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POLYGENIC INHERITANCE AND MULTIPLE ALLELISM
Introduction:
The phenotypic traits of the different organism may be of two kinds, viz qualitative and quantitative.
Qualitative traits are classical Mendelian traits which follow the basic Mendelian pattern of
inheritance, such as round or wrinkled seed of pea. Each qualitative trait may be under genetic
control of two or many alleles of a single gene with little or no environmental modifications to
obscure the gene effect. The organism in qualitative pattern is said to exhibit discontinuous
variation.
The quantitative traits however, are important measurable phenotypic traits of degree such as
height, weight, etc. These are metric trait which shows continuous variation. In contrast to
qualitative traits, the quantitative traits may be modified variously by the environmental condition
and are usually governed by many genes, each contributing such a small amount of phenotype that
their individual effect cannot be detected by Mendelian methods but only by statistical methods.
The genes which affect the phenotype of a single quantitative trait are called polygenes or
cumulative genes. The inheritance of polygenes or quantitative traits is called quantitative
inheritance, multiple gene inheritance or polygenic inheritance. The term polygene was coined by K.
Mather in 1941.This pattern of inheritance i.e. large number of genes, each with a small effect,
which segregates to produce quantitative variation has long been the basic model of quantitative
genetics. Among the varieties of more complex situations, the most common one is multiple alleles.
The term "multiple alleles" refers to one pair of alleles per person, but several possibilities within the
population of that species. The term "polygenic inheritance pattern" refers to several genes at
different loci, or chromosome locations, or genes on more than one chromosome pair, each
contributing to the trait.
Characteristics of Polygenic inheritance:
Polygenes for quantitative traits have following characteristics:
• Each contributing allele in the series of multiple genes produces an equal effect.
• Effect of each contributing allele is cumulative or additive.
• There is no dominance. The alleles at each gene locus lack dominance and each active allele has an
effect on the phenotype that is small and equal to that of each of the other.
• There is no epitasis (masking of the phenotypes) among genes at different loci.
• There is no linkage involved.
• The environmental conditions have considerable effect on the phenotypic expression of polygenes.
• Active alleles affecting the trait. Phenotype is determined by the sum of all the active alleles
present in the individual.
Finally, polygenes are not qualitatively different from other gene.
A polygenic trait varies in populations, as our many shades of hair colour and body weight. Some
genes contribute more to a polygenic trait than others. Within genes, alleles can have differing
impacts depending upon exactly how they alter an encoded protein and how common they are in a
population.
Polygenic traits can also be multifactorial, which means they are influenced by the environment.
Pure polygenic traits that not influenced by the environment are very rare. Multifactorial traits affect
more than 1 in 1,000 individuals and include height, skin colour, body weight, illnesses, and
behavioral conditions and tendencies. A more popular term for “multifactorial” is complex, but we
use multifactorial here because it is more precise and is not confused with the general definition of
“complex.” The genes of a multifactorial trait are not inherently more complicated than other. They
follow Mendel’s laws, but expression of the genes is more difficult to predict because of the
combined actions of genes and the environment.
A polygenic multifactorial condition reflects additive contributions of several genes. Each gene
confers a degree of susceptibility, but the input of these genes is not necessarily equal. Different
genes may contribute different aspects of a phenotype that was once thought to be due to the
actions of a single gene.
Skin colour is one of the best example of explaining polygenic inheritance. We know that human skin
colour is different because of presence of melanin pigment. The more the melanin pigment the
darker the skin colour. And the production of melanin pigment is controlled by gene. Assuming that
3 genes i.e. 6 alleles occurring at 3 loci controls the melanin production, let us assumed that the 3
dominant ABC controls dark pigmentation because more melanin is produced by ABC. On the other
hand the recessive alleles of these 3 genes abc controls the light pigmentation because lower
amount of melanin are produced by these alleles. Finally, a genotype with dominants AA BB CC
which occur at 3 different loci has the maximum amount of melanin and will have a very dark skin
colour. On the other hand the genotype with all recessive small letters genes i.e. aa bb cc has the
lowest amount of melanin and have very light skin colour. Now further assuming that a male with
the genotype AA BB CC marries with a female having the genotype aa bb cc. So when they get
married the gamete produced by male will be A B C only and cannot produce a b c. because his
genotype has only AA BB CC. Similarly the female can produced the gamete having a b c. Now, when
the gamete fused together to form the zygote, the F1 generation offspring will have Aa Bb Cc. So a
cross between white gentleman and a black lady, the skin characteristic of the offspring will be Aa
Bb Cc i.e. carrying 3 alleles for dark skin colour and 3 alleles for light skin colour. Now, when such
two individuals carrying Aa Bb Cc get married then the possible gametes are eight i.e. ABC, Abc then
AbC, ABc, aBC again aBc, abC and finally abc. So, these are the possible gametes that can be
produced by male and the same set of gametes can be produced by female. So, when these gametes
fused together to form zygote there may be 64 possible offspring with 7 different phenotypes. Now
these phenotypes are in such a manner that a particular phenotype may have no capital letter in
each genotype aa bb cc. And one type of phenotype may have only one capital letter and there will
be such 6 cases. And there may be as many as 15 individuals with two capital letter genotype. And
there may be 20 individuals with 3 capital letter genotype. Similarly there may be 15 individuals with
4 capital letter genotype. Likewise there may be 6 individuals with 5 capital letter genotype. And
there may also be only one individual having all the 6 alleles represented by capital letters. These
crossing between two individuals a male and female having Aa Bb Cc with a female of same
genotype will give 7 different phenotypic skin colour sets. These can be very easily exemplified if we
look into the punet square presented below.
The Approximate Shades of Skin Colour Corresponding to each Genotype
Some such other human traits are human height, blood pressure, dermatoglyphics, ridge pattern,
ridge count, circumference of head, intelligence pattern, skin colour, heart diseases, and body
weight. Rh factor etc.
Multiple gene (polygenic) inheritance explains many plant and animal traits where there is a wide
variation between extreme phenotypes, with most individuals having intermediate phenotypes.
Twin studies in humans have been useful in showing how the expression of polygenic traits is
influenced by the environment. For example, two twins may have the potential to become diabetic,
but because of their different diets, one becomes diabetic and the other does not. Because
environmental factors can play such a large part in the expression of diseases that are polygenic,
many people are not aware that they carry these genes until their offspring accumulate enough of
the defective genes to express the disease. This makes these types of disease hard to eradicate.
Multiple Allelism:
Many phenotypic traits of an individual depend on a single pair of genes, each of which occupies a
specific position called the gene locus. Moreover a particular gene has been found to occur in two
alternative forms or allelomorphs, one being dominant and other recessive; one being wild form and
the other mutant form. If the mutant allele has developed from the wild type of allele due to
mutation, one may expect that the wild type of allele can mutate in more than one way. The mutant
form of allele too can mutate once again to give rise to another mutant form of allele. Therefore, it is
possible to have more than two allelic forms, i.e. multiple alleles, of one kind of gene. Though only
two actual alleles of a gene can exist in a diploid cell (and only one in a haploid cell), the total
number of possible different allelic forms that might exist in a population of individuals is quite large.
This situation is termed as multiple allelism.
Characteristics of multiple allele:
• Multiple alleles of a series always occupy the same locus in the chromosome.
• All the alleles of multiple allelism occupy same locus in chromosome, therefore, no
crossing
over occur within the alleles of a same multiple allele series.
• Multiple allele always influences the same character.
• When the two of the mutant multiple alleles are crossed, the phenotype is mutant type and not in
wild type.
• The wild type allele is nearly always dominant, while the other mutant alleles in the series may
show dominance or there may be an intermediate phenotypic effect.
In human blood typing (A, B, AB, and O), the gene for the blood type protein has three alleles (A, B,
and O).
However, sometimes for practical purposes, multiple alleles may be classified together as
“dominant” or “recessive.”
Example of multiple allele:
An excellent example of multiple allele inheritance is human blood type. Blood type exists as four
possible phenotypes: A, B, AB, & O. Human blood types are determined by proteins on the surface of
the red blood cells. Alleles A and B, for A-type and B-type glycoproteins, are co-dominant. Therefore
a person who inherits an A allele from one parent and a B allele from the other parent will have AB
bloodtype. The O allele is recessive. The O allele produces no glycoproteins. Thus, a person with the
genotype AO will produce only type A glycoproteins. Therefore the person will have type A blood. A
person with the genotype OO will produce neither A-type nor B-type glycoproteins. Therefore the
person will have type O blood.
The inheritance of polygenic traits depends upon the interaction of two or more genes. This is
sometimes confused with the idea of multiple alleles, which are just different forms of the same
gene. However, the two concepts are related when discussing the probable phenotype resulting
from a particular genotype. Such diseases as diabetes mellitus or heart disease are not the
consequence of single gene inheritance; likewise physical traits such as height, weight or even
behaviour are all examples of quantitative traits whose expression depends upon several different
factors. Unlike qualitative traits such as blood type or other multiple allelic genes that show an
unambiguous phenotype, quantitative polygenic traits show a range of expression. The distribution
of height, for instance, is expressed as short at one extreme and tall at the other extreme, with the
bulk falling somewhere in the middle. Height can be used again when discussing the role
environment plays in the expression of polygenic traits.
The existence of polygenic variation free in the phenotype must lead to some individuals departing
from the optimum and so showing reduced fitness. Variation is to this extent disadvantageous.
However, it is also essential for prospective adaptive and evolutionary change. If most of the
variability in a population is potential, high current fitness can be combined with the possibility of
great change under selection. In such cases the response of the organism to selection will largely
depend on the fixation of variability as it passes from the undetectable potential to the detectable
Free State. Thus, selection may superficially appear to create its own free directional variability.
The frequency of recombination between polygenes affecting a character will control the rate of
variability release. Consequently the effective recombination frequency is itself an adaptive
character and will be subject to selective action. The evolution of genetic systems is largely the
history of this selective control of effective recombination. Control of recombination is almost wholly
achieved within chromosomes, so that the storage of variability must depend on intrachromosome
adjustment. Natural selection will tend to build up balanced combinations of polygenes within each
of the chromosomes.
Conclusion:
From the present module, it can be said that the inheritance of polygenic traits depend upon the
interaction of two or more genes. This is sometimes confused with the idea of multiple alleles, which
are just different forms of the same gene. Though only two actual alleles of a gene can exist in a
diploid cell (and only one in a haploid cell), the total number of possible different allelic forms that
might exist in a population of individuals is quite large. However, the two concepts are related when
discussing the probable phenotype resulting from a particular genotype. The inheritance of many
significant human behavioural, anatomical and physiological characteristics is best explained by a
polygenic model of transmission. Expression of polygenic traits is often markedly affected by the
environment, causing them to be referred to as multifactorial traits.