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Genetic Recombination and Linkage

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Welcome to MOOC–NPTEL course on bioengineering and interface with biology and engineers.In todays class we will study genetic recombination and linkage the production of off springswith combination of traits that differ from those found in either parental generations.Morgan in addition to making contribution for understanding the chromosomal basis ofthe inheritance he was also making many interesting fundamental observations. If you not thinkabout going back to meiosis at some point, we have said that you know the genes get recombinedand that is what gives lot of uniqueness among the progenies right among the child you seelot of differences because the genes get recombined.So, he was observing this that whether the genes recombination event happened or thereare some genes which are very closely linked to each other and they are not getting recombined.So, the production of off springs with combination of traits which differs from the parents whetherthose are you know passing together or they are getting segregated.So think about the classical Mendel thing first and then we come to Morgan. So, in Mendelianexperiment we have seen this parental type off springs and then when we did this dihybridcross, we also saw some recombinant off springs appeared> Recombinant means some new typeof features appeared along with the original combination of the characters. So, two non-parentalphenotypes were also seen in the off springs.And these are known as recombinant types or recombinance.So, if you remember from the meiosis image these chromosomes are exchanging a small segmentof it at a site which is known as crossing over. So, from this part from the male andfemale gametes they are certain change you know that is interchange is happening. Andnow you can see the recombinant A recombinant small a this part is actually transferred.So, all the genes which are part of this one has now come in this chromosome.And all genes from this part has come to this chromosome. So, this is kind of concept whichwe have briefly a seen in the meiosis part and when Morgan was trying to investigatethis in much more detail. So, just you know refresh you in recombination is an event whenthese genes are getting recombined and they are getting an exchange from these two chromosomesand at a site. which is known as crossing over site.Alright yeah, this part we have talked in the meiosis prophase 1 just to be preciseand we have seen that you know a set of proteins are getting exchanged a part of it which wesee a crossing over event. So, how does if the genes are linked or they are far aparthow that does that information can be transmitted from the one to next generation? And Morganwas always using the fruit fly as a model system and he wanted to study these investigationalalso in the fruit fly.So, lets now increase little bit more information from the fruit fly experiment. And in thiscase now we have two different types of fruit flies one is wild type which is having thegray body and the normal wings. And second is double mutant which is black body and thevestigial wings.Just to simplify this part if you make these kinds of crosses or two properties like thisthat if we are saying wild type it means it is + right. And we are showing the both thechromosomes so b+ and vg+ / b+ vg+ and we are doing a cross with double mutant. So,what is shown here you know b+ b+ it is showing on the both the chromosomes right. So, thisis how the cross should ideally look like.And so now if you take this situation that you have wild type with two features.And what you are looking at studying two genes and are they linked to each other or are theygoing to you know separate in the process of recombination. Now in the f1 dihybrid crossall of them are looking like the same the wild type feature what we had and now if youwant to test cross from this think about Mendelian test cross. So, what would you do the crossnow if you want to do test cross f1 whatever the phenotype shown here.This f1 you want to do a test cross so what will be the Geno type of the other fruit flyyou want to cross off this particular fruit fly and you want to do test cross. So, whatwe did for the test cross in Mendel pea experiment. Anybody recall Homozygous recessive.Is this a homozygous recessive? This is right alright well so now can we see this thing?This wild type of f1 and we do a test cross with Homozygous recessive with double mutant.We will do that.Write down this gamets and make the square now you can write these gamuts. So, now howwill this look okay now you try other. “Professor - student conversation starts” there arefour different types of eggs actually produce it will cross with We will discuss after doingthe cross first alright “Professor - student conversation ends” and now if you can readfrom here there is a gray body and normal wing.And this is black body vestigial wings. Could also write down what will be the phenotypeof these flies. Write down what are the phenotype of these files trust means wild type. “Professor- student conversation starts” yes so that I was telling in all of these problems goodidea to write b+vg+ and b and vg because they are like this because these are not differentchromosomes so ideally same thing.But when it + here it means it is wild type and if it is not + it is mutant. “Professor- student conversation ends” so we have now these four different types of phenotypes.And what we are observing here that one is gray normal one is black normal one is blackvestigial, and one is gray vestigial. So, in addition to the two which were originalphenotype what we had we have two different new phenotypes as well.So, what Morgan was trying to understand from the experiment whether these genes are veryclosely linked to each other or they are very much far apart.So, if you think about you know just imagine that on this chromosome if this part is gettingrecombined and these two genes which we are studying one is this part and one is you knowgoing to get separated it will be not linked. But if that these two genes are here, or thesetwo genes are here these are going to be a linked right here very close to each other.So, then if they are close to each other.So, even if they are getting recombined is still, they are transporting together. So,therefore these two genes you know a higher proportion of this looks like they are linkedto each other. But there are some progenies you know some flies which are also found offother phenotype which may be that you know some recombination event might have happened.So, he was the first time we were looking at not only that how genes are being transmitted.But where they are located on the chromosomes and are, they linked to each other or not.So, this is where he was looking at the linkage phenomenon.And what he concluded that most off springs had parental phenotype. Whereas very smallnumber relatively very small number of the flies were also having the non-parental phenotypewhich means that you know there might be some mechanism which break this particular typeof linkage. And some recombination events might happen which is being transmitted fromone to next generation which happens usually during the meiosis process.Alright let us take a short break and let me show youa clip. Hi, is this is your son yeah (Video starts 10:30 – Video Ends 11:35) (FL) Okayalright it will not be quick question but what is this movie? Paa very good and whatis the character? What is this character? Character name. Auro. And what is this diseasecome from which syndrome? That will be the question. Alright so you know I am sure youknow you have appreciated.When you would have at least watched the trailer of this movie that you know there are somechild who might be suffering from these kinds of chromosomal glitch to say. And those sometimecould just be because of one gene defect and in this case, it was based on a Lamin A genewhich is defective. So, there are many diseases which happened because of certain alterationshappening at the chromosome level.And lets kind of not a discuss some of these kinds of genetic disorders which are happeningbecause of abnormality in the chromosome number or the structure.Certainly want to study about all the chromosomes. If you think about at the some part developmentalstudy, we talk that you want to map the karyotypes you want to look at their all the chromosomepatterns and it is just color coded differently for each pair of chromosomes. So, it is possiblethat. You know while various recombination events are happening in various genetic eventsthat are happening some part of chromosomes get structurally changed.And it do not happen in always do not happen very often. But once in a while it happensand those may happen.And those may happen in a small part of the chromosome get deleted and in this case thisD part is deleted. So, this is known as deletion of chromosome so all the genes which are apart of that part of the chromosome is going to be absent or it can be inverted. For exampleif the gene sequence is ABCDE. Now BCD became inverted so now you have ADCB so many of theactually the cascade of the gene signaling will get changed.Because of this inversion part or a segment of it like B C in this case here gets duplicated.So, you know you have two copies of B and two copies of C.For most situation is translocation a part of these chromosomes move from one to otherchromosome like AB in this case moved to MNO and MNO moved to A B and so now they becamevery different characteristics or sometime it can be nonreciprocal it means it is justgoing to get attached to other chromosome not the vice versa. So, there are many thingswhich can happen at the chromosomal structural level.And these are all different types of operations which result in to different type of diseasesdifferent disorders.For example you see this child here suffering from Down syndrome. One of the very commonlyfound syndrome which is having three copies of the 21st chromosome which is known as trisomy.Trisomy which means 3 copies of the chromosome for Down's syndrome is usually the resultoff an extra 21st chromosome. So now they are total chromosome number 47 so there isa disbalance and these trisomy of 21 will not people do test for this.You know at the prenatal level for testing whether the features might be getting thissyndrome or not.So, another syndrome which is something very similar which is trisomy of 18 chromosomethat is known as adverse syndrome.You can see this particular phenotype here Turner Syndrome which is based on the absenceof the certain genes from the x chromosomes.And now here you can see a phenotype for Patau syndrome which is a trisomy of chromosome13 so three copies of chromosome 13 is present in this case. So, many of these things arein know some abnormality which happen because of the way chromosome would have shown somechanges at the structural level.And we have seen this example that even sometime as specific gene like Lamin A gene in thiscase having some mutation and as a result resulted into this Hutchinson Gilford ProgeriaSyndrome and then this kind of child is being born which now people can do genetic testingand try to find out these kinds of genetic abnormalities beforehand. And that is whereour understanding of molecular biology.And lots of genetic tools are becoming really important in medical field.But what happened at the plant level so many times plants also show this kind of changesbut not always this changes are detrimental for plants. So, in plants what is more commonoccurrence is that certain chromosomes just totally get duplicated or they make 4 copiesor 6 copies or 8 copies and this phenomenon is known as polyploidy. You have multipleploidy level of those chromosomes.So, rather than only 1 chromosome making you know a third copy or 4 th copy the entirechromosome set has become duplicated. And this is what is shown you here that 2n 4nor 6n of the whole chromosome sets.Look at this particular watermelon I am sure you would love to eat watermelon without seedsright that is seedless watermelon and those are actually polyploids those are having extraset of chromosomes. And sometimes those results in to beneficial properties in plants.For example many of the high yielding wheat variety those are head haploid wheats, soit is not always that you know if you have some extra chromosomes that are going to havesome problems in this case it is much more advantageous. But what is important here tonote in plants we have a full set of chromosomes getting duplicated and that is usually ismuch more easy to retain for the cell as compared to only one chromosome getting extra.So, that is where I think you can see these are still giving us some positive resultsin the plants.Let us now solve few sums for genetic problems based on this concept but before that thesequestions will help you revise your concepts. If you do not get this right, please referback to the s before you begin the problem sums.Determine the sequence of genes along a chromosome based on the following recombination frequencies.A-B 8 map units; A-C 28 map units A-D 25 map units and B-C 20 map units B-D, 33 map units.The farther apart two genes are what could be the probability that a cross over willoccur between them and therefore what will be the recombination frequency?Chromosomal abnormalities occur due to missing, extra or irregular portion of ChromosomalDNA. Which of the following would not disrupt genetic balance?Let us now begin with some problems based on chromosomal basis of inheritance. My TAwill assist you in this process.In some dogs, the gene for calico (multicolor) is codominant Females that receive a B andan W gene have black and white spots on brown coats. Males can have black or white spots,but never calico. Here is what a calico females genotype would look like: X B XW. Show thecross of a female black dog, with a male white dog. What percentage of the pups will be calicoand female?Red- green color blindness is caused by a sex - linked recessive allele. A color blindman marries a woman with normal vision whose father was color- blind. What is the probabilitythat they will have a color–blind daughter? What is the probability that their first sonwill be color blind?A man with hemophilia which is a recessive, sex linked condition has a daughter of normalphenotype. She marries a man who is normal for the trait. What is the probability thata daughter of this mating will be hemophiliac? That a son will be hemophiliac? If the couplehas four sons, what is the probability that all four will be born with hemophilia?Now let us solve two sums based on genetics linkage.Assume that genes A and B are linked and are 50 map units apart. An animal heterozygousat both loci is crossed with one that is homozygous recessive at both loci. What percentage ofthe offspring show phenotypes resulting from this crossovers?Two genes of a flower, one controlling blue B verses white b petals and the other controllinground R verses oval r stamens are linked and are 10 map units apart. You cross a homozygousblue oval plant with a homozygous white round plant. The resulting F1 progeny are crossedwith homozygous white oval plants and 1000 F2 progeny are obtained. How many F2 plantsof each of the 4 phenotype do you expect?So, in the conclusion in today class we studied recombination of unlinked genes and the independentassortment of chromosomes the concept of crossing over and recombination of linked genes. Wealso looked at how linkage between the two genes affect inheritance of characters basedon Morgan experiments on fruit flies. We then discussed a few chromosomal abnormalities.And how alteration of chromosome numbers or structure can cause some genetic disorders.In the next class we will understand the molecular basis of inheritance with the help of someclassical genetic experiments. Thank you and see you next week.