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Mendelian Genetics Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States. In the middle 1800’s Gregor Mendel, an Austrian monk, did famous experiments that allowed him to understand quite a bit about how heredity works. Of course he knew nothing of the existence of DNA or anything about chemistry, actually, but from his work he was able to make out some mathematical patterns that then allowed him to make good inferences about how the system works. We will look at two principles he discovered and then tie them to what we know of DNA structure and function. Mendel studied peas because he could grow them under controlled conditions in the abbey’s garden, control their pairings (by hand pollination), and get more than one generation per year – something not possible with most animals. Among the characteristics he studied we will examine two: shape and color. Mendel saw that there were two shapes to the peas when they were dried: smooth (left) and wrinkled (right). When Mendel sorted his dried peas after crossing them he always found the two shapes (which we call phenotypes) in approximately the ratio 75% round and 25% wrinkled (3:1) after the second crossing. Make sure you realize that this observation is not based on four peas, but on many thousands of peas. Statistically the ratio consistently holds, and the bigger the sample the closer to a perfect 3:1 ratio it gets. Mendel reasoned that any consistent ratio of phenotypes indicates that 1) whatever causes heredity comes in unit “packages”, and 2) these packages are of two types. He reasoned further that one version of each type came from each parent, creating what we see as a blend of parental characteristics in the offspring. He was the first to see that it wasn’t a blend in the way it had always been considered – like making orange paint by mixing red and yellow. Rather it was more like making orange color on a page by mixing red dots and yellow dots of small size. (Mixing pixels on a screen, as was done to make the shading on the images on this page is a better example.) The fact that the ratio is not 50:50 (or 1:1) suggested to Mendel that one trait was dominant and the other recessive. That is, if the code for one trait (in this case smooth shape) is present in either of the parental genes in the package then the peas are smooth, regardless of the other parent’s gene. To illustrate this he assigned a letter to both possible versions of the code: R from a parent (capital to indicate dominance) coded for a round pea and r (lower case for recessive) for a wrinkled one. Each parent contributes one letter to the code. There are exactly four possible combinations: RR, Rr, rR, and rr. Let’s go a little deeper. Assume that the first letter is the code supplied by the father and the second by the mother. “RR” means that both parents contributed the code for round peas. Rr means that the father contributed code for round peas and the mother for wrinkled ones. Work out the other two if the system is not crystal clear to you. Now, which code pairs make round peas and which make wrinkled ones? Remember that as long as there is one R in the code the pea will grow round. Only one combination (rr) can make a wrinkled pea.all the other three make round peas. That is what causes the 3:1 ratio of shapes. The visible traits of an organism (including this one for peas -- round or wrinkled) is called the phenotype. The code that grows the traits is called the genotype. A contingency table is a good way to illustrate this. The cells of the table effectively list all the possible combinations. Dad’s version R The gene from Dad can be either the dominant or the recessive version, and the same is true for the one from Mom. Notice that we are not talking about any particular genes in one offspring; we are talking about all possible versions in all possible offspring. R r R from both parents r R r r Mom’s version R There are exactly four possibilities in the offspring as shown. We’ll see them one at a time. Dad’s version R r from Dad and R from Mom. R r r R r Mom’s version R R from Dad and r from Mom. r R r R RR rR Rr rr r R r Mom’s version R r Dad’s version r from both parents So the possible phenotypes are: And they occur in a 3:1 ratio Mendel dubbed this system of coding for characters in two alternate states the principle of segregation. The code for round, in other words was a separate thing for the code for wrinkled – the codes were segregated. Mendel, of course, knew nothing of DNA – even its existence. We now know the architecture of the principle of segregation. When the homologous chromatids in the two gametes come together during meiosis the one from Dad has a version of a large number of genes on it. The one from Mom has versions of exactly the same genes, but different versions. This explains the part of Mendel’s first principle that says the traits come in two mutually exclusive versions. In characters like pea color and shape, one version will be preferentially expressed over the other in the phenotype – it will be dominant. This was the part of the principle that explained the ratios. If both versions of the gene were the dominant version (a) then the dominant version will appear in the phenotype. D D D D X d a D b Indeed, if either version is the dominant version then the dominant phenotype is expressed (b & c). Only if neither parental gene version is the dominant version then the recessive version will be expressed by default (d). D d a X D d d d a Mendel found a number of other traits in his peas that followed the principle of segregation. One was color. The peas (when dry) were either a bright green (as frozen peas are in the store) or greenish yellow. Given the picture below, which shows the typical ratio of phenotypes, determine which trait is dominant and which recessive. Then assign a code letter (the first letter of the dominant color – capital if dominant and lower case if recessive). Then list all the possible combinations and determine what the phenotype would be for each genotype. The possibilities are shown below. Note that in any four randomly selected peas you wouldn’t be able to tell what the genotypes of three of them are. (You’d always be able to recognize yy.) This figure just shows what the possibilities are. In a large population you’d expect equal numbers of all four genotypes, yielding the Yellow : Green = 3:1 ratio of phenotypes. yy yY Yy YY Y y Y YY yY y The contingency table and phenotype possibilities are: Yy yy Mendel also wondered if the traits were connected to each other in some way. Were green peas more likely to be smooth, or green ones more likely to be wrinkled? The solution to this he reached in a similar way: if they are independent then there should be a specific ratio of the four possible phenotypes (green/smooth, green/wrinkled/yellow smooth/yellow wrinkled) in the population after some number of generations. If they are connected then the ratio should be different from that – still a specific ratio, but a different one. What he found was that the ratio of phenotypes was exactly that predicted by the independent hypothesis. This is known as Mendel’s principle of independent sorting. To simplify the calculations Mendel started with peas he knew the genotypes for. His round yellow peas were RRYY (which he had to work to isolate) and his wrinkled green ones were rryy. He did not have to work at all to isolate these. Because wrinkled and green are both recessive then any wrinkled green pea must have this phenotype. We could go either way, but let’s assume that the Dad’s pollen was RRYY and the Mom’s ovum rryy. In the first generation the offspring will always get a dominant version of both genes from the Dad and recessive versions from Mom, so the only possible combinations are various shufflings of RrYy: RY, Ry, rY, and ry. Follow this through and make sure you see it. If these peas are then used as Mom (ovum) and Dad (pollen) then the next generation will have more genetic complexity and a contingency table is useful. And the phenotypes are: RY Ry rY ry The genotypes are: RY Ry rY ry RrYy Rryy rrYy rryy RrYY RryY rrYY rrYy RRYy RRyy rRYy rRyy RRYY RRyY rRYY rRyY Expected ratio: 9 : 3: 3 : 1 Mendelian genetics can only go so far. The traits that Mendel studied were the easy ones – the ones with complete dominance and perfect segregation. Many other traits are much messier, for several reasons. I’ll only mention a couple of very general ones: Some traits are influenced by more than one gene. Brown and blue eyes can be explained by Mendel’s simple model, but green ones result from the activities of other genes – how much brown gets mixed with the blue. A little and they’re green, more and they’re hazel, more and they’re just brown. Similarly, some genes code for more than one character – the characters are linked, in other words. In some cases this means that a dominant trait is nowt often expressed because it is linked to another trait that isn’t so great for the bearer. Mendelian models of how many genes in a population behave are very useful for things like predicting how genetic diseases would spread in the population, or how traits of a certain type would spread through the population under certain conditions. These population genetic models have been used in the past to study how evolution would proceed under various environmental scenarios. Indeed, it was the rediscovery of Mendel’s work in the early 20th century that led to the resurrection of Darwin’s hypothesis of Natural Selection as the driving mechanism for evolution in the “Modern Synthesis”. As it turns out, if “evolution” just means spreading some trait through an existing population (like, “the evolution of drug resistance”, or of “pesticide resistance” then the models work well. For those of us who think evolution encompasses the origin of new species and of higher taxa, population genetics turns out not to be so interesting. All the published examples of “natural selection” (including the one in your book) leading to evolution are actually not interesting because they don’t explain where the alternative traits came from in the first place. They only explain (very well) why one is more adaptive in some conditions and the other in some other conditions, and so why some populations have more phenotypes of one sort rather than the other. We’re are left to assume that the same thing that spreads them around differently also created them, which I think is not wise because it doesn’t fit the model of “critical thinking”.