3. Inheritance and Variation

-CREATED BY-

PROF. DESHMUKH A. B.

ASST. TEACHER

AGASTI ARTS, COMMERCE AND DADASAHEB RUPWATE SCIENCE JUNIOR COLLEGE, AKOLE

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3.3 Mendel’s Laws of Inheritance

3.4 Back Cross and Test Cross

BIOLOGY (botany)

• Day : Monday Prof. Deshmukh A. B.
• Date : 05/10/2020
• Time : 7:45pm to 8:15 pm
• Class : 12^th Science
• Division: A
• Lecture : 4
• Topic : 3 Inheritance and Variation

3.3 Mendel’s Laws of Inheritance

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3.3 Mendel’s Laws of Inheritance :

• Mendel proposed three basic postulates based on which three laws were formulated.
• These are as below:
• 1. Law of Dominance
• 2. Law of segregation (Law of purity of gametes)
• 3. Law of Independent Assortment

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1. Law of Dominance :

• In monohybrid and dihybrid crosses,
• Phenotypic characters are controlled by discrete units, called factors.
• In a dissimilar pair of factors,
• One member of the pair dominates (i.e., dominant) over the other (i.e., recessive).
• The law of dominance is used to explain-
• the expression of only one of the parental characters of a monohybrid cross in F1 and
• the expression of both in F2.

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Statement of Law of Dominance :

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• “When two homozygous individuals with one or more sets of contrasting characters are crossed, the alleles (characters) that appear in F1 are dominant and those which do not appear in F1 are recessive”.

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing of F1:

Genotype:

Gametes:

Round Seeds (♀)

X

Wrinkled Seeds (♂)

RR

rr

R

r

Rr

Round Seeds

Round Seeds (♀)

Round Seeds (♂)

Rr

Rr

R

r

R

r

F2 Generation:

R

r

R

r

RR

Rr

Rr

rr

Phenotypic

Ratio:

Round Seeds

Wrinkled Seeds

:

=

3:1

Genotypic

Ratio:

=

1:2:1

RR : Rr : rr

(Selfing is allowing self pollination in same flowers on same plant…)

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing

of F1:

Genotype:

Gametes:

Round Yellow Seeds (♀)

X

Wrinkled Green Seeds (♂)

RRYY

rryy

RrYy

Round Yellow Seeds

RrYy

Round Yellow

Seeds (♀)

Round Yellow

Seeds (♂)

X

RrYy

RY, Ry, rY, ry

RY, Ry, rY, ry

RrYy

R

r

Y

y

Genotype of F1

Hybrid:

Segregation:

Gametes:

Let us see how alleles combine to form gametes

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F2 Generation:

Phenotypic Ratio:

RY

Ry

rY

ry

RY

Ry

rY

ry

RRYY

RRYy

RrYY

RrYy

RrYy

RrYy

RrYy

RRYy

RrYY

Rryy

Rryy

rrYy

rrYy

rryy

RRyy

rrYY

9 : 3 : 3 : 1

Genotypic Ratio:

RRYY : RRYy : RrYY : RrYy : RRyy : Rryy : rrYY : rrYy : rryy

1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1

2. Law of segregation (Law of purity of gametes) :

• This law is based on following facts-
• The alleles do not show any blending (mixing) and
• Both the alleles (characters) are recovered as such in the F₂ generation, though one of these is not seen at the F₁ stage.
• During formation of gametes, these two alleles (factors) obviously separate or segregate, otherwise recessive type will not appear in F₂.
• The gametes which are formed are always pure for a particular character (trait).
• A gamete may carry either dominant or recessive factor but not both.
• That’s why it is also called as law of purity of gametes.

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Statement of Law of Segregation :

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​

• The law states that “When hybrid (F1) forms gametes, the alleles segregate from each other and enter in different gametes”.

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing of F1:

Genotype:

Gametes:

Round Seeds (♀)

X

Wrinkled Seeds (♂)

RR

rr

R

r

Rr

Round Seeds

Round Seeds (♀)

Round Seeds (♂)

Rr

Rr

R

r

R

r

F2

Generation:

R

r

R

r

RR

Rr

Rr

rr

Phenotypic

Ratio:

Round Seeds

Wrinkled Seeds

:

=

3:1

Genotypic

Ratio:

=

1:2:1

RR : Rr : rr

(Selfing is allowing self pollination in same flowers on same plant…)

3. Law of independent assortment:

• This law can be explained with the help of dihybrid cross and dihybrid ratio.
• Appearance of new combinations in F₂ generation proves the law.

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Parents (P):

Phenotype:

Genotype:

Gametes:

F1

Generation:

Selfing

of F1:

Genotype:

Gametes:

Round Yellow Seeds (♀)

X

Wrinkled Green Seeds (♂)

RRYY

rryy

RrYy

Round Yellow Seeds

RrYy

Round Yellow

Seeds (♀)

Round Yellow

Seeds (♂)

X

RrYy

RY, Ry, rY, ry

RY, Ry, rY, ry

RrYy

R

r

Y

y

Genotype of F1

Hybrid:

Segregation:

Gametes:

Let us see how alleles combine to form gametes

DIHYBRID CROSS

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F2 Generation:

Phenotypic Ratio:

RY

Ry

rY

ry

RY

Ry

rY

ry

RRYY

RRYy

RrYY

RrYy

RrYy

RrYy

RrYy

RRYy

RrYY

Rryy

Rryy

rrYy

rrYy

rryy

RRyy

rrYY

9 : 3 : 3 : 1

Genotypic Ratio:

RRYY : RRYy : RrYY : RrYy : RRyy : Rryy : rrYY : rrYy : rryy

1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1

Parental Combinations

Round Yellow

Wrinkled Green

New Combinations

Round Green

Wrinkled Yellow

= 9

= 1

= 3

= 3

Statement of Law of Independent Assortment:

• The law states that,
• “When hybrid possessing two (or more) pairs of contrasting factors (alleles) forms gametes, the factors in each pair segregate independently of the other pair”.

OR

• “When two homozygous parents differing in two pairs of contrasting traits are crossed, the inheritance of one pair is independent of the other”.

OR

• “When a dihybrid forms gametes, assortment (distribution) of alleles of different traits is independent of their original combinations in parent.”

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3.4 Back Cross and Test Cross :

• a. Back cross :
• The F1 individuals obtained in a cross are usually selfed to get the F2 progeny.
• They can also be crossed with one of the two parents from which they were derived (either recessive or dominant).
• Such a cross is known as back cross.

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Phenotype:

First Filial

Generation (F1):

Pure

Tall

X

Pure

Dwarf

All Tall

Parents: (P)

Back Cross

F1 Hybrid

Any

Parent(P)

X

OR

• b. Test cross :
• The cross of F1 hybrid with the homozygous recessive parent is known as a test cross.
• It is used to test whether an individual is homozygous (pure) or heterozygous(hybrid).
• Test cross is easy, simple, repeatable and predictable.

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• Test cross can be used to find out genotype of any plant with dominant expression.
• But it is not known whether it is homozygous (pure) or heterozygous for that trait.
• For example,
• A pea plant having violet (purple) flowers is crossed with a pea plant with white flowers.
• If all flowers produced are violet, we can conclude that plant is pure or homozygous and
• If we get violet and white flowers in 1:1 ratio, we can conclude that plant is heterozygous.
• Test cross is also used to introduce useful recessive traits in the hybrids of self pollinated plants during rapid crop improvement programs.

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