🧬 Concept: Mendel’s Laws of Inheritance

Mendel selected pea plants for his experiments due to several scientific advantages:

• Pea plants show clear contrasting traits (e.g., tall/dwarf, round/wrinkled seeds)
• They are naturally self-pollinating but can be cross-pollinated easily
• They have a short life cycle
• They produce a large number of offspring, allowing statistical analysis
• Pure breeding lines available

Term	Definition
Gene	Unit of inheritance controlling a trait
Allele	Alternative form of a gene
Genotype	Genetic makeup (TT, Tt)
Phenotype	Observable trait
Homozygous	Same alleles (TT / tt)
Heterozygous	Different alleles (Tt)
Dominant Trait	Expressed in heterozygous
Recessive Trait	Expressed only in homozygous

Mendel conducted hybridization experiments using seven pairs of contrasting traits in pea plants, such as:

• Tall vs Dwarf
• Round vs Wrinkled seeds
• Yellow vs Green seeds
• Purple vs White flowers

Based on these experiments, Mendel proposed three fundamental laws of inheritance.

Statement When two contrasting alleles are present together in an organism, only one expresses itself, while the other remains hidden. Explanation

• The expressed allele is called dominant
• The hidden allele is called recessive

Monohybrid Cross Example (Height of Pea Plant)

• Tall plant (TT) × Dwarf plant (tt)
• F₁ generation: All plants are Tall (Tt)

Although dwarf allele (t) is present, it does not express itself in F₁ generation. Conclusion The dominant allele masks the effect of the recessive allele in heterozygous condition. When two contrasting alleles are present, only one is expressed.

Example: TT × tt → F₁: All Tt (Tall)

Statement The two alleles of a gene separate (segregate) during gamete formation, so each gamete carries only one allele.
Explanation:

• Alleles do not blend
• Each parent passes only one allele to the offspring
• This law explains the reappearance of recessive traits

F₂ Generation (From Monohybrid Cross)
F₁ plants (Tt) self-pollinate:

• Genotypic ratio: 1 TT : 2 Tt : 1 tt
• Phenotypic ratio: 3 Tall : 1 Dwarf

The dwarf plant reappears in F₂ generation, proving that alleles separate during gamete formation.

F₂ Ratio: 3 Tall : 1 Dwarf

Statement Alleles of different genes assort independently during gamete formation.Genes of different traits assort independently (if unlinked).
Explanation:

• This law applies when genes are located on different chromosomes
• One trait does not influence the inheritance of another

F₂ Ratio: 9 : 3 : 3 : 1

Proposed by Sutton and Boveri, this theory states:

• Genes are located on chromosomes
• Chromosomes segregate during meiosis
• Behavior of chromosomes Explains Mendel’s laws

This theory provides a cytological basis for Mendel’s laws.

Traits Studied:

• Seed shape: Round (R) / Wrinkled (r)
• Seed color: Yellow (Y) / Green (y)

Cross:

• RRYY × rryy

F₁ Generation: All plants are Round Yellow (RrYy) F₂ Phenotypic Ratio:

• 9 Round Yellow
• 3 Round Green
• 3 Wrinkled Yellow
• 1 Wrinkled Green

Phenotypic Ratio = 9 : 3 : 3 : 1
This ratio confirms the Law of Independent Assortment.

Not all traits follow Mendelian inheritance. Some exceptions include:

• Incomplete dominance
• Codominance
• Linkage
• Polygenic inheritance

• Foundation of genetics
• Helps in breeding
• Understanding disorders
• Basis of modern biotechnology and genetic engineering

Who is the father of genetics?
Gregor Johann Mendel is known as the father of genetics.
How many laws of inheritance did Mendel propose?
Mendel proposed three laws: Dominance, Segregation, and Independent Assortment.
Which generation shows recessive traits again?
Recessive traits reappear in the F₂ generation.

Mendel’s Laws of Inheritance form the backbone of genetics and are essential for understanding how traits pass from one generation to the next.