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Chromosomes Crossing Over - Linked Genes

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

When two genes are located on the same chromosome they are called linked genes because they tend to be inherited together. They are an exception to Mendel's law of Segregation because these genes are not inherited independently. When chromosomes cross over, two different chromosomes trade pieces of genetic information during prophase I of meiosis. If the linked genes are far apart on the chromosome, it is more likely that crossing over will separate them.

When my students start trying to study linked genes initially they get kind of confused but when you first after you get past that first confusion and you start to learn what it really is it's actually pretty simple concept.

The Human Genome Project has determined that humans have somewhere in the range of about 25,000 genes now if we only have 23 different kinds of chromosomes that tells you could do the same simple Math that we have roughly a 1000 genes plus or minus a bit on average per chromosomes so linked genes is simply the idea that if you have two genes on the same chromosomes, when that chromosome goes into a sperm or an egg those two genes travel together and windup being inherited together so they are "linked." This would violate Mendel's second law, the law of independent Assortment, but luckily there's this process during meiosis during the first step called prophase I that does this process called crossing over where physically a chromosome will break and exchange parts with its homologous pair so that even though two genes were right next to each other, maybe they get swapped over so your mum's version of a particular allele will get swapped over to the DNA that originally came from your dad and vice versa so that all of your children don't look the same.

Now, one thing about this is that genes that are far apart on the chromosome they tend to get crossed over very easily because they have lots of room for those breaks and exchanges to occur but if they're right smacked up next to each other it's very unlikely that you'll get the crossing over even between them so they tend to be inherited together. A lot of genetic test where they test your DNA to see if you have a particular disease or a chance for a particular disease, they're not actually testing for that gene they're actually testing for a linked gene that they found tends to be passed on along with the disease because they are still trying to figure out where exactly the disease gene is, so let's take a look at this and see how it works.

Now, here is a standard Mendelian example here we have somebody's homozygote recessive for two genes the r gene and the e gene I don't really care in this context what they stand for and this person is having a crossing event with somebody who is big R little r big E little e heterozygotes for both traits. Now this person can only create one possible kind of gamete, a gamete sperm or egg, that has little r and little e in it, this person on the other hand through the process of meiosis can create four possible gametes for example here is their chromosomes with the big R's and the larger I'm sorry the r gene is on the larger chromosomes the e gene is on the smaller chromosomes and if they are aligned this way when these separate we windup getting our r's and our e's like this. Now that was with the big r's and big e's together on one side. What if they work the other way? What if the big e happens to be on that side? On the opposite side from the big r what that means happens is that when they separate I windup creating a different possible combination than with the earlier combo, so what I'm going to do is I'm going to create for you here the four possible gametes that this one individual could create so they could make the big r little e that we see right here, they could see the big r big e version that's right here we could create a little r little e right here and our last one our little r big e right there and so when we do this cross when we have these two individuals have their offspring we would expect 25% would be this way 25, 25 and 25 so we'd have equal distributions of our genotypes and phenotypes make sense?

Alright, let's take a look at what happens when they're not on different chromosomes when instead the r and the e are on the same chromosome so here I have again same individuals same genotype but now the little r and little e are on physically the same piece of paper or DNA molecule similarly over here we have them together on the same DNA molecule so now when they pair up yes the red one could be on my left or on the right or can swap around it doesn't matter however they windup only creating two kinds of gametes what we see here alright? So we have only our big r big e gametes being formed these guys here absolutely none of the big r little e, fail, we have some little r's little e's right here but none of our little r big e's so 50% of our offspring will be this way 50% that way.

Now what's their crossing over stuff well let's put them back you recall meiosis during prophase I, the homologous chromosomes come together and then randomly they get broken by enzymes and then other enzymes specifically the enzyme used in your cells is an enzyme called ligase in this case I call it the enzyme tape and so they cross over like that and we windup getting this being formed and this being formed and this being formed, and our tape is running out too bad so sad, this being formed but wait even though these two chromosomes are now different from those the crossing over didn't occur between my r's and my e's so my eventual outcome winds-up being the same and that's because these guys are so close to each other. What if they were further apart? Let's take a look at that, here I have again two chromosomes again the r's and the e's are together on the same chromosome but now there's much further distance between them which gives, if I make the break at roughly the same spot there's a greater chance that the crossing over can occur so now I make the break at roughly the spot and now when I separate them I wind up going back to having something approaching a quarter or a quarter or a quarter so now I do start to see these sets of offspring when I do the breeding now it may not be 25, 25, 25, 25 percent it maybe 15% and 15% of this and then hmm let's see of 40 oh sorry, let me do my Math, 35 and 35% there but again we've de-linked them because of this crossing over event so that's linked genes.