases they would be generally neutral in the same way that having blue eyes in generally neutral.
Both.
Nyeehhhh, sorta. There is certainly such a things as a recessive allele . An allele of course is one form of a gene. So yeah, it’s perfectly correct to refer to a recessive gene although possibly confusing to laypeople because they may not understand that you are referring to a recessive alelle.
Yes, and for conditions like having 5 fingers and toes.
Obviously someone has done genetic tests on faile dpregnacies in order to determine that >50% show chromosomal abnormlaities.
This has nothing whatsoever to do with recesisve genes. Both recessive and dominant genes are equally prone to mutation.
No mutations in the gonads will also carry over to the next generation. Mutations in somatic cells can result in cancer, but >99% of the time do not. Morevover this is totally unrelated to genetic dominance in any way.
Yet you seem to be making it unnecessarily complex by introducing topics such a anucleate cells and genetic diseases.
The problem is that in many cases mutaiuons don’t knock out the functionality, it alters the functionality. I’ve tried to get this across several times now. Especially in the case of cancer it is jut as often enhanced functionality that is the problem.
The probelm is that most diabetes doesn’t have a genetic explanation of the type that you propose. you’re giving the kids a really bad example. You might just as well use blindness or paraplagia as an example since those thinsg are also caused by simple genetic causes in a small minority of cases.
It’s your class, but if the kids can’t gain an understanding of diseases they are unfamliar with then I sure wouldn’t be giving them the impression that diabetes is primarily genetic.
How is Phenylketonurkia a neutral condition? or albinism? I am not trying to be a jerk. You definitely have an interest in this topic. I am definitely stuck on this idea that there are homozygous recessive conditions that are determental. If, as you say, they are few and far between, I can live with that.
An absolutely amazing phenomenon which is worthy of another thread. How is it that the dominant allele is not more prevalent in populations? What is the great disadvantage to having more than five digits?
Right! These are a lot easier to find. Take pictures of chromosomes and arrange them. Nailing point mutations as the source of miscarriages would be a monumental task. It took ten years to sequence one human’s genome. As I wrote, I’m not done with this yet. I have an inside line to a genetics department at a hospital that may be of help.
Either way, a mutation can have a deletrious effect. I need to make that point known.
Enucleated cells are discussed as part of cloning. Cloning is a huge ethical issue that can bring up topics such as “tampering with nature” and “genetic diversity”. Students connect with this and it allows me to get away from the biology of it for a few minutes at least.
Good point. I never thought of cancer in that way.
You’re good! After further research I bow to you. I was convinced that in 4th year I learned that type I diabetes was caused by a defected enzyme in glycolysis. I wonder if I’m old and research has surpassed me.