Alleles, Genes and Chromosomes, oh my!

[SkepticalScience]

So, I am reading through Talk Origins “Introduction to Evolutionary Biology” paper, and I am having trouble visualizing something. I was hoping someone would be kind enough to explain it to me.

The paragraph reads: “Genetic variation has two components: allelic diversity and non- random associations of alleles. Alleles are different versions of the same gene. For example, humans can have A, B or O alleles that determine one aspect of their blood type. Most animals, including humans, are diploid -- they contain two alleles for every gene at every locus, one inherited from their mother and one inherited from their father. Locus is the location of a gene on a chromosome. Humans can be AA, AB, AO, BB, BO or OO at the blood group locus. If the two alleles at a locus are the same type (for instance two A alleles) the individual would be called homozygous. An individual with two different alleles at a locus (for example, an AB individual) is called heterozygous. At any locus there can be many different alleles in a population, more alleles than any single organism can possess. For example, no single human can have an A, B and an O allele.”

What I am having trouble understandings, is where all these things are and how they interrelate.

This paragraph suggests to me, that we have these units of heredity called genes. But we can have different VERSIONS of those genes, called alleles.

But then, look at this sentence: “Most animals, including humans, are diploid -- they contain two alleles for every gene at every locus, one inherited from their mother and one inherited from their father.” This sentence doesn’t really make sense to me. It makes it seem that alleles are part of a gene, not a version of a gene. I think I need a metaphor to clear this all up.

Also, I still don’t know where all these things located? Inside the DNA? So, if I look through a microscope, where would the genes, chromosomes, alleles, be? Are they all inside the DNA?

Apologies in advance if these are dumb questions, but I think if I had the answers to them, I would get more out of the material I am readings.

Thanks!
SS

[Katachresis]

I would send you to:
http://en.wikipedia.org/wiki/Gene
and more specifically to:
http://en.wikipedia.org/wiki/Allele

I'm afraid I cannot answer your question, but I hope that helps.

[TobiasTheViking]

The way i read it is that we have the code for all three alleles in our DNA, but only two of them are active.

Thus, your DNA will have the code for all three. But [some organ that produces the alleles and pumps them into the blood] can only produce two of them.

One of them is just not active.

If you get two genes from your parents [black hair/blonde hair], you will have both genes, but only one of them will be active. This will also go on to your children, which means that you can get a child with red hair, even though no one in your family have had red hair for 4 generations or more, and it is still from your DNA.

Or i may be completely wrong. I'm just taking a guess based on what i know and how i read your quote's. But i think it sounds plausible.

Sincerely
Tobias.

[TobiasTheViking]

Asked for verification from someone who should know, she agreed with my statement.

All three are in the DNA, only two are being produced in [the organ].

[SkepticalScience]

Thanks a lot Ketyk/Tobias.

I will look through the links Ketyk.

Tobias, I think you opened up a can of worms with me now. So, in our DNA we have these codes that correspond to physical characteristics. Of all the codes we have, only one of them are active.

So, my questions would be, how many codes can we have in maximum for a particular attribute. I see various shades of color in peoples hair. Is there a code for each particular shade? And what is the process that determines which codes are active, and which are inactive?

I've also never heard of an allele pumping organ. What is this thing? I thought our marrow just makes blood, according to the DNA recipie it has built into it. Is this understanding wrong?

Thanks,
SS

[TobiasTheViking]

Originally Posted by SkepticalScience

Thanks a lot Ketyk/Tobias.

I will look through the links Ketyk.

Tobias, I think you opened up a can of worms with me now. So, in our DNA we have these codes that correspond to physical characteristics. Of all the codes we have, only one of them are active.

Actually, two of them are active.

Try this analogy. The female egg contains two X chromosones(afaik). The male sperm contains X and Y chromosones. Yet you (usually) only get either XX or XY. The rest are just passive.

Quote:

So, my questions would be, how many codes can we have in maximum for a particular attribute. I see various shades of color in peoples hair. Is there a code for each particular shade? And what is the process that determines which codes are active, and which are inactive?

I don't think we know of any maximum in general, for blood type there are only three. For hair colour there are probably many places in the DNA it is defined. And not just one.
As for the process that determines which is the active ones. In school i was told that one of them is a stronger trait than the other, and the stronger one wins(what defines stronger and weaker i don't know)

Quote:

I've also never heard of an allele pumping organ. What is this thing? I thought our marrow just makes blood, according to the DNA recipie it has built into it. Is this understanding wrong?

I believe that allele pumping organ would be the heart. Since the allele is, afaik, in the blood.

Again, i'm not sure if this is accurate, not really working with this, but i believe the bone marrow contains a specialized version of the DNA, that only contains just what it needs. Hence we have stem cells, cells that haven't been speciallized yet.

Quote:

Thanks,
SS

np, and remember, i'm not 100% sure about this, i'm just working from guesses and the best my memory can remember.

Sincerely
Tobias.

[Capsid]

Alleles are genes which are DNA inside the nucleus of a cell. They are not present in the blood and there is not an allele pumping organ. The genes transcribe proteins such as blood group proteins.

Remember that we inherit only one of the pairs of chromosomes from our parents. Sperm and egg are haploid containing only half of the chromosme pairs after cell division (meiosis). When the egg and sperm fuse the full chromosome pair set is returned. So for blood groups, although there are A, B and O genes we will only inherit two. So from your parents say who were AB and AO then you could have a blood group of either AA, AO, AB or BO. Look up Mendelian genetics.

[athon]

Originally Posted by SkepticalScience

The paragraph reads: “Genetic variation has two components: allelic diversity and non- random associations of alleles. Alleles are different versions of the same gene. For example, humans can have A, B or O alleles that determine one aspect of their blood type. Most animals, including humans, are diploid -- they contain two alleles for every gene at every locus, one inherited from their mother and one inherited from their father. Locus is the location of a gene on a chromosome. Humans can be AA, AB, AO, BB, BO or OO at the blood group locus. If the two alleles at a locus are the same type (for instance two A alleles) the individual would be called homozygous. An individual with two different alleles at a locus (for example, an AB individual) is called heterozygous. At any locus there can be many different alleles in a population, more alleles than any single organism can possess. For example, no single human can have an A, B and an O allele.”

What I am having trouble understandings, is where all these things are and how they interrelate.

Think of it this way;

A chromosome is a string of genes. You have 23 chromosomes, and each of those has a partner. 23 pairs, 46 chromosomes all up.

If I take my chromosome number 1, and your chromosome number 1, I can expect that the order of the genes will be pretty much the same. So, if I have a gene for 'green pigment' three genes down the line on my chromosome number 1, I can expect to find your gene for 'green pigment' there as well. This 'address system' is called a locus.

Now, when you make sex cells, one of each of the pair are separated from their partner into another cell. Hence, all sex cells have 23 chromosomes (one half of all the pairs). Making a baby means mixing together new pairs (hence sexual reproduction is essentially about increasing variation through recombining chromosome pairs combinations).

Here's the clincher; genes can change. They break, and when they get fixed they sometimes don't get fixed properly. So imagine on one of our pair number one chromosomes, at locus 3 (our green pigment), something broke and the gene doesn't work anymore. It's still a gene, but it won't make green pigment. It's cool because you still have a working copy...but if you pass this on to yur offspring, and they get a broken gene from their mother, then they have no working 'green' gene.

Both genes are responsible for the same thing, but one is broken. Hence, referring to the variation we say they are 'genes for green pigment', with the working one an example of one allele (we call it dominant, because its effect dominates the broken gene's) and the broken one an example of another allele (called recessive).

This is (of course) an incredibly simplified explanation, but hopefully helps a little.

Athon

[Paul C. Anagnostopoulos]

An egg contains one Y chromosome. A sperm contains either an X or Y chromosome. The resulting fertilized egg contains XX or XY. Occasionally something goes haywire.

~~ Paul

[Hawk one]

Damn. By the time I'd written my explanation, people had already written even more. Good thing I refreshed the page before submitting it.

Oh, and just a final tip: Buy and read Richard Dawkins' book "The Selfish Gene". Using layman's language, he spends the two first chapters of the book going into somewhat deeper details than f.ex. Paul () here to explain what genes and chromosomes and alleles are. And the rest of the book's very good as well.

[Paul C. Anagnostopoulos]

Originally Posted by SkepticalScience

But then, look at this sentence: “Most animals, including humans, are diploid -- they contain two alleles for every gene at every locus, one inherited from their mother and one inherited from their father.” This sentence doesn’t really make sense to me. It makes it seem that alleles are part of a gene, not a version of a gene. I think I need a metaphor to clear this all up.

Remember, you have two copies of every chromosome, one from your mother and one from your father. Pick a gene on that chromosome pair. There are two copies of it, one on each chromosome. They may be the same allele (homozygous) or they may be different (heterozygous).

To complicate matters, some genes are repeated in different places on the genome. Assuming multiple copies are active, you could have more than two alleles.

To further complicate matters, occasionally the fetus ends up with three copies of a chromosome. Down Syndrome is caused by three copies of chromosome 21.

~~ Paul

[Capsid]

A metaphor for you is that you go to the supermarket to buy some apples and there are 3 varieties for sale (A, B and O) but you are only allowed to have two. This is what is meant by there are many different alleles, that is the same gene for blood groups can be many different versions but us humans can only have two.

[Paul C. Anagnostopoulos]

Before you read The Selfish Gene, read:

Larry Gonick and Mark Wheelis, The Cartoon Guide to Genetics, updated ed., Harper Perennial, 1991.

~~ Paul