Thinking like Punnett

Don Lee, Presenter Author

10/11/2018 Added

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This video walks through the thought process of a breeder at the level of a gene. You can watch it to see how Punnett squares and chromosomes are considered in the backcrossing process.

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[00:00:08.560]The biology that we would like you to learn
[00:00:11.830]in this little presentation is gonna start here.
[00:00:16.010]Here's a magnification of some biological objects
[00:00:21.380]that is shared in Wikimedia.
[00:00:24.950]I want you to guess what these are.
[00:00:26.440]And the hint that I am giving you is that flowers make them.
[00:00:31.700]So while they look like little brains
[00:00:34.170]with a kind of a brain stem
[00:00:36.370]or a spinal cord attached, to me,
[00:00:41.370]that's not what they are.
[00:00:42.480]They're something much smaller.
[00:00:44.920]And that's one of the keys for this presentation
[00:00:47.430]is we want you to be able to think small
[00:00:49.880]and connect it to things that are bigger
[00:00:53.520]and we can see 'em with our naked eye.
[00:00:55.000]So we want you to understand the biological connection
[00:00:59.600]between genes, chromosomes, these things, and flowers,
[00:01:04.500]with this presentation.
[00:01:06.170]So this is my organizer for this presentation.
[00:01:11.190]We're gonna kind of review
[00:01:12.270]what you should know about flower structure
[00:01:14.330]by going through this Journey of a Gene app.
[00:01:20.520]We want you to then think
[00:01:22.410]about how a plant reader manipulates flowers,
[00:01:26.340]predicts gene inheritance,
[00:01:28.100]and then uses that to justify and plan
[00:01:31.930]their crossing methods.
[00:01:32.763]So we're gonna start with these two.
[00:01:36.490]Okay, this is a review.
[00:01:40.040]From the animation we learned
[00:01:42.080]that the flower is a highly organized part of the plant
[00:01:45.170]and it's got specific reproductive structure.
[00:01:47.980]So the stamen is the male part of the plant.
[00:01:50.777]So the anther part of the stamen
[00:01:54.140]is where the sex cells, or gametes,
[00:01:56.710]these are cells that have the special role
[00:01:59.010]of being involved in reproduction.
[00:02:00.730]So what will a plant reader do
[00:02:02.550]if they're gonna use this flower as a male?
[00:02:06.550]They'll remove that stamen
[00:02:11.570]when it's shedding pollen
[00:02:13.270]and then they will introduce it to the female part
[00:02:16.580]of another flower.
[00:02:17.800]Okay, so we call that female the pistil
[00:02:20.520]and so the key is for the breeder
[00:02:24.070]to bring the sex cells from the male
[00:02:26.650]together to where the sex cells of the female parent are at.
[00:02:32.570]And so if we think about it,
[00:02:34.820]the pollen that was added will carry the male genes
[00:02:38.930]and the chromosomes down to where the female gametes
[00:02:44.400]that will have the female genes and chromosomes are.
[00:02:46.720]So the flower structure is designed
[00:02:49.620]to support sexual reproduction
[00:02:52.460]and the development of next generation, the seed.
[00:02:56.380]All right, so you can probably guess what these are now.
[00:03:01.140]Yeah, these are pollen, the male sex cells,
[00:03:04.170]the male gametes.
[00:03:05.370]And inside the pollen would be a nucleus
[00:03:08.220]that would have all the male,
[00:03:10.440]or one copy of every chromosome from that species,
[00:03:15.160]that plant species,
[00:03:16.210]and then the genes that are part of that chromosome.
[00:03:18.800]Okay, so flower structure and then the manipulation of that
[00:03:25.330]to make a controlled cross.
[00:03:26.860]So now we need to think about gene inheritance.
[00:03:30.210]So you're gonna see where a lot of you will say,
[00:03:33.400]oh, I've been learning this in my biology class
[00:03:36.380]and I can see where it can be applied
[00:03:38.880]to what the plant breeder's doing.
[00:03:40.536]So let's start here.
[00:03:42.430]What am I showing you a picture of?
[00:03:44.160]This is DNA and so the letters here would represent
[00:03:48.930]a stretch of DNA that we designate as a gene.
[00:03:52.100]This would be a stretch of DNA sequences
[00:03:56.130]that tell the cell how to make a certain protein.
[00:03:59.180]So we're gonna just designate
[00:04:01.910]that this gene, we'll call it the little e version
[00:04:05.810]of a particular plant gene.
[00:04:07.530]The same chromosome,
[00:04:08.610]it's a big, big molecule
[00:04:09.800]so it also has another gene here.
[00:04:12.490]We'll call it the little g gene.
[00:04:13.890]And we'll get back to the e and the g genes later
[00:04:17.149]in this presentation.
[00:04:18.829]So let's remember what a genetic engineer does,
[00:04:22.820]who's a transformation specialist.
[00:04:25.900]What they do is take a gene
[00:04:27.340]that could come from any other living thing,
[00:04:30.810]we'll call it the f gene.
[00:04:32.150]In this we'll say this is a gene
[00:04:33.970]that helps the plant fight off disease.
[00:04:36.040]So we'll use the big F for a disease fighting ability.
[00:04:39.460]That's the transgene 'cause it came from some other place.
[00:04:43.440]And what is the genetic transformation specialist
[00:04:47.090]need to do?
[00:04:48.370]Yes, they need to get that gene introduced
[00:04:51.080]and we've learned about one method
[00:04:54.620]where you use natural genetic engineering
[00:04:59.400]to introduce this F gene into the same chromosome
[00:05:04.870]that already has many other genes,
[00:05:06.600]such as a little e and the little g.
[00:05:08.800]That's what the genetic engineer
[00:05:11.520]who's a transformation specialist has to accomplish.
[00:05:15.560]Okay, so now we've to remember that they do these
[00:05:18.760]in plant cells where these chromosomes are in pairs.
[00:05:23.150]So while one of the chromosomes would have gotten
[00:05:26.400]this new gene, this big F, introduced,
[00:05:29.930]the other chromosome doesn't have it yet.
[00:05:32.690]So this is a plant that we'd call hemizygous
[00:05:36.530]if you'd like to use the technical terms.
[00:05:39.420]Or we could even call it heterozygous.
[00:05:41.240]So now we're gonna think about this
[00:05:43.470]just the way a geneticist would.
[00:05:46.120]We're gonna designate this cell
[00:05:50.250]and then if this divides into an entire plant,
[00:05:53.260]this plant,
[00:05:55.120]and give it the genotype big F, little f.
[00:05:57.830]What that means is that in every one of its cells
[00:06:00.900]it's got this transgene,
[00:06:03.290]but it also carries a chromosome
[00:06:05.070]that doesn't have the transgene.
[00:06:06.410]So we're using the little f to designate that.
[00:06:09.130]Okay, so everybody question, do you remember?
[00:06:10.940]Do transformation scientists have a greenhouse?
[00:06:13.940]The answer is yes they do
[00:06:15.980]because they need to make sure
[00:06:17.240]that the plants they generate can sexually reproduce.
[00:06:20.730]And when they do that
[00:06:24.090]they can get offspring from this big F, little f plant.
[00:06:27.580]So let's move ahead and take a look at one type of offspring
[00:06:32.040]they could get.
[00:06:33.070]They could get offspring in the seeds from the plants
[00:06:37.720]in their greenhouse that have chromosomes,
[00:06:40.780]that each have a copy of this big F transgene.
[00:06:44.900]So we would call that a big F, big F homozygous genotype.
[00:06:50.650]That's the goal of the genetic engineer
[00:06:52.640]is to obtain this homozygous transgenic plant,
[00:06:55.840]and then who are they gonna pass it off to next?
[00:06:58.600]That's correct,
[00:06:59.433]they're gonna pass it off to the plant breeder.
[00:07:03.100]So now we're ready to take a look at how the breeder
[00:07:07.720]will work with these plants.
[00:07:09.610]So I've drawn out a diagram here, this series of squares,
[00:07:13.330]and you're gonna see where that's gonna help me
[00:07:15.170]organize my thinking the way a plant breeder would think.
[00:07:19.330]So the plant they got from the transformation specialist,
[00:07:23.860]that carries the transgene, we're calling big F, big F,
[00:07:27.680]because we're just interested in how these genes
[00:07:29.960]are being passed on.
[00:07:31.640]And then the plants that are their good, elite varieties,
[00:07:36.440]don't have the transgenes,
[00:07:37.640]so they're little f, little f.
[00:07:38.870]So that's the first cross they'll make.
[00:07:41.350]All right, so there was a scientist named Punnett
[00:07:45.850]who was one of the first geneticists
[00:07:49.080]who followed up on Mendel,
[00:07:52.170]Gregor Mendel's ideas of how that genes exist
[00:07:55.430]and they control the traits
[00:07:58.470]that he was observing in his pea plants.
[00:08:01.580]Punnett discovered that he could apply the same ideas
[00:08:05.400]that Mendel discovered to the organisms
[00:08:07.390]and the traits that he was working with.
[00:08:09.510]But what Punnett liked to do was use this series of squares
[00:08:12.890]to predict how the genes were gonna be inherited.
[00:08:16.550]So what Mendel said is that genes are in pairs
[00:08:21.799]in sematic cells,
[00:08:23.350]and all the cells that make up the body
[00:08:26.190]of a multicellular organism,
[00:08:28.030]but then when the sex cells, the gametes, are made,
[00:08:31.100]those pairs split.
[00:08:32.510]So we remember our pollen?
[00:08:34.330]The pollen won't carry both big F's, just one.
[00:08:37.990]Okay, if this was our male parent
[00:08:40.250]we would then show that the gametes just carry one F gene,
[00:08:44.470]the big F.
[00:08:46.050]If this is our female parent,
[00:08:48.610]then the same thing's gonna happen in the female gametes.
[00:08:52.440]Those pair of genes split, okay?
[00:08:54.950]And then the way the gametes come together is random.
[00:08:59.070]So the square diagram takes care of that for us.
[00:09:02.030]It just shows us all the combinations.
[00:09:04.030]So in the case of this first cross,
[00:09:06.590]only one combination that can be made, Ff.
[00:09:09.380]Every seed produced from this controlled cross
[00:09:13.670]that breeder would make between the transgenic parent
[00:09:16.500]and their elite variety, would be big F, little f, okay?
[00:09:20.580]So they'll select those plants
[00:09:22.600]and then they will continue the breeding process.
[00:09:27.080]So we're gonna see the next cross they make
[00:09:29.240]is what's called a back-cross.
[00:09:30.620]They take the offspring,
[00:09:31.800]cross them back to the elite variety.
[00:09:34.700]So in your brain, go ahead and make a prediction.
[00:09:38.530]How many kinds of gametes will the female make here,
[00:09:42.390]or the elite parent?
[00:09:45.240]How many kinds of gametes will this parent
[00:09:47.880]that has the transgene,
[00:09:49.650]we'll call it the donor parent, make?
[00:09:53.630]Yeah, so if you're mentally fill in this Punnett's square
[00:09:56.350]this is what you would have come up with.
[00:09:57.950]And then the gametes, once they get made,
[00:10:00.740]they can randomly come together in the controlled cross.
[00:10:05.040]And we can see we get two different kinds of offspring.
[00:10:08.860]It's the offspring that have the transgene
[00:10:11.360]that the breeder would be interested in.
[00:10:13.440]So then at some point they'll take these offspring
[00:10:16.860]and then they'll do what's called a self-pollination.
[00:10:20.071]In the case of soybean, or a plant with a perfect flower,
[00:10:23.130]they can let it self-pollinate.
[00:10:25.510]And so now we can use our Punnett's square here
[00:10:28.370]and we can see this is where they would get
[00:10:30.740]the homozygous offspring that have the transgene
[00:10:34.500]on both chromosome and will always pass on the transgene.
[00:10:39.300]They could get these other combinations as well.
[00:10:42.910]So we can use these Punnett's squares
[00:10:46.010]to predict the expected outcome from any cross.
[00:10:49.860]And we're using Mendel's principles when we do that.
[00:10:53.560]Okay, so predicting gene inheritance
[00:10:55.950]is something we want to be able to do.
[00:10:57.840]Now we want to justify crossing methods.
[00:11:01.990]Okay, so if you remember from learning
[00:11:05.100]about the work of our plant breeder,
[00:11:07.530]they do a series of these backcrosses
[00:11:10.940]where they cross the donor parent that has a transgene
[00:11:13.370]with the recurrent parent which is the elite variety.
[00:11:16.230]We want to understand why the breeder does that.
[00:11:19.960]And so now I'm gonna, in order to help us understand,
[00:11:23.540]I'm gonna bring in those E genes and G genes
[00:11:26.420]that I talked about when we first introduced
[00:11:28.950]the idea that genes are on chromosomes here.
[00:11:31.684]So the variety that was transformed to get the big F gene
[00:11:37.200]had little e and the little g versions of this gene.
[00:11:40.570]But the elite variety has the big E and the big G versions.
[00:11:44.860]So if we think about all the genes being passed on,
[00:11:49.450]this would be the genotype of the F one,
[00:11:52.290]and then when we backcross we're gonna see
[00:11:55.790]that we get a number of different combinations
[00:11:59.000]of genes being passed on from the two parents.
[00:12:03.910]But every combination is a result
[00:12:06.210]of thinking about that Punnett's square that we went through.
[00:12:09.760]If we cross big E, little e,
[00:12:11.400]with big E, big E,
[00:12:13.090]you know, we'd get two different genotypes.
[00:12:14.800]Big F, little F, with little f, little f, two genotypes.
[00:12:18.180]Big G, little g, with big G, big G, two genotypes
[00:12:21.810]if you draw the Punnett's square.
[00:12:23.420]So if we think about the overall combinations,
[00:12:26.400]yeah, half of 'em have the big F, little f.
[00:12:28.860]Half of 'em have the little f, little f,
[00:12:32.500]but if we can get those same two combinations
[00:12:37.480]at every gene pair
[00:12:39.060]that gives us all of these different genotype combinations
[00:12:42.410]that we would get.
[00:12:43.243]If we just think about these three pairs of genes
[00:12:45.810]that are different between the two parents.
[00:12:48.530]Eight different combinations.
[00:12:50.290]Now we can see where sexual reproduction
[00:12:53.460]generates lots of different genetic variation,
[00:12:57.870]genetic combinations,
[00:12:59.400]because of the way genes are passed on.
[00:13:02.240]Now the breeder's not gonna be interested
[00:13:03.890]in all of these plants, right?
[00:13:05.490]They're gonna eliminate those
[00:13:07.500]that don't have the trans gene, all right?
[00:13:10.140]And then they want lines that are the same genotype
[00:13:15.410]as the recurrent or elite parent.
[00:13:18.490]So they would be able to eliminate more.
[00:13:22.120]In fact there's only one that has a gene combination
[00:13:25.470]that is as close as possible to what they want,
[00:13:28.690]where they have the elite variety
[00:13:30.230]that's also got the transgene, one out of eight.
[00:13:34.500]And if the breeder worked hard enough
[00:13:37.420]they could identify this one out of eight.
[00:13:40.200]So let's do just a little bit of math
[00:13:42.360]to see why we got eight combinations.
[00:13:46.060]Okay, there's three different gene pairs
[00:13:49.766]and half the offspring would have one genotype
[00:13:52.660]in this particular cross.
[00:13:53.810]Half would have a different genotype.
[00:13:56.990]And so 1/2 to the third power,
[00:13:59.700]since there's three different gene pairs,
[00:14:01.450]gives us 1/2 times 1/2 times 1/2,
[00:14:04.030]or 1/8 is the frequency of the genotype
[00:14:08.550]that has the combination that we would most prefer,
[00:14:12.600]1/2 to the third.
[00:14:15.390]So what if we recognized
[00:14:20.260]that there's not just three gene pairs
[00:14:22.530]that are different between the parents.
[00:14:24.640]The donor parent and the recurrent or the elite parent,
[00:14:28.350]they have dozens or even hundreds of gene pairs
[00:14:32.420]in which there's a difference between
[00:14:35.480]what the parent has that's got the transgene,
[00:14:39.930]in this case the big F,
[00:14:41.790]and the variety that's already been proven
[00:14:44.800]to be a good variety for farmers to grow.
[00:14:48.470]Well if there's more genetic differences,
[00:14:50.600]the probability in one cross of finding what you want
[00:14:54.510]becomes very, very low,
[00:14:56.060]because there's all of these gene pairs that are different.
[00:14:59.740]You know, you'd have 1/2 to a much bigger number,
[00:15:02.940]which would give you a much lower probability.
[00:15:06.240]A really small chance that in one backcross
[00:15:08.680]you're gonna be able to generate
[00:15:10.970]and then find the genotype
[00:15:16.940]that the breeder would most prefer.
[00:15:18.980]So what does a plant breeder do
[00:15:21.170]when there's a very low probability of getting it
[00:15:23.710]with this one cross?
[00:15:25.240]That's why they do this backcrossing scheme.
[00:15:29.400]So in backcrossing,
[00:15:31.140]what it does is it starts to increase the average occurrence
[00:15:36.830]of the variety that has the combination of genes
[00:15:40.630]that they want from the recurrent parent.
[00:15:44.030]So instead of finding that one rare type,
[00:15:48.310]they can more or less select an average plant
[00:15:52.090]and it'll be genetically very close to the genetic makeup
[00:15:56.110]of the elite parent,
[00:15:57.700]but it can also carry the transgene.
[00:16:00.040]So the backcrossing just increases the chance
[00:16:03.060]that they're gonna get to the breeding goal
[00:16:05.800]that they're looking for.

Thinking like Punnett

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Thinking like Punnett

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