As I understand it, a gene is a location on a chromosome and what differs from person to person is the sequence of base pairs that occur there. Is this accurate? Is the length of the sequence the same for each gene?
Thanks,
Rob
Genetics talks about genes and alleles.
Think of a gene as a physical location, such as a space on a library shelf. Typically you will have a maternal and paternal gene for any given somatic gene.
Alleles are the different versions of DNA that could be in that location, such as the different possible titles of Mexican cookbooks that could be on that shelf.
For the most part we all have the same genes (except the Y chromosome in males has some genes not present on the X chromosome). A few people do have deletions or duplications of short pieces of chromosome which might result on having 1 or 3 genes instead of the normal 2.
Not all genes are the same length. The gene for insulin is much shorter than he gene for CFTR that governs chloride ion transport across cell membranes.
Not all alleles for a given gene are exactly the same length. The most common allele that results in cystic fibrosis is three base pairs shorter than the normal allele for CFTR.
Some alleles may be exactly the same length. The various alleles responsible for the ABO blood type groups are the same length - just differing by a single letter in a few points.
And, of course, the X chromosome has a heck of a lot of genes not present on the Y.
Some genes even have variable lengths; like the repeating genes that cause some genetic diseases (example).
Yes, but we all have X chromosomes, so what Iggy said was correct. (Some of us have two alleles of each X chromosome gene, and some of us do not, but that is not having different genes.)
Sort of. The gene is actually the chunk of DNA that is present at a particular point on the chromosome. And actually, genes can move - it’s the sequence of DNA that is important more so than where it is on which chromosome. But, yes, nearly all the time, a particular chunk of my chromosome 1 will have the same gene as the same chunk of your chromosome 1.
Yes, pretty much. This is as good a place as any to say that in biology, if you look hard enough, you can find an exception to any and all rules. But, yeah, that’s the basic idea.
I’m not quite sure what you mean by this. Each gene in your body will have its own length, and the length can change a lot from gene to gene. Examples have been given in this thread already. If, however, you’re comparing the CFTR gene (for example) in my body to the CFTR gene in your body, they should be the same length. Although, again, there are exceptions to this. The most common CFTR mutation that causes cystic fibrosis is called delta F508. The version of the gene carrying this mutation has three base pairs deleted, so it’s three “letters” shorter than the normal version.
Generally speaking, though, if you select a random gene and sequence it in a population of people, nearly all of the sequences will be the same length. There will be some differences here and there, which may or may not be important.
It’s also worth pointing out that genes make up only a relatively small chunk of your genome. Most of your DNA doesn’t code for proteins. It does other stuff. We know some of what it does, but we’re still working on understanding all of it.
Yes, I was referring to the number of base pairs for a given gene in healthy individuals (and healthy cells). Now as I understand it, one gene makes one protein (prions excepted) and these proteins combine to form enzymes which catalyze all the various reactions necessary for life. What does the remaining DNA do? I often hear this referred to as junk DNA, or does that refer to DNA segments which have no function whatsoever? I know some of the DNA is viral detritus. Is this the result of a viral infection by some distant ancestor, possibly pre-human? Does some of it come from infections I have had? Also, do I have what Smeghead said right, that genes can appear in different orders in chromosomes? Is this from cell to cell or individual to individual? Can a gene appear on chromosome 3 in me and chromosome 6 in you? If so, and you have children with a person whose DNA doesn’t “line up” with his or hers, will the child suffer a birth defect?
Thanks,
Rob
All of us have deletions, duplications, inversions, copy number variations, and other structural variations. In fact, these variations are estimated to comprise 5% of the genome of normal individuals, and may be the predominant determinant of genetic diversity in humans.
This was not known 10 years ago.
See, for example: Structural variation - Wikipedia
As we learn more about the genomics, the term “junk DNA” has fallen on hard times. It is known that much of the supposedly “junk” DNA is transcribed into RNA, probably for a reason. We also know that a lot of the formerly “junk” DNA is now understood to be part of the machinery that regulates which parts of the genome get transcribed into protein. A major thing that separates us from simpler organisms is not the number or size of our genes, but the amount of DNA that is used to regulate the expression of those genes.