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So with all this variation going on .... is there any way of predicting what we might inherit?? Yes - thanks to an Austrian monk called Gregor Mendel. Thanks to him we now have "Mendels laws of inheritance"
Step 1 : Time for a bit of a history lesson - he was born in 1822 - lets go and see what he did that gave him the title "Father of modern Genetics" (Learn)
Mission 1 : This should be revision ..... but have ago at the questions on Mendel's Peas - If you stumble over some of the terms - dominant, heterozygous etc then remind yourselves by watch the following clips. (Learn)
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Step 2 : In the Bozeman Science clip above he mentions Punnett squares - once again it should be familiar to you. They are the most useful tool for predicting inheritance. Notice that it says useful for predicting = a Punnett square indicates the likely possibilities ..... but as each fertilisation is a random event, actual results will differ from what is predicted. The more results you have to work with the closer to the predicted ratios you'll get - that's why Mendel grew 1000's of pea plants.
If you are a bit rusty watch the following. (Learn)
Mission 2 : Have a quick practice run - as we are going to be using Punnetts a lot in this topic. (Learn)
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Step 3 : Now you have these letters in your Punnett square ( the genotype) you need to be able to translate them into what it looks like (the phenotype). Need some reminding - it does take a bit of practice = its also a common exam problem type. Have a look at this. (Learn)
Mission 3 : Your turn to try and create some genotypes and then figure out the phenotypes - it is an important skill so take the time to get confident with it. Make sure you get your answers double checked when you complete the genotypic and phenotypic ratio review work. (Learn)
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Step 4 : If something is pure or true breeding for a particular trait = then you will only get that trait no matter how many crosses you do = its homozygous for that trait - dominant or recessive as long as its homozygous you'll only get those features showing up. This can be quite useful as we'll see later on.
Here is someone else explaining that too - if you need more clarification. (Learn)
Mission 4 : Its more Punnett square practice for you - and genotypes to phenotypes - see what happens when something is pure breeding. Complete the work on pure bred peas. (Learn)
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Step 5 : This is the kind of puzzle question that very commonly turns up in exams - how do you determine the genotype of an individual? We need to use something called a test cross (or a back cross). It makes use of an individual that is homozygous recessive for a particular allele that we we want to test for ..... we know it must be pure breeding for that allele. if it shows the dominant trait it could be either homo or heterozygous. But by mating the known with the unknown we can determine the genotype of the unknown. A farmer I know used this method to prove that his neighbours bull had jumped the fence and ..... well the calves that arrived weren't what he wanted to have.
Sometimes it easier to see in action than to read about it. (Learn)
Mission 5 : More Punnett practice as you do some test crosses in your workbook. (Learn)
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Step 6 : In my family we have a history of left handedness, heart disease and baldness. I've been able to figure out where they come from (do I blame Mum or Dad or both) and how likely it is that any will effect me and my kids. I've been able to do this by constructing a Pedigree chart - a useful tool = I'm not worrying about left handedness or baldness but am being careful about my heart.
Have a look as Pedigree charts are explained to you. (Learn)
Step 6 : Here it comes - yes more Punnett practice and some pedigree chart creating for guinea pigs and rats. (Learn)
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Step 7 : Up until now you've only been dealing with dominant and recessive ..... unfortunately its not always that straight forward. Time to meet incomplete dominance = this is where the heterozygous individuals are a blending or combination of both the dominant and recessive alleles. Plus we can't forget its "colleague" co-dominance where we have a gene with more than one dominant allele. In this case both the dominant alleles are seen = NO blending.
You need to be clear on the difference between the two.
These clips should help too. (Learn)
Mission 7 : So are you going to be in-between or co-operating on this? There are some pages of practice questions on both incomplete and co-dominance for you to work, and yes more Punnett square practice. (Learn)
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Step 8 : Okay so far we've had "simple" dominance, in-complete dominance and co-dominance - with me so far? Also we've only had 2 alleles - if it was simple that would always be so - sorry its not always "simple". We have cases where we have multiple alleles - he most common one that you will be a bit familiar with is blood type = what type are you? O, or A, or B, or AB ..... A and B are dominant over O, but are co-dominant with each other ..... with me so far? Shall I add in the Rhesus factor + or - that each type has eg I'm O -
Here is a look at that explained visually. (Learn)
Mission 8 : Before we had DNA "fingerprints" lots of paternity disputes were resolved by using blood types - could they be fully resolved? Your turn to be the detectives/doctors - try out the multiple allele and paternity problems - and a few more Punnett squares. (Learn)
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Step 9 : Now hopefully you now comfortable with what a"normal" monohybrid cross phenotypic Mendelian ratio is = the classic 3:1 ratio. If this is isn't appearing then something a bit interesting is going on. (think of the cases we've just met) There is an interesting case called a lethal allele - where if you get this in the lethal combination = its fatal = often even before birth.
Here is an example to look at .... (Learn)
Mission 9 : Time for you to look at a common lethal allele example - ever seen a Manx cat? (Learn)
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Step 10 : Up until now you have been working with monohybrid crosses = 1 gene, 2 alleles. The next step is 2 genes, 4 alleles = the dihybrid cross. This also means a step up in the Punnett square department too. You'll be working with 4 letter genotypes = 2 letter gametes = lots of combinations.
Its easiest to see in action - here is monohybrid first leading into dihybrid. (Learn)
Mission 10 : This is a lot more Punnett practice - but with dihybrid crosses. First a practice then a pea problem, then guinea pigs, before rabbits and finishing with a dihybrid test cross example (= same as monohybrid just with 2 sets of recessive alleles = homozygous recessive for both genes).
When ever you get the chance - use a Punnett square = a "Picture" saves a thousand words ... (Learn)
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Step 11 : Final (?) complication - remember back to Meiosis and Variation and we had "segregation" where the different alleles ended up in different gametes. Well what would happen if they didn't? What if the alleles being looked at were on the same chromosome = they are less likely to under go segregation = not be separated = this is called being linked genes. The closer they are on the same chromosome, the more likely they are to be linked = less likely that crossing over willseparate them. It messes with the "classic" Mendelian ratios = no 9:3:3:1 but something else. We also get sex linked traits = they are linked on the sex chromosomes - more common for boys to be colour blind than girls.
Once again its easier to see than just read .... (Learn)
Mission 11 : Okay see how much of that made sense by completing the questions on linked genes. (Learn)
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Here is a review power point slide show that covers all of this section and more - should fill in any gaps and add extra knowledge (Learn)
Inheritance.pptMission 12 : At the end of this section of your workbook you'll find (maybe) a puzzle page or two, a review planning page then a practice copy of the end of topic test. Complete these pages plus don't forget the definition page found at the start of the topic = you'll be ready for the test. (Learn)
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Predicting inheritance review questions - you should have completed these by now = we'll go over the answers together. This should give you an idea of where you are at with your understanding of Predicting inheritance = let you know what you need to do the get ready for the practice exam at the end of the whole section on Genetic variation and Change.
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