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Unformatted text preview: Bio 105 Guide to Concepts & Problem Solving The basics The physical process of meiosis underlies Mendel's laws, so you need to know how it all works. If you understand how meiosis occurs, you can explain a great number of the concepts we've covered in class. Mendel's 1st Law of Segregation During gamete formation (aka meiosis), the parent's alleles get segregated. That is, each gamete contains only one of the parent's two alleles. Mendel's 2nd Law of Independent Assortment Alleles of different genes assort independently during gamete formation (meiosis!). In other words, where one gene's alleles end up do not affect where another gene's alleles end up. Punnett squares are one way to depict Mendel's laws. You should be intimately familiar with them and be able to do classical Mendelian Punnett squares in your head by now. Transition to pedigrees Alleles can have a dominant or recessive phenotype, and they can be found on autosomes or one of the sex chromosomes. This means there are four combinations for any given allele: 1. Autosomal dominant 2. Autosomal recessive 3. Sexlinked dominant 4. Sexlinked recessive Note that sexlinked refers to either X or Ylinked genes. These traits get inherited in different patterns, as seen by the pedigrees we've done. Each type of allele behaves differently: Autosomal Sexlinked Males and females should be affected with equal If Xlinked, more affected males than females; frequency affected males transmit the gene to all daughters and no sons If Ylinked, males are exclusively affected Dominant Recessive Dominant Recessive All affected males have Males are hemizygous If rare allele, see two Every affected person for the trait unaffected parents with affected mothers has at least one one/few affected affected parent child/ren Two affected parents have all affected kids The reasoning behind the various modes of inheritance goes back to Mendelian genetics. Convince yourself why the statements in the table make sense. Knowing them is also how you solve pedigrees. 1 Extensions to Mendelian genetics Not every trait behaves according to the `dominant / recessive' binary. There are many traits that get inherited in a Mendelian fashion, but their phenotypes don't quite add up that way. When alleles are codominant, it means that there is more than one dominant allele for a given gene. An example is the triallelic ABO blood group. The A and B alleles both produce a sugar on the surface of the red blood cell, while the the O alleles produces no sugar. A and B are considered to be codominant because an AB individual will have both A and B type sugars on their RBCs. The A allele doesn't mask the effect of the B allele, and vice versa. Alleles can also exhibit incomplete dominance, which means that the dominant and recessive alleles blend phenotypes. The classic example is 4 o'clock plants. Red flowers (R) are dominant to white flowers (r), but an Rr plant has pink flowers. Here there is overlap between phenotypes, whereas with codominance both phenotypes are expressed together. A visual example I just thought of while changing words to bold and italic type... You have a bold allele and an italics allele. In the heterozygote: codominant incomplete dominance I hope that makes sense Recessive lethality is a concept that confuses. I make sense of it by regarding lethality as a second phenotype, with the first being agouti coat color, red eyes, montezuma coloring, etc. My mantra: dominant for phenotype, recessive for lethality. This means that the heterozygote will be viable and have the agouti coat, red eyes, etc. The homozygote, however, will not be viable, so you don't see these progeny (it is recessive for the lethality phenotype). Again, all of these Mendelian `extensions' do NOT change the way the alleles are inherited. Segregation and independent assortment are still happening here; the difference is in the F2 phenotypic ratios. If you have two otherwise `normal' alleles and you see nonMendelian F2 ratios, most likely it's one of these special cases. For solving problems... Codominance and incomplete dominance o crosses here generally start with true breeding parentals and F1 x F1 crosses o heterozygotes have their own phenotype distinct from the homozygous parentals o the F2 phenotypic ratios are still 1:2:1 for one gene Recessive lethality o you can't have a true breeding organism bearing two lethal alleles here, so the informative cross here is two heterozygotes (e.g. Ee x Ee) o that cross will get you a 2:1 phenotypic ratio why? because the EE is recessively lethal and dies On the off chance you get asked about two genes, and only one of them falls under these special cases, remember to deal with one trait at a time! The genes are still assorting independently. But not for long... 2 Linkage it gets more complicated Up until now, Mendel's laws have held true. But if you understand meiosis (and you should), you might ask whether independent assortment occurs for genes located on the same chromosome, since that's what's really being passed onto the gametes, not individual genes. It turns out that genes on the same chromosome do NOT assort independently. Actually there is a range of how independent the assortment is, and that depends on how far apart the two genes are on the chromosome. This distance (one might call it a map distance...) separating two genes is important because there is a physical exchange of DNA between sister chromatids during prophase I (one might call this recombination...). This makes things a bit complicated, but it's nothing we can't handle. There are four scenarios to ponder: Tightly linked two genes are so close they behave as one gene (RF = 0%) AC AC/AC x ac/ac AC/ac (F1) x ac/ac (backcross) F2: AC/ac, ac/ac (two genes inherited as a single unit) Linked recombination occurs between the two genes (RF between 149%) AC AC/AC x ac/ac AC/ac (F1) x ac/ac (backcross) F2: parental AC and ac gametes with equal freq, recombinant Ac and aC gametes with equal freq `Unlinked' far apart on the same chromosome (RF "equals" 50%) AC AC/AC x ac/ac AC/ac x ac/ac F2: expected Mendelian ratio of gametes: Ac and aC, AC and ac all equally frequent Not linked two genes on different chromosomes (RF = 50%) A, C AACC x aacc AaCc (F1) x aacc (backcross) F2: same as `unlinked' parental AC and ac, recombinant Ac and aC with equal freq The last two scenarios will generate the same proportions of F2 progeny. This means that if you have 50% recombination, you can't tell these two apart (hence the quotation marks). Why can't you get >50% recombination frequency? (I'm asking you. This is a great exam question...) Solving the 3 point cross The short version 1. Identify the two parental combinations of alleles 2. The two rarest classes usually represent the DCOs 3. Using 1 and 2, you can establish the gene order, i.e. you can determine which gene has to be in the middle to generate the DCO progeny seen 4. When calculating map distance, remember: 1 DCO event = 2 SCO events 3 The long version These are difficult because you have to know how several of the previous concepts work in order to make sense of the information you get from a 3 point cross. Fortunately, it's fairly easy if you understand those concepts. Here's a general list, in order, of what you need to know or derive to solve a 3 point cross: 1. the F2 spread (how many, and what they all look like) 2. the parental phenotypes 3. the parental genotypes (on each chromosome!) 4. the recombinant phenotypes (both SCO and DCO) 5. the recombinant genotypes (both SCO and DCO) 6. the gene order 7. recombination frequency between the three genes 8. interference, if any All 3 point crosses start you off with some part of #13. #3 and #4 are the two critical things you must find out. You can figure out everything else if you know the parental genotypes and recombinant phenotypes. Identifying the parentals and recombinants: P > SCO > DCO The parental phenotypes are usually the most common in the F2 spread. The SCO progeny are found less often than the parental, and the DCO are the rarest progeny. An aside: if you see another pair of phenotypes with equal frequency, this should tip you off that the gene involved in this second pair is not linked to the other genes. Rewrite the F2 data If the various pairs of F2 progeny aren't conveniently written next to each other, rewrite the data so the parentals are grouped together, the SCOs, etc. The way you figure out which are the right pairs is simple the two phenotypes in the pair will be `complementary' to each other. That is, if one looks like A B c, then its partner will look like a b C. This is because recombination creates two types of gametes which must `add up' to the original parental gametes. Gene order You can determine gene order once you have the parental genotypes and DCO phenotypes. In the DCO progeny, you know that two crossovers occurred. This means that whichever gene got recombined is located between the other two. Think of it as driving on a freeway. You know that you switched freeways twice, and that you started and ended on the same freeway. This means you must have switched somewhere in the middle. It's the same with a DCO. There's one gene that's been switched from the parental, and It has to be in the middle of the other two. Recombination frequency and interference Finding RF values is plug and chug RF = # recombinants / total F2's. Just remember to count in the DCOs once for each of the SCOs. Interference = 1 (# observed DCOs / # expected DCOs). This tells you how much effect one crossover event had on subsequent crossovers on that chromosome. 4 ...
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This note was uploaded on 04/07/2008 for the course BIO 105 taught by Professor Sullivan during the Spring '08 term at University of California, Santa Cruz.
- Spring '08