Polygenic Inheritance

Polygenic Inheritance

Polygenic Inheritance

Polygenic Inheritance The final topic in our discussion on the principles of inheritance will be polygenic inheritance now in humans there are many different types of traits, for example,  

the high trait or the skin color trade these different types of traits are affected or controlled by many different pairs of alleles found on a different low sigh Polygenic Inheritance. 

so polygenic inheritance is this phenomenon that  basically describes this idea that multiple different allele pairs actually  affect and control and have an additive effect on a given type of trade, for example,  the high trait and the skin color trait now to demonstrate what we

mean by polygenic inheritance let's begin by assuming that the skin color  trait in humans is controlled by three gene pairs 

so we have gene pair B and we have gene pair C let's suppose that these three different gene pairs are on linked genes and what that means is they're located on different homologous chromosomes

 so let's suppose we have homologous chromosome pair 

1. Homologous chromosome pair 
2. And homologous chromosome pair 
3. Now in pair number one we have 

the traits 

so this can either be an uppercase a or lowercase a and this can also be  either an uppercase a or lowercase a now the second allele pair contains the B  gene and likewise,

 we can either have uppercase B or lowercase B on each one  of these individual chromosomes and finally 

we have allele pair Number 3  homologous pair Number 3 that contains the final gene 3 or gene C that  also controls and affects and have an ad and has an added effect on the skin  the color trait in human 

so we can also have uppercase e or lowercase C now what do we mean by an uppercase C and a lowercase C well notice that a B and C  are not complete dominance traits, in fact,

 they're in completely dominant genes and we'll see what that means in just a moment if you are not sure what we mean by incomplete dominance go back and watch my lecture on incomplete dominance and what it means for genes to be incompletely dominant 

so the capital of the capital letters represent incompletely dominant genes that Express  a dark color while our lowercase letters were present incompletely dominant genes that Express the light skin color now since each gene as we said earlier has an additive effect on what that means is the more uppercase letters that we have in a given person the darker the skin color of that person is

 so since each gene has an added effect on the skin color the more capital letters we have the darker the skin is and vice versa the smaller the Number of capital

letters are the lighter the skin color of that particular person

 so for example, if in a given individual all of these genes contain the uppercase letter 

so these are uppercase a these are uppercase B and these are uppercase C then we have uppercase and uppercase BB uppercase CC and this represents the darkest possible skin color and likewise, if all of these are lowercase letters the lowercase a lowercase B lowercase C then we have the lightest possible skin color now to further demonstrate how this added effect in polygenic inheritance actually works let's carry out the following experiment let's suppose we have a male individual that contains this genotype 

so we have these three pairs of homologous chromosomes and on each one of these individual chromosomes we all have uppercase letters 

so we have uppercase and on this set of alleles we have uppercase B on the second set of alleles an uppercase e uppercase E and let's suppose we mate this male individual with the female that is homozygous recessive for all of these they're from

genes 

so we have up to a lowercase an on the first set of chromosomes lowercase B in the second set and lowercase C lowercase enow if we made these two individuals then what we see is the offspring that  is produced that f1 generation will be uppercase a lowercase a and that's  because one of this G are one of these crows one of these chromosomes uppercase a will come from the male and the other one lowercase a will come from the female and 

so we'll produce uppercase a lowercase a the same thing happens here when meiosis takes place the sperm cell receives one of the uppercase B the Excel receives one of the lowercase B they combine to form uppercase B lowercase B and finally these combine to form uppercase e lowercase e and 

so exactly half of the letters are uppercase and what that means is since this is a very dark individual this is a very light individual because we have an intermediate case here this individual the offspring will have a skin color that is somewhere in between the dark skin color and the light skin color  because here we have six uppercase letters here we have zero uppercase  letters here we have one two three uppercase letters 

so that means three is somewhere in between six and zero and 

so, this will have an intermediate skin  color now let's suppose we take our individual the f1 generation and we made  it with another individual that has the same exact genotype for the skin color genes 

so what exactly will be the distribution of the genotypes of our offspring the f2 generation? 

so what we basically have to  do is this relatively long Punnett square 

so essentially we have  different possibilities for the egg cell and different possibilities for the

sperm cell 

so each one of these basically describes its own unique sperm  cell that contains its own unique genoa genotype, for example, this firm cell  contains all uppercase letters uppercase and uppercase B uppercase E and this one  contains uppercase an uppercase B and lowercase e and 

so forth and these are

Polygenic Inheritance

individual unique cases 

so just like in any Punnett square we basically have to  combine the egg cell with the sperm cell and we basically combine our genotypes 

so we have uppercase an uppercase B an uppercase C uppercase e to form this zygote 

so uppercase an uppercase B uppercase e uppercase e and we continue the process with all these different types of combinations, for example, let's suppose we look at this one 

so far in this particular zygote we combine an Excel that contains all  lowercase letters a sperm cell that contains all uppercase letters and we form three uppercase letters and three lowercase letters 

so uppercase a lowercase an uppercase B lowercase B and uppercase C lowercase C and we do this with every single one of these squares

 so once we carry out the process let's actually, count up the squares that have all six uppercase letters and notice if

we go through each and every one of these only ones of these zygotes so one  out of 64 possibilities will contain case 

so this is the only zygote the only  square that contains all uppercase six letters and

 so we place a six now, by the way, there are a total of 64 cases because 8 x 8 gives us 64 

now if we  continue the process and count up all the zygotes all the possibilities where  five letters are uppercase letters we're going to get 1 2 3 4 5 6

 so we see that

6 out of 64 possibilities will produce an offspring that has 5 uppercase  letters and we continue the process let's continue with 3 so in the case  of 3 we have let's see 1 we have 2 3 4 we have 5 6 7 we have 8 9 10  we have 11 12 13 14 15 16 17 18 19 and 20 to 20 out of 64 cases 

so there is a  likelihood 20 out of 64 that the offspring that is produced will have a  genotype in which 3 of the letters are uppercase and 3 of the letters are  lowercase if that means we're going to have an intermediate skin color and us  continue the process with two uppercase letters 

so we have one we have two we  have 3 4 5 we have 6 we have 7 8 9 10 11 12 13 14 15 so 15 out of 64 will have  only to uppercase if we continue with one upper case we'll see that we have 1  2 3 4 5 6 so 1 2 3 4 5 6 so 6 out of 64 we'll only have one uppercase letter and  finally just like there's a 1 out of 64 possibilities of having all uppercase letters there's a 1 in 64 possibilities of having no uppercase letters

and if we plot this if we create a bar graph of this distribution we get a  normal distribution 

so we basically get this normal distribution that looks like  a bell curve and 

so we have one out of 64 chance that six of them are uppercase  and one out of 64 that none of them are uppercase six out of 64 gives us the  likelihood that 5 will be uppercase and 6 out of 64 it gives the likelihood that one  will be uh that only one will be uppercase and we continue and notice  that this highest bar basically describes the greatest likelihood it  tells us that there is the greatest likelihood that the offspring produced  from these two mating processes will give us an individual that contains

three uppercase letters and three lowercase letters

 so it is most likely  that the offspring that is produced between these two individuals that have the same exact skin color that individual will also have that same skin  color 

so in this particular case, this is the f1 generation the f2 generation is this entire distribution here 

so from this distribution, we see that the f2  generation offspring will be most likely the same exact skin color as these two individuals as the parents 

so this is what we mean by polygenic inheritance 

so we see that polygenic inheritance refers to the phenomenon by which multiple different allele pairs have a similar and an additive effect on the given trait and by added we simply mean there is the genes are incompletely dominant with respect to one another so the greater the Number of uppercase letters is the darker the skin is and vice versa if we have less uppercase letters that means the light of the skin color will be

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