what is polygenic inheritance?
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| Polygenic Inheritance |
Polygenic Inheritance The final topic in our discussion on the principles of inheritance will be what is polygenic inheritance now in humans there are many different types of traits,
for example,
monogenic vs polygenic inheritance biology discussion human body
Polygenic inheritance skin color
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.
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
- Homologous chromosome pair
- And homologous chromosome pair
- 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 has 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,
what polygenic inheritance means
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 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
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 an 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
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
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
the 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
have zero uppercase letters here
we have one two three uppercase letters
genotype for the skin color genes
so what exactly will be this 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 cell and different possibilities for the
sperm cell
B uppercase E and this one contains uppercase an uppercase B and lowercase e
what polygenic inheritance means
so forth and these are individual unique cases
so just like in any Punnett square
we basically have to combine the cell with the sperm cell
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
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
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
we continue the process let’s continue with 3 so in the case of 3
we have let’s see 1
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
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
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
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 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 fewer uppercase letters that mean the light of the skin color will be