Genetics-Deviation-Mendelian-Inheritance | UKsir-notes | Genetics-4
Deviation from Mendel’s Principles:
-
 |
| Genetic material |
Want to know about History of Genetics and Mendel , Click the link below:
Want to know about Mendel's work , Click the link below:
https://uksirnotes.blogspot.com/2021/07/Genetics-Mendel-experiment-Uksir-Notes.htmlThe
work of Mendel was based on Garden pea plant (Pisum sativum).
-
Fortunately
that doesn’t show much deviation in inheritance.
-
However
in later stage, some deviations were found from Mendel’s work, on different
crosses.
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According
to Mendel’s law, one allele of one pair should completely dominant over others.
-
But
in some cases it was different, like :
-
Heterozygous
hybrid may show one intermediate phenotype, or may be equal expression of both
alleles occurs.
-
Some
time seen that, more than one gene may control a single character or trait.
(Polygenic inheritance)
-
Or
one gene may control many characters (Pleiotropic effect)
GENE INTERACTION
Gene interaction is of two types :
(i) Allelic interaction/ Intragenic
interaction- Allelic interaction
takes place between allele of same gene which
are present at same locus. (Incomplete dominance, Co-dominance, Multiple
allelism, Pleiotropic gene)
(ii) Non allelic interaction/ Intergenic
interaction- When interaction takes
place between non-allele is called non allelic gene interaction. It changes or
modifies
other non allelic gene. (Complementary Gene,
Epistasis)
Exceptions to Law of Dominance
1) Incomplete Dominance/Blending Inheritance :
- Observed
by Correns, 1903.
- It is also
called as intermediate or Partial or Mosaic inheritance.
- When F1
hybrids exhibit a mixture or blending of characters of two parents, it is
termed as blending inheritance.
- Here the two genes of allelic pair are not dominant or recessive, Rather each of them expresses themselves partially.
 |
| Incomplete Dominance |
Example - 4 O'clock plant (Mirabilis jalapa),
snapdragon (Antirrhinum)
- In 4
O’clock plant, cross between dominant (red) and recessive (white) variety
should result in Red color hybrid.
- But,
presence of both Rr heterozygous form, produces Pink flower
- F2
generation shows deviation from law of dominance.
- In this
case both phenotypic and genotypic ratios are same = 1 : 2 : 1 (Red : Pink :
White.)
2) Co dominance:
-The
phenomenon of expression of both the alleles in a heterozygote is called co-dominance.
- Or It is
the condition in which both the alleles of a gene pair in one heterozygote are
fully expressed or equally expressed.
- The
alleles are able to express themselves independently when present together are called
co-dominant allele.
- Example
: Coat color in short horned cattle’s, AB blood group in humans, MN- blood
group in humans
- In short horned cattle, cross between white (dominant) and red (recessive) variety, produce Roan offspring in F1 generation.
 |
| Codominance |
- in F2 generation show co-dominance (white,
roan and red in 1: 2 :1 ratio)
- both the hybrid show roan colour.
- The roan
coloured F2 individuals have both red and white hairs in the form of patches.
- The intermediate
colour hair can’t be seen.
-
Example-
AB blood group in humans.
 |
| AB blood group |
In AB blood
group of Human Both IaIb
expressed equally.
Example : MN- blood group in humans :
 |
| mn grouping |
Homozygous individual with M allele - (RBC surface contain M
antigen)
Homozygous individual with N allele - (RBC surface contain N
antigen )
Heterozygous Individual with both allele – (RBC contain both Antigen)
3) Multiple Allelism :
-
In
diploids (2n) two copies of a gene normally occurs.
-
The
two copies present in both chromosome at same loci/ position.
-
If
more then two alternative form of a gene present in the same locus, it can be
called as multiple allelism.
-
Or
the presence of more than two alleles for a gene, can be called as multiple
allelism.
-
Example : ABO blood group in human (3
allele), Coat colour in Rabbit (4 allele), Colour in Drosophila ( more then 100 allele)
ABO Blood group:
-
Blood
group in human beings is controlled by three alleles, but only two of these are
present in an individual.
-
In
this case we can find both complete dominance as well as co-dominance.
-
The
3 alleles found are- Ia, Ib, Io. (I allele
produces a particular cell surface antigen.
Ia Produces – enzyme which add Galactosamine to the RBC of A-
blood group individual. Ib produces enzyme – which add galactose to
the RBC of B- blood group individual.)
Here Ia is dominant over Io, Ib is dominant over Io, but Ia and Ib are co- dominant to each other.
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| ABO blood group |
-
|
|
Allele Present |
Blood Group |
|
a |
Ia Ia / Ia
Io |
A-Group |
|
b |
Ib Ib / Ib
Io |
B- Group |
|
c |
Ia Ib |
AB- Group |
|
d |
Io Io |
O- Group |
-
If
“Ia Ib” both alleles present, then both the antigen added
to RBC, which is due to co-dominance.
-
If
“Io Io” present then no antigen added to the RBC.
·
Thus
all Blood groups are unic to each other.
·
O-
Blood group is considered as Universal donor.
·
Whereas
AB+ is the Universal acceptor.
Another Good example is Coat color in Rabbit:
 |
| Rabbit color |
4) Pleiotropy ( Pleiotropic Gene):
-If a single
gene controls/ contribute to multiple phenotype/traits – that may be called as
Pleiotropy.
- Or Ability of a gene to have multiple phenotypic
effects, as it can influences a number of characters simultaneously,
is known
as pleiotropy.
 |
| Pleiotropy |
-Extracted
from Greek word : Pleio means many.
- Such genes are called pleiotropic genes.
- In many plants
various characters are co-related.
- It is not essential that all traits are equally influenced, sometimes it is more expressed in case of one trait (major effect) and less expressed in other (minor effect).
- Example : in garden Pea, the gene controlling flower
color, also controls the color of seed coat and the presence of red spot on
leaf axil.
-Example
: Phenyl Keton Urea (in Human)
-
This
is a genetic disorder caused due to deficiency of enzyme phenylalanine
hydroxylase.
-
This
enzyme is necessary to convert one essential amino acid Phenylalanine to
Tyrosine.
-
If
there is problem in expression of the gene producing this enzyme, then Phenylalanine
accumulate in body fluid, resulting the decrease of Tyrosine.
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| Poly Ketone Urea |
-
As
tyrosine deficiency occurs in body, there will be less synthesis of
neurotransmitters like Dopamine, norepinephrine etc.
-
This
cause serious mental disorder as well as health issue.
-
Thus
a mutation in single gene affects not only Tyrosine synthesis but also multiple
body system.
-Example: Sickle cell anaemia (also called as Autosomal linked recessive disorder)
-
Its
a good example of pleiotropy.
-
It
not only causes haemolytic anaemia but also results increased resistance to one
type of malaria,
caused by Plasmodium falciparum.
-
The
sickle cell HbS allele also show pleiotropic effect - development of many tissues and organs (bone,
lungs, kidney, spleen, heart)
-
Defect
starts due to single base substitution at the sixth codon of the beta globin
gene from GAG to GUG.
-
This
result in substitution of Glutamic acid (Glu) by Valine (Val) at sixth position
of beta Globin chain of The hemoglobin molecule.
-
The
mutant hemoglobin molecule undergoes polymerization (under low oxygen tension) change the shape of the
RBC. (biconcave disc →sickle like
structure)
-
Now
this impact all the organs of body directly, as Oxygen supply slow down.
Also may be fatal for the patient
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| Sickle cell anaemia |
Polygenic inheritance /quantitative inheritance :
- Here Quantitative inheritance refers to number or quantity
of gene control the character.
Polygenic inheritance influence qualitative characters only. (Traits which can easily classified to distinct phenotypic categories e.g. Height, skin color etc.)
 |
| Polygenic inheritance |
- Mendel's studies mainly based on traits
that have distinct alternate forms like flower color which are either purple or white.
- But we can find that many traits which
are not so distinct in their occurrence and are spread across a gradient.
- Example, in human case not just tall or short people can be seen as two
distinct alternatives, but a whole range of possible heights.
- Such traits are generally controlled by number of genes and are
thus called as polygenic traits.
- Polygenic inheritance also influenced by
environment.
- Human skin color is another example for
this.
Example-1:
Multiple gene model was developed by Swedish
genetics Nilsson Ehle in 1910 to explain the kernel (grain/ seed) color of
wheat is regulated by two polygene.
Red AA BB × aa bb White
↓
F1 Aa
Bb Intermediate
↓
F2
Generation
|
Genoype |
AABB |
AABb AAbB AaBB aABB |
AAbb AaBb AabB aABb aAbB aaBB |
Aabb aAbb aaBb aabB |
aabb |
|
Number of Dominant Gene |
4 |
3 |
2 |
1 |
0 |
|
Color |
Full Red |
Medium Red |
Light Red |
Very Light Red |
White |
Phenotypic Ratio - 1 : 14 : 1 (Full red :
Intermediate: Full White)
Genotype become- 1: 4: 6: 4 : 1
Example-2 :
Skin color in Human:
 |
| Skin color |
- Skin color
inheritance in human was studied by Devenport.
- Human skin color is regulated by three poly genes.
- Genes A, B and C control skin color in human
with the dominant forms A, B, and C responsible
for dark skin color and the recessive forms a, b, and c for light skin color.
-
The
genotype with all
the dominant alleles (AABBCC) - darkest skin color and that with all the
recessive alleles (aabbcc) - lightest
skin color.
-
As
expected intermediate skin color may also produced.
-
The
number of type of alleles in the genotype -
determine the darkness or lightness of the skin
in an individual.
-
When
a Negro Black (AA BB CC) phenotype is crossed with white (aa bb cc) phenotype,
intermediate phenotype is produced in F1
generation.
Negro Black × White
AA BB CC × aa bb cc
↓
F1-generation Aa Bb
Cc
[Intermediate brown/mullato]
↓
(Inbreeding)
F2 – generation
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| Skin Color In Human |
Main Difference between Multiple alleles and polygenic inheritance:
-
In
Multiple allele same DNA strand involved with multiple alternative form of a
gene.
-
In
polygenic inheritance multiple DNA strand may involved may regulated through
group of non-alleleic genes.
However the polygenic inheritance also influenced by environmental factors.
Exceptions to Law of Segregation
1)Complementary Genes
2) Epistatic Gene or Inhibitory Gene :
(a)
Dominant Epistasis
(b) Recessive Epistasis
Exceptions to Law of Independent Assortment
(i) Supplementary Gene
(ii)
Duplicate Gene
(iii) Collaborator Gene
Want to know about History of Genetics and Mendel , Click the link below:
Want to know about Mendel's work , Click the link below:
https://uksirnotes.blogspot.com/2021/07/Genetics-Mendel-experiment-Uksir-Notes.htmlThis was all about deviation from Mendel's Law of Inheritance. Thanks for visiting, feel free to ask doubts in comment section... UK Sir
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