Gene Mapping, Percent Recombination and Map Units

gene mapping is the process that involves finding the positions of genes on chromosomes and also determining what the distance is between the genes on a given chromosome now typically in genetics and biology we express the distance between any two genes on a given chromosome by using special units known as map units or recombination units so these two terms are basically used interchangeably they mean the same exact thing now as we'll see in just a moment to actually calculate the map units the distance between our two genes in map units we have to calculate the percent recombination between those two genes so to see exactly what we mean by that let's take a look at the following example in this example we're going to discuss how to calculate the percent recombination between two genes and how to use the percent of recombination to find what the map units are what the distance is in map units between those two genes so let's begin by taking a look at the following diagram so in this diagram what we're basically doing is we're taking two types of fruit flies so in this example we're going to study fruit flies now we're going to study two types of traits and we're going to assume that the traits are in fact linked so we're going to study the color trade which is linked to our wing type trait so we have two types of colors and two types of wings we have the color gray which is dominant over the color black and the color gray is given by uppercase uppercase G the color black is given by lowercase G by the same exact token we have two types of wing times we have normal wings and we have vestigial wings now normal wings which are functional are given by uppercase n and vestigial wings which are non-functional are given by lowercase ad so in this example in this experiment we're basically mating a female individual that is homozygous dominant for the color trade and homozygous recessive for the wing trade with a feet are with a male individual so we're mating this with a male individual that is homozygous recessive for the color and homozygous dominant for the wing type now in a what is the genotype of the f1 generation offspring that is produced when we make these two individuals to actually determine what the genotype is we first have to answer the question what are the gametes produced but what are the gametes that are produced by these two types of individuals so let's begin with our female individual so we have uppercase G uppercase G lower case and lower case and so upper case G upper case G lower case and lower case n and before they actually mate they have to produce our gametes the sex cells and in this case because we have female these are going to be excels now because we're assuming these genes are linked that means they're located on the same exact chromosome and in this particular case if you carry on the process of meiosis we only have one type of gamete that can actually form and that Gami will have a chromosome that contains an uppercase G a lowercase n so this is our Excel and this is equivalent to basically redrawing it in the following diagram so we have this chromosome that contains lowercase and G and uppercase G lowercase and gene and uppercase G gene and so 100% of our gametes will look like this okay now we're crossing it with a male that is lowercase G lowercase G uppercase and upper case and and likewise by the same exact reasoning if we carry out the process of meiosis we'll see that only one type of gamete can actually be formed in this particular case in fact 100% of the gametes will have this genotype as shown so lowercase G uppercase and so two sperm cells so let's designate that with this squiggly line so we can either designate it this way or by using the chromosome symbol so we have lowercase G uppercase n okay so one hundred percent of these genes will basically look like this so this produces this sperm cell this produces digs are this Excel when they combine to form the zygote we basically form M so this chromosome combines with this chromosome and we form the following zygote that contains uppercase G lowercase n so uppercase G and lowercase n and then we have lowercase G uppercase and that comes from this right so we have lowercase n so let's write that like so I'm sorry uppercase n then we have lower case n here so lowercase and uppercase G that came from the female and lowercase G that came from the male and so this will be the genotype of all the offspring produced in the f1 generation so f1 generation genotype okay now let's move on to Part B in Part B when we make an f1 generation female so what that means is we take a female that has the same genotype as the f1 generation and what that basically means is this is the f1 generation so we have a female that has a genotype that is uppercase G lowercase G uppercase and lowercase M and we make this with a homozygous recessive male that is homozygous recessive for both traits so homozygous recessive for both traits means we have lowercase G lowercase G lowercase and lowercase M so when we mate or cross an f1 generation female this individual here with a homozygous recessive male this individual here we obtain 2,000 offspring so we have 2,000 individual fruit flies now if we assume that the traits the color train and this wing type trade are lengths that means they are located on the same chromosome but we assume no crossing over actually took place what will be the expected genotype distribution between those 2,000 offspring that are produced so basically the entire point of Part B is to note that no genetic recombination actually takes place because no crossing over takes place so once again to determine what the genotypes of the offsprings are we have to find what the gametes that are produced are so we have two types of gametes in this particular case the question is why well because no crossing over actually took place and what that basically means is the same gametes that were combined to produce this f1 offspring so namely this guide me and this guy me will be produced in this particular case because no new recombinant gametes are actually formed because no crossing over actually took place and so when meiosis actually takes place so we replicate these then they divide to form haploid cells those haploid cells divide what we form our a gammy that contains uppercase G lowercase M and a gamete that contains up a lowercase G uppercase n so one of these gametes will contain a chromosome that has uppercase G so uppercase G lowercase n right why well because this individual contains these two chromosomes meiosis takes place separates them and so we have uppercase G lowercase n and then this one has the other one lowercase G uppercase and so lowercase G uppercase n and so 50% of the gametes will have this unit I'm and the other 50 will have that genotype so 50% this 50% that now in this particular case things are quite simple because we only take we only form one type of sperm cell that contains uppercase G lower lowercase G lowercase n so lowercase G lowercase n and that is 100% of the offspring and so this always forms this but this can form too now if this combines with this what we basically form is offspring number one that contains well we basically have uppercase G lowercase G or um it should be uppercase G with this green color and lowercase G with this green color and then lower case and lower case n so lower case and lower case n the second type of offspring that is produced is so if this combines with this we have lower case G lower case G upper case and lower case n and because this is 50% and 100% so 0.5 times 1 gives us point 5 so 1/2 of the 2,000 or 1000 of the offspring will have this and the other thousand are going to have this genotype right over here so a thousand of the offspring will have this genotype here the other thousand will have this genotype here now what exactly is the phenotype of this well upper case G is dominant over lowercase G so that means we have gray wingless because we're going to have vestigial non-functional wings and then we have lowercase G lowercase G is black and uppercase and lowercase n is normal wings because uppercase n is dominant over lowercase and so we have functional wings so a thousand are gray wingless the other thousand are black and winked now this is if we assume that no crossing over took place but crossing over does normally take place and that's exactly what we discuss in part 3 suppose that the actual f2 distribution was as follows instead of having this hypothetical distribution because crossing over does take place we produce this distribution so notice that now not only do we have the a and wingless and the black and winged as we have in this case we also have the gray winged and the black winged and these two here are actually the recombinant offsprings and they're produced as a result of crossing over as a result of the production of recombinant chromosomes so we are given that 8 9 5 are grey wingless 9:05 our black wing but 110 our gray wing and 90 are black wingless to make a total of if we sum these up we obtain 2,000 offspring so the question is what is the recombination frequency between the two traits the color traits and that wing type trait and to find the recombination frequency also known as percent recombination what we basically do is we sum up all the offspring that are recombinant so 110 plus 90 so 110 plus 90 this is the total number of offspring that are recombinant and we divided by the total number of offspring so 2,000 offspring and what we get is 200 divided by 2,000 and that gives us once we reduce it to 110 and that's equivalent to 0.1 now this is our recombination frequency and to find the percent recombination we basically multiply 0.1 so we multiply 0.1 times 100% and we get 10% is our percent recombination between those two genes it's basically it basically tells us how many of those offspring are a result of the process of crossing over now we can use this to calculate what the recombination units are or the mapping units and to basically do that we have to remember that one map unit or one recombination unit is equal to 1% 1% recombination so we have 1% and so by using this ratio this proportion we know that because we have 10% recombination that means we have 10 map units or 10 recombination units between those two genes so what that means is if we examine our chromosome okay along that chromosome we have let's say this gene right here that is

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Keywords: gene mapping, percent recombination, recombination frequency, map units, recombination units, map units example, recombination units example, recombination frequency example, determining recombinationg frequency, genetic recombination

Id: asNgHpOuJmY

Length: 16min 9sec (969 seconds)

Published: Fri Jan 09 2015