[Music] Well, we're back on planet Jurassinome. In this sketch, we're taking on one of the most urgent topics in sociological research, the institution of dinosaur marriage in space. And where better to start than the wedding reception, a touching celebration of eternal love and open bars. Okay, confession. This isn't exactly a sociology sketch. We're really here to boogie down on genetics, specifically recombination and linkage. But don't worry, it's still going to be a party. Let's get started. Oh, good. There's already dancing. Notice that these dinosaurs are lined up in two straight lines. That's because they represent homologous chromosomes lining up during meiosis. After meiosis is complete, the chromosomes on the left will end up in one pair of gametes. and the ones on the right will end up in a second pair of gametes. That means the dinos on the left will end up on one side of the dance floor and the dinos on the right will end up on the other side. The color of these chromosomeshaped scarves represents which parent contributed that chromosome. Dinos wearing blue scarves are the chromosomes that came from one parent. So dinos are from the other parent. The fancy tulip glasses these dinos are holding represent alals of individual genes. Two drinks each. Wow, they don't mess around. Just like the scarves, blue al drinks come from one parent and yellow al drinks are from the other parent. You'll notice that all the yellow scarves and drinks aren't in the same line. This represents the law of independent assortment. Homologous chromosomes are arranged randomly. So in any given game, you cannot predict which parent a chromosome will come from. This means that the alals inherited on one chromosome are independent of the alals inherited on other chromosomes. To put it more simply, the alals inherited on one chromosome don't influence the alals inherited on any other chromosome. As you can see, the color glass one dancer is holding doesn't influence what color glass their next friend in line has. Now, you might be wondering, what happens to genes on the same chromosome? This is where things get a little more complicated. Sometimes genes on the same chromosome also follow independent assortment. In gametes, the alals on a chromosome are not always from the same parent. Wait, what? How? Oh, right. Recombination. Recombination is the exchange of parental DNA during meiosis 1. While chromosomes are lined up, homologous chromosome regions are cut and swapped in a process called crossing over. Check out these two groovy pals. They're in the process of swapping drinks to represent recombination of homologous chromosomes. Soon there will be a dino and a yellow scarf with a blue drink and a dino and a blue scarf with a yellow drink. They're creating new scarf drink color combos. Just like recombinant chromosomes, reocombinant chromosomes carry al combinations that don't match the combo of alals found on the maternal or paternal chromosomes. Offspring who carry these new mixes of alals for genes of interest are called recombinant offspring. Even though you might think a chromosome is from only one parent, crossing over makes it so that many chromosomes have a mix of maternal and paternal DNA. In this case, which parent contributes an alil at one locus doesn't predict which parent contributes the alil at a different locus. And voila, still independent assortment. But this isn't always the case. Let's now take a look at genes that do not independently assort. Linked genes are so close together on a chromosome that they are unlikely to separate during crossing over. Check out these two dancers with three alil glasses each. Uh-huh. I'm sure it's just water. They take hydration super seriously on planet Jurassinome. Well, anyways, when dinosaurs have two drinks super close together in the same hand, they can't swap just one of these drinks with a buddy. Instead, they have to swap both alo glasses together. This represents linked genes, which are likely to stay together during recombination. Linked genes do not follow independent assortment. When genes are linked, gametes are likely to inherit alals for both of those genes from the same parent. Which means in this situation, the combo of alals is not random. Historically, to determine if genes on the same chromosome are linked or unlin, scientists use recombination frequency. Recombination frequency measures how likely it is that two alals on the same chromosome will be separated onto different chromosomes during crossing over. Now, we all know that on planet Jurassinome, dope party and math are basically synonymous. So, we've represented recombination frequency with partygoers playing two super fun, so exciting probability games, a coin toss and dice. When two genes are super far apart on a chromosome, there's a 50/50 shot those alals will end up on different homalues due to crossover. This corresponds to a recombination frequency of 50%. These genes are unlin because the probability of the alals for those two genes being inherited together is the same as if they were on separate chromosomes. Notice these dinosaurs with their alil glasses held far apart. They are having a coin toss to see if they'll do a recombination drink swap or not. Since the odds of them switching drinks are 50%, this pair represents unlin genes. On the other hand, take a look at this duo playing dice. Their odds of rolling the correct number are less than 50%, so they are less likely to swap drinks. These dinos represent that the recombination frequency of linked genes is always less than 50%. Now, this is a wedding after all. Where's the bride? I hope she didn't run off again. Oh my, isn't she lovely? This organized line of alil flowers on her veil represents a gene map. A gene map describes the organization of multiple genes on a chromosome. If scientists know the recombination frequencies of all the pairs of genes they want to map, they can use the distances between each pair to figure out the order of all genes. Knowing where genes fall along a chromosome helped scientists understand how genes interact and are regulated. Gene mapping using tools called genetic markers has been used to identify mutations that cause disorders like cystic fibrosis. Ah, Mr. Morgan is here to make sure the bride stays lawless. He represents centmorggans. So naturally, he's measuring the distance between the al flowers on her veil. Centmorggans are the unit of distance on a chromosome. The number of centmorggans between two genes is equal to 100 times their recombination frequency. Genes on the same chromosome can be 0 to 50 centmorggans apart. One centmorgan corresponds to a 1% recombination frequency, while 50 corresponds to unlin genes with a 50% recombination frequency. Now, the bride sure is looking smitten with Mr. Morgan. I don't think her new spouse would be too happy about that. Oh. Oh my. Is that her better half sulking at the bar? So much for newlywed bliss. Well, that puts a damper on this celebration of not so eternal love. But everything's already been paid for, so let's stick around for a quick review. The law of independent assortment states that homologous pairs are divided randomly into gametes during meiosis. So the alals inherited on one chromosome do not influence the alals inherited on any other chromosomes. Furthermore, recombination allows alals on the same chromosome to be separated if homologous parental chromosomes exchange DNA by crossing over. Unlin genes follow independent assortment because they are on separate chromosomes or are very far apart on the same chromosome. Conversely, linked genes are close together on a chromosome. The alals for these genes are unlikely to undergo recombination and will typically be inherited together. Recombination frequency is the probability that two genes on the same chromosome will be separated onto different chromosomes during crossing over. Genes with a recombination frequency of 50% are unlin. Linked genes have lower recombination frequencies. Gene maps use recombination frequencies to show the order of genes on a chromosome and the distance between them. These genetic distances are measured in centmorggans. Yikes. If there's one thing I've learned at this wedding, it's that space dinos are fun. But they aren't always faithful. Only chromosomes were supposed to recombine at this party, not couples. Well, I hope the bride and Mr. Morgan stay linked for a long time to come. Maybe there's a second wedding on the horizon.