Genetics-Mendels-Law-Inheritance | UKsir-notes | Genetics-3
Mendel’s Law of Inheritance:
 |
| Garden Pea |
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Mendel
performed various cross like Mono hybrid cross, Di hybrid cross etc. to give
conclusion.
They are
discussed below:
Mono Hybrid Cross:
-
When
only one trait/ character taken into consideration, then the cross is called as
the Monohybrid cross.
-
He
took height of the plant (tall vs dwarf) as the parameter in this experiment.
-
He
got pure parents by self cross.
-
After
that- when he crossed pure parents, he found F1 generation as tall plants.
-
When
tall F1 generation self crossed/ pollinated, he found the F2 generation.
-
F2
generation result was interesting.
-
He
found Total 4 Phenotype offspring as well as 3 genotype combination.
-
Out
of the 4 Phenotype combination - 3 were
tall where as one became dwarf.
 |
| F1 Generation |
 |
| Monohybrid cross |
-
Phenotypic
Ratio- 3:1 (Tall : Dwarf) , Genotypic
Ratio – 1:2:1 (Pure tall : Hybrid tall : Pure dwarf )
Explanation:
-
Parents
were pure Tall as well as dwarf.
-
In
F1 generation due to mixing of character, he found – Tt combination (Tall
Plant).
-
However
the F1 were hybrid heterozygous tall, not pure.
-
Here
one character expressed called Dominant (Tall).
-
Whereas
other suppressed called as Recessive (Dwarf).
Laws of Inheritance:
-
On
the basis of Monohybrid cross – He proposed two laws, as follows:
(1)Principle of Dominance :
-
Every
organism transfers some character from one generation to next generation by
Factor/ Gene.
-
The
Factor/ Gene have two alternative form called as Allele.
-
One
of the Allele is fully expressed called as dominant allele, where as other is
suppressed/ can’t express itself called as recessive allele.
-
When
two homozygous parents crossed and form heterozygous condition, then Dominant
allele always express itself suppressing other one.
-
This
is called as Law of Dominance / Principle of Dominance.
Explanation:
-
In
F1 generation Heterozygous (Tt) condition, always express dominant character
i.e. Tallness is expressed.
-
Same result can be seen in F2 hybrids.
-
The
law of dominance explains why individuals of F1 generation express the trait of
only one parent and the reason for occurrence of 3:1 ratio in F2 generation.
Other
Examples in Animals:
-
Length
of hair in Cat : Short hair are dominant over long hair.
-
Skin
color in Dog : Grey color is dominant over black color.
-
Eye
color in Drosophila : Red color is dominant over white.
 |
| Drosophila |
(2)Principle of Segregation : (Law of Purity of Gametes)
-
In
diploid organisms two factors of given character remain together, but didn’t
mix with each other. They also show their individuality.
-
The
Rule of Segregation - A gamete receives only one allele from the pair of
alleles possessed by an organism; fertilization reestablishes the double number
OR
-
In
F1 hybrid, the dominant and recessive characters though remain together for a
long time, but do not contaminate or mix with each other and separate or
segregate at the time of gamete formation.
-
Thus,
the gamete formed receives either dominant or recessive character out of them.
-
The
hybrid formation is considered from pure line homozygous parents, through monohybird cross.
-
However
the purity of gametes again established in F2 generation, for which it is
called law of purity of gametes.
Explanation:
-
During
gamete formation from Pure parent 2n (TT/ tt), each allele separate to form T
or t Gamete (n).
-
Likewise
in F1 hybrid (Tt) diploid heterozygous parent , during gamete formation again the
separation occurs by forming T/t Gamete (n)
-
One
gamete get pure tall character and another pure dwarf character.
Di hybrid Cross (Inheritance of Two Genes):
-
When
two pair of contrasting character taken in to consideration in a cross, it is
called as di hybrid cross.
-
The
contrasting characters may be a any two of the 7 pairs of contrasting character
choose by Mendel.
-
For
Example: we can take 2 characters like:
-
(a)
Round seed (R) is dominant over Wrinkled seed (r) in case of pea plant.
-
(b)
Yellow seed (Y) is dominant over Green seed (y) in pea plant.
-
Here
Round seed and Yellow seeds are dominant characters = (RRYY)
-
Whereas
Wrinkled seed and Green seeds are recessive character = (rryy)
-
When
the pure Parent crossed we can find F1 generation.
Dominant
Parent (2n) × Recessive
Parent (2n)
(Round
Yellow seed) RRYY × rryy (Wrinkled Green seed )
↓ ↓
Gametes (n) RY ry
Fertilized ↘ ↙
F1 Hybrid RrYy (Diploid 2n)
(Round
Yellow Seed)
When F1 hybrids are self crossed/ fertilized , they produce 4 type of gametes and 16 combinations in F2 generation.
 |
| Dihybrid cross |
Phenotypic
Ratio: (9:3:3:1)= 16
|
Plant With RY allele |
9 |
Round Yellow seed |
Pure dominant |
|
Plant With rY allele |
3 |
Wrinkled Yellow seed |
Hybrid |
|
Plant With Ry allele |
3 |
Round Green seed |
Hybrid |
|
Plant With ry allele |
1 |
Wrinkled Green seed |
Pure recessive |
Genotypic
Ratio: (1:2:1:2:4:2:1:2:1) = 16
However the
genotypic ratio differs from Phenotypic ratio, according to the arrangement of
alleles. They show different combination.
|
RRYY Combination = |
1 |
|
|
|
RrYY Combination = |
2 |
rrYy Combination = |
2 |
|
rrYY Combination = |
1 |
RRyy Combination = |
1 |
|
RRYy Combination = |
2 |
Rryy Combination = |
2 |
|
RrYy Combination = |
4 |
rryy Combination = |
1 |
3. Principle of Independent Assortment
- This law
states that, ‘the inheritance of one character is always independent to the
inheritance of other character within the same individual’.
Or
- According
to this principle a cross between parents with two or more contrasting
characters is not dependent to each other, i.e. inheritance of one pair of
character is independent of another pair.
- The two
characters are transmitted independently.
- For which
there will be formation of different combination of characters in F
2generation.
- The dihybrid cross of Mendel can be a very
good example of independent assortment.
Explanation:
-
The
two factors for seed color (Y & y) and Factors for shape of seeds (R
& r ) present together in F1- hybrid.
-
But
they assort independently and randomly during gamete formation.
-
Also the gametes contain different factors.
-
Like
: A single gamete contain only one of the two factors for a single character.
-
Result
= RrYy (Diploid hybrid 2n) will
form gametes (n) like RY, rY, Ry and ry.
-
These
4 types of gametes unite randomly and give rise to 4 type of F2 offspring as
per probability.
Conclusion:
-
Thus
Mendel explained about principle of inheritance.
-
But,
the Mendel’s law can’t be applicable in all cases, as we can find many
deviations from Mendel’s ration.
-
However
Mendel was lucky choosing garden pea plant for which, he could able to
postulate the principle of inheritance.
BACK CROSS
A back cross is a cross in which F1 individuals are crossed with any of their parents.
(1) Out Cross :
 |
| Outcross |
-
When
F1 (heterozygous) individual is crossed with dominant parent then, termed as
out cross.
-
The
generations obtained from this cross, all possess dominant character.
-
So
the any analysis cannot possible in F1
generation.
F progeny (hybrid) Tt ×
TT Dominant parent
(2)
Test Cross:
When F1 progeny
(heterozygous) is crossed with recessive parent then it is called test cross.
[A]
Monohybrid Test Cross:-
-
The
progeny obtained from the monohybrid test cross found in equal proportion, (50%
dominant and 50% recessive)
It can
be represented in symbolic forms as follows.
F1 progeny(hybrid) Tt × tt Recessive parent
 |
| Testcross |
-
Monohybrid
test cross ratio = 1 : 1
[B] Di hybrid Test Cross:-
- The
progeny obtained from di hybrid test cross - four types (each with 25% probability).
F1 – di hybrid Recessive parent rryy ×
RrYy (Dominant Phenotype)
 |
| Testcross 2 |
The ratio of Dihybrid
test cross = 1:1:1:1
Conclusion:- In test cross phenotypes and genotypes ratio are same.
Test cross helps to find out the genotype of dominant individual.RECIPROCAL CROSS
- When two parents are used in two experiments in such a way that in one experiment "X" used as the female parent and "Y" is used as the male parent, in the other experiment "X" will be used as the male parent and "Y" as the female parent.Reciprocal cross show
impact in case
of cytoplasmic inheritance and sex linkage.
(karyogene present on autosomes are not affected)
 |
| Reciprocal Cross |
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.html
This was all about Mendel's Law of Inheritance. Thanks for visiting, feel free to ask doubts in comment section... UK Sir
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