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Class 12 Biology Chapter 5: Principles of Inheritance and Variation

By Arvind Sharma, B.Pharm, M.Pharm, Assistant Professor, MUIT

QUICK REVISION

Mendel's Laws	Description	Significance
Law of Dominance	In a heterozygote, one allele (dominant) masks the expression of the other (recessive).	Explains why F1 generation resembles only one parent and F2 shows a 3:1 phenotypic ratio in monohybrid cross.
Law of Segregation	During gamete formation, the two alleles for a heritable character separate (segregate) from each other so that each gamete receives only one allele.	Ensures genetic purity of gametes; universal law.
Law of Independent Assortment	Alleles for different characters assort independently of each other during gamete formation.	Explains formation of new combinations in dihybrid and polyhybrid crosses (9:3:3:1 phenotypic ratio in F2 of dihybrid cross).

Genetic Term	Definition	Example
Gene	A segment of DNA that codes for a functional product (e.g., protein, RNA).	Gene for height, gene for seed color.
Allele	Alternative forms of a gene.	Tall (T) and dwarf (t) alleles for height gene.
Genotype	The genetic constitution of an individual.	TT, Tt, tt.
Phenotype	The observable characteristics of an individual resulting from its genotype and environment.	Tall, dwarf.
Homozygous	Having two identical alleles for a particular gene.	TT (homozygous dominant), tt (homozygous recessive).
Heterozygous	Having two different alleles for a particular gene.	Tt.
Dominant Allele	An allele that expresses its phenotypic effect even when heterozygous with a recessive allele.	T (Tall).
Recessive Allele	An allele that expresses its phenotypic effect only when homozygous.	t (dwarf).
Monohybrid Cross	A cross involving a single pair of contrasting characters.	Tall x dwarf.
Dihybrid Cross	A cross involving two pairs of contrasting characters.	Round-yellow x wrinkled-green.
Test Cross	Crossing an individual with dominant phenotype but unknown genotype with a homozygous recessive individual.	T_ x tt (to determine if T_ is TT or Tt).

Punnett Square Overview: This is a graphical representation to calculate the probability of all possible genotypes and phenotypes of offspring in a genetic cross. It's a simple tool to understand genetic inheritance patterns.

Mnemonic for Chromosomal Disorders:
Down's = Disomy 21
Klinefelter's = King (XXY)
Turner's = Tomboy (XO)

MAIN CURRICULUM

Introduction to Principles of Inheritance and Variation

Genetics is the branch of biology that deals with the study of heredity and variation. Heredity refers to the transmission of characters from parents to offspring, leading to resemblances. Variation refers to the differences among individuals of the same species, arising from genetic recombination, mutation, or environmental factors. Understanding these principles is fundamental to comprehending the diversity of life and the mechanisms of evolution.

Heredity: The process by which genetic information is passed on from parent to offspring.

Variation: The degree by which progeny differ from their parents.

Mendelian Genetics

Gregor Johann Mendel, known as the 'Father of Genetics', conducted hybridization experiments on garden peas (Pisum sativum) for seven years (1856-1863) and proposed the laws of inheritance. His work remained unnoticed for many years until it was rediscovered in 1900 by three scientists independently: Hugo de Vries, Carl Correns, and Erich von Tschermak.

Reasons for Mendel's Success with Pea Plants:

Pea plants are easy to cultivate and have a short life cycle.
They produce a large number of offspring, making statistical analysis reliable.
They exhibit several clearly distinguishable contrasting characters (seven pairs).
They are naturally self-pollinating, but can also be cross-pollinated, allowing controlled experiments.
The flowers are bisexual, making self-fertilization easy.

S. No.	Character	Dominant Trait	Recessive Trait
1	Stem height	Tall	Dwarf
2	Flower colour	Violet	White
3	Flower position	Axial	Terminal
4	Pod shape	Full	Constricted
5	Pod colour	Green	Yellow
6	Seed shape	Round	Wrinkled
7	Seed colour	Yellow	Green

Mendel's Laws of Inheritance

1. Law of Dominance

This law states that in a cross involving two parents differing in one pair of contrasting characters (alleles), only one form of the trait (the dominant trait) will appear in the F1 generation, while the other (the recessive trait) remains unexpressed. Upon self-pollination of the F1 generation, both traits reappear in the F2 generation in a specific phenotypic ratio.

Monohybrid Cross Example (Tall × Dwarf Pea Plant):
Parental Generation (P): TT (Tall) × tt (Dwarf)
Gametes from P: T, t
F1 Generation: Tt (All Tall)

Selfing of F1: Tt × Tt
Gametes from F1: T, t (from each parent)
F2 Generation (via Punnett Square):

♀ \ ♂	T	t
T	TT	Tt
t	Tt	tt

F2 Phenotypic Ratio: 3 Tall : 1 Dwarf
F2 Genotypic Ratio: 1 TT : 2 Tt : 1 tt

The Law of Dominance is not universally applicable, as seen in cases of incomplete dominance and co-dominance.

2. Law of Segregation (Law of Purity of Gametes)

This law states that the two alleles for a heritable character separate (segregate) from each other during gamete formation so that each gamete receives only one allele. These alleles do not blend or contaminate each other. This is a universal law.

Consider the F1 hybrid (Tt) from the monohybrid cross. During gamete formation, the 'T' and 't' alleles segregate, so that 50% of the gametes carry 'T' and 50% carry 't'. When these gametes unite randomly during fertilization, the F2 generation exhibits a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.

3. Law of Independent Assortment

This law states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters. In simpler terms, the alleles for different genes assort independently of each other into gametes.

Dihybrid Cross (Round-Yellow x Wrinkled-Green Pea Plant):
Parental Generation (P): RRYY (Round-Yellow) x rryy (Wrinkled-Green)
Gametes: RY | ry
F1 Generation: RrYy (All Round-Yellow)

Selfing of F1: RrYy x RrYy
Gametes from RrYy: RY, Ry, rY, ry (due to independent assortment)

F2 Generation (via Punnett Square - 16 squares)
Phenotypic Ratio (F2): 9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green
Genotypic Ratio (F2): Complex (e.g., 1 RRYY : 2 RRYy : 2 RrYY : 4 RrYy : 1 RRyy : 2 Rryy : 1 rrYY : 2 rrYy : 1 rryy)

The Law of Independent Assortment is applicable when the genes for the two traits are located on different chromosomes or are far apart on the same chromosome (unlinked). If genes are linked, this law does not hold true.

Deviations from Mendelian Inheritance

While Mendel's laws provide a foundational understanding, many genetic phenomena do not strictly follow his patterns. These are called non-Mendelian inheritance patterns.

1. Incomplete Dominance

In incomplete dominance, the F1 hybrid exhibits a phenotype intermediate between the two parental phenotypes. Neither allele is completely dominant over the other.

Example: Flower color in Mirabilis jalapa (Four o'clock plant) and Antirrhinum majus (Snapdragon).
Cross: Red flower (RR) x White flower (rr)
F1: Rr (Pink flowers)
F2 (selfing of Rr): 1 RR (Red) : 2 Rr (Pink) : 1 rr (White)
Phenotypic Ratio (F2): 1:2:1
Genotypic Ratio (F2): 1:2:1

In incomplete dominance, the phenotypic and genotypic ratios in the F2 generation are identical (1:2:1).

2. Co-dominance

In co-dominance, both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows characteristics of both parents simultaneously, without blending.

Example: ABO blood group system in humans.
The gene for blood group (I) has three alleles: I^A, I^B, and i.
I^A and I^B are dominant over i.
When I^A and I^B are present together (heterozygous), both express themselves, resulting in AB blood group.

Alleles from Parent 1	Alleles from Parent 2	Genotype of offspring	Blood group of offspring
I^A	I^A	I^AI^A	A
I^A	I^B	I^AI^B	AB
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