9.2.1 Mendelian Genetics


ok you are now going to start our treatment of genetics and let’s preface by
pointing out that genetics really is the oldest branch of biology and that is because humans, homo sapiens has been practicing genetics in the form
of domesticating animals and plants by selective breeding for over 10,000 years and this ranges from horses in very early times in native americans for example and Europe and the steps of Europe to Egyptain cultivation of plants date palms for example and also obviously dairy cattle the pharaohs of Egypt also were domesticating donkeys and show here are the burial domesticated donkeys in the Americas the sunflower was an early cultivar cultivated in the western United States and Mexico apparently corn is one of the
best examples corn is derived from a very small plant that grows in the Mexican Highlands called Teosinte and modern maze, our modern corn varieties are all derived ultimately from the domestication of this plant and it’s not that the original farmers who took Teosinte had in mind that they would ears of corn like this but they selected for features of this plant that did yield increasingly large kernels of corn for eating and this is what we ended up with after many hundreds and thousand years of cultivation and then there’s a classic example peas that Mendel used to discover the laws heredity so we will be, we will be studying Mendel’s experiments
with peas shortly and I only show this to again emphasize that genetics really is the oldest branch of biology. what is
genetics years here is where our treatment of genetics is going over the next little while over the next few weeks genetics really is information flow that’s what the science of genetics is there is transmission genetics which
describes the properties of information flow between
generations the way the genes are inherited and passed
on from parents to offspring through gametes or through cell division if it’s bacterium and molecular genetics describes information
flow within cells and organisms that is, this is the central dogma of what we would say molecular biology is
the central dogma of molecular genetics is that information stored in DNA copied in RNA is translated into protein and naturally
within this paradigm DNA replicates as part of this
so we have DNA replication transcription of DNA and RNA and translation of RNA into protein and so we have two real branches of genetics that will be
treating over the next few weeks we’re going to start out by talking about
transmission genetics information flow between generations so let’s look at what, the types of experiments that Mendel the discoverer of the modern laws of heredity were, lets look at the techniques he was using and people have been using these techniques for very long periods of time we have male parts of P flowers and female parts and what can be done is the removal by
cutting of the male part so that self-fertilization cannot occur you could prevent self-fertilization and in that case you can grab pollen using a fine brush or human hair even you can obtain from
another flower the male gametes the pollen and you can
place those on the female parts of stigma and the pollen grains will migrate down the pollen tube and fertilize eggs in the ovary of the recipient and
those fertilized embryos will develop into peas in case the pea plant and those peas can be planted to give rise
to plants which in this case have purple
flowers so you can do these types of crosses then at will why did take so long we’ve been practicing practical genetics applied genetics for
thousand years why did it take so long for the discovery of the basic laws that govern transmission of information from one generation to
the next and thats partly because the intent of applied geneticists that is domesticated, domestication breeders was to develop better agricultural
plants and animals and their goals really weren’t to understand the basic biology of the process so let’s look at an experiment that was
reported by an investigator Goss in 1824 well before Mendel did his work and let’s look at his data and I’m going to paraphrase, paraphrasing the approach that I’m sorry I’m paraphrasing the data from his paper and this is how we reported them so he grew a plant from green pea a green seed and he grew a plant from a yellow pea, green I mean yellow pea and he crossed those two plants that grew up from the seeds and he notice
that all the seeds from that cross all the peas in other words were yellow so he took those yellow peas and
he grew them and he allowed them to self-fertilize he did not cut off any of the male parts he allowed self-fertilization to proceed and
this is the results that he got in terms of what his pea’s look like after self-fertilization of these individuals he had some pods had all yellow seeds there were many pods with both yellow and green seeds that are mixtures and there were some pods with all green
seeds so he took green peas from these pods these mixed pods and green
peas from these all green pods and he grew those plants up and allowed those to self-fertilize and he noticed that in that case all the progeny plants had green peas only exclusively green peas whereas if he took pods with all yellow seeds and he took those yellow
peas grew those into plants and allowed them to self-fertilize he obtained mixture again some pods with all seeds yellow some with green and yellow seeds mixed well what do we see in this, Goss was interested in pea breeding but not necessarily interested in the basic laws of heredity or if he was he certainly was missing a key ingredient and what do we know, what do we see rather, what do we observe about this type of date what is missing in this description of data I’ll let you think about that for a moment and it’ll probably come to you is that there is a lack of quantitative data there are no numbers associated some, many, some there’s no quantitative data whereas Mendel an Austrian Monk who clearly had a clerical bent and was a an excellent experimentalist and data recorder Mendel let’s look at his data which was basically derived is an initial
experiment was initially derived from the exactly the same type of cross so let’s look at the way that Mendel recorded his data and Mendel published in 1866 much later
than Goss did so here we have, the first thing that Mendel did is he was after the laws of heredity and he established true breeding lines of plants that is plants that only produced yellow peas a line of plants that only produced yellow peas and a line of plants that only produce green peas and having these true breeding lines then allowed him to breed with confidence and would allow him to determine with confidence the results of his experiment in terms of the properties of inheritance so he calls the first generation, the first filial generation or the F1 so all seeds are yellow just like Goss when he crossed a pea plant that was grown from a green pea and a pea plant that was grown from a yellow pea, all the peas resulted were yellow and he grew those into plants and he allowed those to self-fertilize just like Goss had done and then heres where Mendel gets
quantitative he counted the number of green and yellow peas here and out of 8,000 total seeds in one experiment 6,000; approximately 6,000 were yellow and approximately 2,000 were green that is there was a 3:1 ratio of, 3:1 ratio yellow to green, 3:1 of yellow to green peas in this F2 generation in the F2 generation so what did he do he took some of these green peas and grew those into plants and he allowed those to self-fertilize and just like Goss when that was the case he got all green peas produced in other words he had established a new true breeding line of plants grown from green peas and allowed to self-fertilize whereas if he took some of these yellow peas he took 519 he would, Mendel grew lot of plants and looked at a lot of data so he took 519 of these yellow peas grew those into plants allowed them to self-fertilize and of those 519 plants 166 gave all yellow seeds that is they bread true for the yellow pea appearance but 353 did not they did not bread true that is to say when those 353 plants were allowed to self-fertilize they give rise to progeny peas that were mixed yellow and green so there in this case then what we have is we compare these what we have is a
1/3 true breeding and 2/3 of the yellow of the plants grown from yellow seeds were not true breeding we’ll come back to these 1/3 and 2/3 shortly but keep that in mind for now so let’s see what Mendel did with
this data but before we do that lets just point out that in fact he could do this with other traits he could cross now not looking at pea color but rather flower color he would cross true breeding purple flower
plant from true breeding purple line and a white flower plant from a true breeding white line and he got all purple offspring in the F1 generation and when those self-crossed self-fertilized he got a three to one ratio of purple flower plants to white flower plants in the F2 generation let’s not worry about the F3 generation right now and let’s look at what Mendel, Mendel’s model was that he used that he derived from his data and here’s what his model involved, he said
that for every trait that you’re looking at for example yellow peas verses green peas that there are two factors two elements in the case of the true breeding yellow he
designated those as two capital A symbols (AA) representing
those two elements those two factors whereas he used little A’s (aa) to designated the two factors in the true breeding green plants and Mendel said he could explain his data if when, if there are two factors that govern a particular trait like the pea color that when plants produced reproductive cells, egg cells or pollen cells or in animals egg cells and sperm cells those gametes those reproductive cells
those sex cells only obtain 1 of the two factors that are
present in a given individual so that little a, little a (aa) plant would only produce pollen cells that contain the little (a) factor which would specify green peas whereas eggs cells from a true breeding plant would only obtained one of the big (A) factors that would specify yellow and so that when these were when these combined by fertilization that when he did his parental cross, he obtained an F1 generation that had one large (A) factor and one small (a) factor and as you know these, all these yellow, all these F1 peas were indeed yellow they were not green and we’ll talk about dominant and recessiveness in a moment but right now we have a mixture an F1 generation that has the large (A) factor and a small (a) factor and when those yellow seeds were grown into plants and self-fertilized we can ask what — what kind of pollen eggs they would produce well remember Mendel is saying that he can make his data work if when sex cells are produced only one of the
factors that are present in individual finds its way into a very reproductive comedic cell so here if we have individuals that are combination of these factors big (A) and little (a) they will produce for example pollen
that half of which contains a big (A) and half
which contains little (a) and they will produce eggs half which contain a big (A) factor and half which contain little (a) factor and then if we allow the random mixing of this pollen, these pollens with these eggs we would get the
following one quarter of our progeny (1/4) of the progeny would contain the two big (A)’s they would result from the fertilization of a big (A) containing egg with the big (A) containing pollen. One half we look at these two together 1/2 of the progeny peas would contain one little (a) factor and one big (A) factor and they would result from the fertilization of a big (A) egg by a little (a) pollen or alternatively by a little (a) egg by a big (A) pollen but they would be equivalent in terms of
the factors that they inherited they would be a mixture just like their parents would, would have been and then one quarter of the progeny would inherent a little (a) from a little (a) pollen and a little (a) from a little (a) egg and they would have two little (a)’s so the, this constitution of factors is just like the true breeding green grandparent up here and this combination the two big (A)’s is just
like the grandparent from the true breeding yellow line whereas these mixture F2 plant peas would be just like their F1 parents so what does this result in this results in a 3:1 ratio of yellow to green, so we have yellow and green in a three to one ratio that is 3 yellow to 1 green and we can immediately see that of the
yellow peas in the F2 generation only one-third of them will be true breeding in the sense that they are self-fertile, if they are grown into plants and
self-fertilize the will give rise to a only yellow peas as their grandparents were true breeding big (A) big (A) whereas of the yellow peas in the F2 generation, of the yellow, ones two-thirds when self-fertilized will give a mixture of green to yellow peas in the same ratio
exactly the same ratio 3:1, yellow to green so these ratios spoke to Mendel in a profound way and like (excuse me) and allowed him to develop his first law and here is his first
law each trait whenever it is you’re talking about curly hair, straight hair, brown eyes, blue eyes purple flowers, white flowers, green peas, yellow peas every trait is governed by two particles and one is inherited from each parent and the two particles do not influence each other anyway in an offspring but when when that offspring grows up and becomes sexually mature those particles separate uncontaminated they do not influence each other, in other words, into gametes when reproductive cells form and an unstated corollary is that any for plants for peas any pollen cell can fertilize any egg cell there’s random fertilization consider if that were the case, if there weren’t random fertilization let’s say that big (A) pollen could only fertilize big (A) eggs and little (a) pollen can only fertilize little (a) eggs well then you would not get 3:1 ratio of the following combinations you would get little (a) little (a) and big (A) big (A) in the F2 that is you’d get half yellow and half green but we have a 3:1 yellow to green ration from a monohybrid cross we call this a monohybrid cross because we’re looking at one character trait and we have hybridized, we have produce
hybrids in the F1 because we’ve crossed two plants or two
individuals that differ in that trait so we’ve hybridized and its a monohybrid cross because it’s we’re only looking at one character trait at this point so now we can introduce modern terms we can talk about, we can take Mendel’s law and put it into modern terms so let’s talk about his particles the particles are known as alleles, so for every trait then there are two alleles that govern that trait and as we’ve seen those alleles can be dominant or recessive in the case of the peas here the big (A) allele the big (A) version of the gene in other words is dominant over the recessive little (a) because in this combination we have the appearance of the big (A) the big (A) character which is yellow seeds in this case and we call the little (a) here which specifies green peas we call that a recessive allele because it recesses for a generation
in a monohybrid cross its present in the parental generation here it disappears in the F1 and then it appears again in the F2 when the F1 is self-fertilized so the the green pea trait which is specified by the small (a) allele recesses for a generation and we
call that the recessive allele and the one that dominates over that we call that the dominant allele and now we can introduce the terms phenotype and genotype and phenotype and genotype genotype, lets take genotype first. genotype can be refer to two things one is that you can have the genotype of an individual so an individual let’s say could be big A
big A and or it could be a little a little a and or it can be big A little a so all these are genotypes and we refer to these two genotypes as homozygous genotypes because both alleles both alleles are the same in these homozygous genotypes whereas in this genotype we say that we
have a heterozygous situation because there are two different alleles present now genotypes govern phenotypes so phenotypes which an phenotypes we can find as the appearance of a trait in a given individual it is the genotypes that determine the phenotypes so in the our
previous cases the homozygous big A genotype and the heterozygous genotype gave a yellow phenotype whereas the homozygous small a genotype gave rise to the green
phenotype so genotypes determine phenotypes and we have heterozygous genotypes or homozygous genotypes but we shouldn’t forget that there’s
also the genotypes of the gametes so we could say that this pollen grain, this pollen cell has a little a genotype and this one has a big gametes have genotypes as well so here
we have a little a genotype for this pollen cell big A genotype for that pollen cell a big A genotype for that egg cell and a little a genotype for that egg cell so gametes have genotypes as well now Mendel went on to test his law he wasn’t satisfied with just coming up with an explanation or a law that would would explain his three to one ratio on recesses phenotypes to dominant phenotypes in the offspring of a monohybrid self-fertilization he tested the law for example by crossing
let’s say heterozygous by homozygous recessive so for example he would cross big A by big A as he originally did little a little a so these lets say are yellow and these are green and he would get these F1 heterozygous which were yellow these were yellow and he would cross those, he would grow those up into plants and cross them by green plants like these and he asked or he predicted what ratios a of phenotypes he would get in the offspring of this cross and think about that for a moment is
there anything magical about that three to one ratio that Mendel came up with no there isn’t and Mendel used his understanding, his law to predict accurately what would happen here well let’s look at it let’s draw it out so if we use our little pundit square method so called pundit square after geneticist named Pundit to see what kind of gametes genotypically speaking this individual would produce it would only produce little egg gametes we don’t even need to make a second column because 100% of the gametes will contain little a there’s no other alleles present to find their way into gametes whereas these heterozygous individuals will produce half on their gametes with the big A genotype and half of their gametes with the little a genotype and that then should produce 1/2 of the offspring of this cross should be heterozgyous big A little a and the other half of the offspring should be homozygous little a little a so we should get 1/2 yellow that have big A little a heterozygous genotype and 1/2 green that have a little a little a homozygous genotype and that is exactly what he
got so Mendel actually became the first person
in history to be able to accurately predict the ratios of phenotypes of offspring from a given genetic cross, despite the thousands of years of domestication which people were interested in being able to predicted or determined what what percentage of phenotypes would be available in a particular cross Mendel was the first one to discover a law of heredity that would allow the accurate prediction on this and he tested other traits in addition
to doing the experiment that we just talked about in which he made specific predictions based on his law and had those predictions confirmed he
tested other traits in fact he looked at seven traits he looked at flower color purple and
white, and yellow and green peas we’ve already
talked about that round and wrinkled peas, green and yellow
pea pods inflated pea pods verses constricted or wrinkled pea pods, axial flowers that would grow from the branches on the side branches of the plant verses terminal flowers which would be growing from the end of the plant the top of the plant plant height as well tall verses short and in all seven of these he found when he made monohybrid crosses he would get a 3:1 ratio roughly of the dominant phenotype to the recessive phenotype in the self crosses on the F1 hybrids and there’s always the dominant allele that he could assign to the dominant
phenotype so for purple he would have lets say big P big P as the purple flower little p little p as the white flower the F1 hybrids
would be big P little p heterozygous and when those were self-fertilized he would get at three to one ratio of purple to white and we would be able to predict what the genotypes of those would be and we’ll run through some examples of
that in a little while so we tested this with other traits and therefore became convinced to the universality of his first law but he didn’t stop there in addition to testing all these traits that we’ve just mentioned and here are the F2 ratios he tested what would happen if two character traits are followed simultaneously that is instead of doing monohybrid crosses he looked at the that phenotypic ratio produced by the self-fertilization of dihybrid two hybrid crosses so now what Mendel did is to follow the inheritance of two traits simultaneously so here we have a round yellow plant where round is dominant to wrinkled peas big R big R and yellow peas we already know are dominant to green peas we specify yellow with big Y green
with little y wrinkled peas with little r and round peas with big R so here we have a homozygous big R Big Y genotype and we have a true breeding line of plants like that Mendel was careful to establish true breeding lines and here we have little r little r little y little y double homozygous genotype that produces wrinkled and green peas in a true breeding line of plants so Mendel crossed these and observed in the F1 generation not surprisingly given his previous results that the dominant alleles were dominant and the recessive alleles recessed so in this double heterozygote now big R little r, big Y little y, in the
double heterozygous we had a double dominant phenotype that is a round peas and yellow peas now what would happen then if we self-fertilize if we self-fertilizes these and this is what Mendel got these are his ratios that he observed in the F2 generation and let’s see why that is
true well we just have to ask ourselves what type of gametes these F1 individuals would produce and so we can say well in one type of gamete we’ll inherit a big R allele and that gamete could inherit also a big Y allele remembering that only one allele from each gene that governs a particular character trait only one allele finds it’s way into any given reproductive cell any given gamete so we
can have big R going into a gamete with Big Y we can have big R going into a gamete with little y here we can have little r into a gamete with Big Y and little r with little y that is they’re four genotypes of gametes made possible by this genotype and so if we self-fertilize then we will have sperm with those four same genotypes as these eggs and if we do if we look at random fertilization we will get the following genotypes and there are 16 possible combinations
note that only 1/16th here 1/16th are will have a wrinkled green phenotype that is these are that is the
double recessive phenotype and that is because if only one-quarter
of the sperm are little r little y and onle one-quarter of
the eggs are little r little y then it would be one-quarter the probability of getting that type of
fertilization of that is one-quarter of the eggs times one-quarter of the sperm equals 1/16th and indeed Mendel saw that one-sixteenth of the progeny in these dihybrid crosses had the double recessive phenotype well many more have the double dominant phenotypes so here we have the
round and yellow are the double dominant phenotypes in 9/16th and you can add these up and see why that why that is true of those nine-sixteenths though only one-ninth are grown into plants and self-fertilized will be absolutely true breeding that is one-sixteenth are homozygous for the for both alleles for both recessive alleles likewise only one-sixteenth are double homozygotes their doubly homozygous for the dominant alleles in this case here they were doubly homozygous for the recessive alleles here they are doubly homozygous for the dominant alleles all the other that on the double dominant phenotypes are heterozygous for at least one of the two genes that we’re talking about so all
these other all of the other eight sixteen that have doubled dominant phenotype have at least one are heterozygous for at least one of the two genes that we’re looking at alright so there’s another one there’s another one and there’s another one alright now what about the other 6, sixteenths well of those we have three-sixteenths that are domaint for have the domaint phenotype from one trait and recessive for the other that is three-sixteenths a round which is the dominant phenotype and green which is the recessive and the other three-sixteenth are recessive for the first trait and dominate for the second rate that is these are are reciprocals each other we can see why that’s true for the round green peas we have this genotype which is homozygous for big R and homozygous for little y here’s another one of those three-sixteenths heterozygous for big R homozygous for little why and here’s yet another heterozygous for r and homozygous little y of course
because they have, of these three-sixteenths we’re talking about ones that have the
recessive phenotype for one of the two character traits of course those have
to be homozygous recessive in order to have that recessive phenotype likewise you can go through
this on your own for the wrinkled yellow seeds that would be here your homozygous for the little r allele that is you have a wrinkled phenotype and you are either homozygous big Y yellow or heterozygous big Y little y and you’ll also have a yellow phenotype in that regard so Mendel could explain his 9:3:3:1 ratio 9 we commonly draw this as 9:3:3:1ratio this is a result of of a dihybrid self cross the characteristic ratio of a dihybrid self cross hybrid for 2 traits and we self-fertilize he could explain his 9:3:3:1 ratio by stating his second law and that is as follows: it’s the law of independent assortment in
that it reads as follows: during the formation of gametes then the segregation of alleles at one locus and
for locust let’s just write in gene is the same thing as a locus the segregation of alleles at one gene is independent of that of the segregation of alleles at any other what does that mean well what that means is that in the formation of gametes from these heterozygous let’s say there’s an equal probability that if a gamete receives a big R and a big Y that a gamete will receive a big R and a little Y that is the y alleles which govern color and the texture allele which govern round or wrinkled the alleles for those for those traits segregate into gametes completely independently of each other let’s imagine that weren’t the case if that weren’t the case what if when we of a genotype a double heterozygous genotype like this that is a dihybrid heterozygote if let’s say when this form gametes if a gamete received an r it always received a y a big Y big R always went with big Y for some reason and likewise let’s imagine little r always went with little y well then there would only be two types of gametes formed and if you look at sperm and eggs you would have big R big Y and little r little y and then you would have, end up with
three to one ratio of double dominant phenotypes two recessive phenotypes so you would have big R big R big Y big Y, here big R little r big Y little y heterozygote here big R little r big Y little y, here thats a little y there so we here we would get a three to one
ratio of dominant double dominant to recessive, double recessive for both traits here we would have round I’m sorry we would have wrinkled green and all the other three here would be round yellow so if this were the case if weren’t independent assortment of the alleles into gametes such that in the case drawn you could get four possible genotypes of gametes if that weren’t the case you would get something other than the 9:3:3:1 ratio yet that is what Mendel got repeatedly when he repeated these experiments for different traits monitored in dihybrid crosses and therefore his second law seemed like a universally applicable law and we now know the bases with this and as we see when we study when we draw together the behavior of chromosomes with Mendel’s alleles we will see that to
be true and that’s what we’ll cover in the next part of this lecture

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