Thinking Like Punnett - Enviropig

Grace Troupe, Presenter Author

02/22/2016 Added

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Introduction to breeding and Punnett squares For more information visit the Enviropig website: https://ge.unl.edu/enviropig/

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[00:00:06.506]In this video,
[00:00:08.055]we're going to learn how to think like a breeder,
[00:00:09.685]and see how they make their mating decisions.
[00:00:11.966]And to do that, we'll use tools like Punnett squares
[00:00:14.206]and pedigrees.
[00:00:15.526]So first, let's think,
[00:00:17.926]what is the goal of the breeder
[00:00:20.086]in the process of genetic engineering?
[00:00:24.461]Well they have their pig,
[00:00:26.462]and they want to get the transgene in, right?
[00:00:28.901]But they don't just want the transgene in one pig,
[00:00:31.822]they want to be able to pass this on
[00:00:33.462]for generations and generations,
[00:00:35.262]so that all the pigs that they're producing
[00:00:37.062]include this transgene with the beneficial trait.
[00:00:40.461]So how do they do that?
[00:00:42.301]Well they want to get to
[00:00:44.222]a homozygous genotype.
[00:00:46.142]That's the most efficient way,
[00:00:48.181]it's not the way they do it in every project,
[00:00:49.861]but in this project that's what we want.
[00:00:51.782]We want this homozygous genotype.
[00:00:53.901]So we'll talk more about why coming up.
[00:00:58.711]So this, this is a pedigree,
[00:01:00.769]and this is a massive pedigree.
[00:01:02.768]Each circle or square in this picture
[00:01:06.387]represents a single pig, and so,
[00:01:09.142]if you look over here,
[00:01:10.942]each of these brackets represents one generation.
[00:01:13.062]And so starting with the generation zero,
[00:01:16.302]where the transgene first got introduced into the pig,
[00:01:20.303]we have seven generations listed here.
[00:01:23.662]Tons of pigs,
[00:01:25.342]I'm not going to make you read this pedigree.
[00:01:28.383]Way too big.
[00:01:29.903]But what I want you to think about is
[00:01:31.903]why are there so many pigs?
[00:01:34.183]This is a ton of pigs,
[00:01:35.583]couldn't we get what we wanted a lot sooner?
[00:01:38.583]Well there's several reasons why.
[00:01:41.063]So first we wanna integrate the transgene,
[00:01:43.303]but that's not why we have tons of pigs.
[00:01:48.212]It's more because we want to avoid inbreeding.
[00:01:51.178]Because if you mate with relatives that are too close,
[00:01:54.698]you can start to have detrimental effects
[00:01:57.017]called inbreeding depression.
[00:01:58.937]By losing the diversity in your gene pool.
[00:02:02.657]Another reason there's so many pigs
[00:02:04.977]is because the breeder is not only trying
[00:02:07.738]to introduce a transgene,
[00:02:09.377]but continue to make genetic advancements,
[00:02:11.618]and the way they do that is by bringing in genes
[00:02:14.418]from other pigs that help advance the population.
[00:02:18.178]So they're bringing in genes from all sorts of sources,
[00:02:20.937]they're not just trying to get the transgene,
[00:02:23.177]so when a breeder is making their decisions,
[00:02:25.138]they're thinking big scale like this.
[00:02:29.137]So before we start thinking large scale
[00:02:31.254]like that pedigree,
[00:02:32.654]let's think about a single pig and what it looks like
[00:02:34.813]to have the transgene there.
[00:02:36.813]So pigs are like us in that they have
[00:02:38.693]two of every chromosome.
[00:02:40.413]And in this story, the transgene was inserted
[00:02:44.774]into chromosome 4.
[00:02:47.250]So let's take a closer look at what that chromosome
[00:02:49.890]could look like.
[00:02:51.650]So like I said they're like us,
[00:02:54.010]they have two of every chromosome,
[00:02:55.649]they got one chromosome 4 from their mom,
[00:02:58.610]and one chromosome 4 from their dad.
[00:03:00.490]So while each of these has the same types of genes,
[00:03:04.689]they can have different versions called alleles.
[00:03:07.529]So they each have an A gene,
[00:03:10.050]but they can be different copies.
[00:03:14.170]So if this was eye color,
[00:03:15.890]both of these are eye color genes,
[00:03:17.370]this one could be a brown color
[00:03:19.130]and this one could be a blue color,
[00:03:20.609]but they're both eye color.
[00:03:22.289]So that's very important
[00:03:23.970]as we look closer at them.
[00:03:27.330]So, what I want you to think about
[00:03:31.210]is what the genotype is
[00:03:33.330]for this individual with further chromosome 4.
[00:03:37.380]So here's the genotype gene,
[00:03:40.290]this, you can break this down to gene,
[00:03:43.434]and type, what types of genes do they have?
[00:03:46.555]So you have a capital A, a small a,
[00:03:48.755]two big Bs, big T,
[00:03:50.914]and then nothing over here, Cs and Ds.
[00:03:54.555]So we have words to describe each of these.
[00:03:58.989]And they're at the bottom of the screen here.
[00:04:00.949]So if you had to tell me which of these root words
[00:04:03.670]best describes the A locus,
[00:04:06.229]capital A, small a,
[00:04:08.310]which of these describes it?
[00:04:11.860]Well I'd say that big A and little a are different,
[00:04:14.356]so it's called heterozygous.
[00:04:17.275]The next allele for the Bs,
[00:04:20.435]they're both capital Bs, so what word describes that?
[00:04:24.145]Homozygous.
[00:04:25.956]Homo, they're the same.
[00:04:27.912]So then we get to T, which I used to represent
[00:04:30.992]the transgene,
[00:04:33.071]and we have something on one chromosome
[00:04:37.431]but nothing on the other.
[00:04:39.791]And that's because when we inserted this transgene,
[00:04:42.352]it only really inserted once,
[00:04:46.031]and in order for it to insert both here and on this other
[00:04:50.831]corresponding chromosome,
[00:04:52.831]the chance is pretty much zero that that's gonna happen.
[00:04:56.472]So when you initially insert it,
[00:04:58.071]only one chromosome has it.
[00:04:59.952]So we represent this with a dash or a blank,
[00:05:03.392]showing that we have a gene on this side,
[00:05:06.552]but no corresponding allele over here.
[00:05:10.892]So what would we
[00:05:13.747]so what would we call this?
[00:05:15.898]If we only have it on one chromosome?
[00:05:19.537]This is what we call hemizygous,
[00:05:21.537]and this is how every transgene starts.
[00:05:24.858]So then let's practice just a couple more times,
[00:05:27.818]what would the C allele be?
[00:05:30.337]That's different, so it's hetero,
[00:05:32.218]and same with the D allele.
[00:05:35.898]Alright, so,
[00:05:38.678]if we have the transgene in one chromosome,
[00:05:42.397]but no corresponding allele,
[00:05:44.518]because this was an insertion, right?
[00:05:46.557]It didn't exist before.
[00:05:48.638]We call that hemizygous.
[00:05:51.598]So when a breeder is working with a hemizygous pig,
[00:05:55.038]they can think of it like a heterozygous one.
[00:05:57.757]And so we're gonna use a couple Punnett squares
[00:06:00.038]to see how this works.
[00:06:02.118]So here's a Punnett square,
[00:06:04.958]and let's talk about how we actually use this,
[00:06:07.197]how it works.
[00:06:08.638]So on the top, these are the genes
[00:06:10.757]that the dad has, right?
[00:06:12.198]Every person, or every pig
[00:06:14.957]has two of every gene,
[00:06:16.998]but when they pass on their sperm to make their offspring,
[00:06:19.478]they're only going to be able to pass on
[00:06:21.757]one of these versions,
[00:06:23.318]because a sperm only contributes
[00:06:25.625]half the genetic information, right?
[00:06:27.464]Because you get the other half from your mom.
[00:06:29.585]So this dad had two different versions,
[00:06:32.825]and so we write each version that the dad has,
[00:06:36.025]he is heterzygous.
[00:06:37.865]On the side here, we can see that the mom
[00:06:41.545]also had two different versions.
[00:06:43.544]And in her egg, she'll pass on one of these or the other.
[00:06:47.064]Then this center part shows us all the possible offspring
[00:06:50.824]that could come from these two parents.
[00:06:53.385]So let's say this dad passed on his big A,
[00:06:56.425]and the mom passed on her big A,
[00:06:58.745]that means that there's a possibility
[00:07:00.705]of having a child that's big A, big A.
[00:07:03.104]And so because this is one square out of the four,
[00:07:06.665]we say there's a 1/4th chance
[00:07:08.705]of getting a big A, big A child.
[00:07:10.824]And so these are all the possible children.
[00:07:13.504]For two heterozygous parents.
[00:07:16.425]So let's draw another Punnett square,
[00:07:19.424]and let's say we have two hemizygous parents.
[00:07:23.384]So go ahead and pause this video,
[00:07:25.345]and fill out the Punnett square.
[00:07:29.425]Alright, this is gonna look about the same, right?
[00:07:32.904]This big T from the dad, a big T from the mom,
[00:07:36.545]so you get big T, big T.
[00:07:38.385]Let's trace this down,
[00:07:39.825]you've got a,
[00:07:41.544]no transgene from the dad for this top square here,
[00:07:44.185]a transgene from the mom,
[00:07:45.944]so you get a hemizygous child,
[00:07:48.385]hemizygous, and then, no transgenes,
[00:07:52.624]so these are all the possibilities
[00:07:54.145]if both parents are hemizygous.
[00:07:57.504]And it looks the same as this Punnett square
[00:08:00.385]for the heterozygous,
[00:08:02.305]so they can think of them roughly the same.
[00:08:06.915]So,
[00:08:08.659]what parent genotype will help the breeder
[00:08:10.980]keep the transgene in the population most effectively?
[00:08:14.580]Well at the beginning of this video,
[00:08:16.300]I told you that a homozygous parent would.
[00:08:19.830]So along the top of each of these Punnett squares
[00:08:22.190]I put a homozygous parent,
[00:08:24.311]and I just put the father.
[00:08:26.030]So on the side, I showed every possible
[00:08:29.391]mother genotype,
[00:08:31.830]so let's see what all these Punnett squares look like,
[00:08:34.031]and why it's best to have a homozygous parent.
[00:08:38.150]Alright, so if this parent was homozygous,
[00:08:40.471]and this parent had no transgene,
[00:08:42.430]every child still has the trait.
[00:08:47.600]Same here, every child gets the trait
[00:08:50.446]even though the mother is not homozygous.
[00:08:54.405]And then obviously if the mother is homozygous
[00:08:56.686]and the father, then every child is homozygous.
[00:08:59.606]So if you have one homozygous parent,
[00:09:01.525]that means no matter what you do,
[00:09:03.606]you're going to have children that display this trait.
[00:09:07.126]Which is why it's really rather efficient
[00:09:09.325]to be able to get a homozygous parent.
[00:09:17.676]Alright, in the beginning I showed you a pedigree,
[00:09:19.761]and this is how breeders represent
[00:09:21.681]the crosses that they make.
[00:09:23.481]So let's look at a small piece of that pedigree.
[00:09:27.921]And determine how it's used.
[00:09:30.441]So this, with all the shapes and lines,
[00:09:33.521]this is what a Pedigree is.
[00:09:35.360]And here's the key on how to read it.
[00:09:38.281]So each of these symbols is an individual, a pig,
[00:09:42.880]and circles are females because
[00:09:46.161]ladies are curvy, right?
[00:09:48.121]And squares are male.
[00:09:49.841]And the coloring shows the genotype of each individual.
[00:09:54.720]So white being non transgenic,
[00:09:57.881]red being homozygous transgenic,
[00:10:01.600]and then this 50/50,
[00:10:03.480]that shows that hemizygous.
[00:10:06.521]So what I want you to do is use this pedigree
[00:10:09.280]to make this Punnett square.
[00:10:12.041]So show how this father and this mother
[00:10:15.040]can make a Punnett square to represent these offspring.
[00:10:18.920]So pause this video and fill out the Punnett square.
[00:10:25.871]Alright, so on the top we have the father,
[00:10:28.376]so he is hemizygous, right?
[00:10:31.416]So we represent that with a big T,
[00:10:33.617]and a dash,
[00:10:35.496]and the mother, she's non transgenic,
[00:10:37.736]so she's homozygous for not having a transgene.
[00:10:42.537]So then all the possibilities for their offspring
[00:10:47.006]are that they could be hemizygous
[00:10:50.202]or have no transgene.
[00:10:51.961]And obviously if we want the offspring to have the trait,
[00:10:54.682]these hemizygous ones are what we're aiming for
[00:10:58.202]in our breeding program here.
[00:11:00.362]As we start out incorporating the transgene.
[00:11:03.641]So that's how these work together.
[00:11:07.042]So then, that's one individual cross,
[00:11:10.601]we scaled this up to lots of crosses
[00:11:13.521]to meet all these goals
[00:11:15.322]of integrating the transgene,
[00:11:17.201]avoiding inbreeding by bringing in more
[00:11:20.442]genetic diversity with more pigs,
[00:11:22.524]and then making genetic advancements.
[00:11:24.393]And so you don't need to be able
[00:11:26.434]to work out this whole pedigree,
[00:11:28.073]but just know that this is done lots of times,
[00:11:31.034]a breeder makes a lot of these decisions.
[00:11:34.153]And in the end, the breeder is working
[00:11:36.857]to meet his goal of that
[00:11:38.783]breeding pig plus the transgene,
[00:11:40.623]so that way he can produce all sorts of pigs
[00:11:42.903]that have the new trait,
[00:11:44.383]and the way they can most efficiently do that
[00:11:46.543]is if they get their pig to be homozygous,
[00:11:50.183]and that's the goal of the breeder
[00:11:52.423]in the genetic engineering process.

Thinking Like Punnett - Enviropig

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