🧬 Concept: Inheritance of Two Genes (Dihybrid Inheritance)

Inheritance of two genes explains how two different traits are transmitted from parents to offspring simultaneously. This concept is known as dihybrid inheritance and was first experimentally explained by Gregor Johann Mendel using pea plants (Pisum sativum).

This topic builds on monohybrid inheritance and helps us understand genetic variation, which is essential for evolution, breeding, and heredity studies.

Dihybrid inheritance refers to the inheritance pattern in which two different characters, controlled by two separate gene pairs, are studied together. Example:

• Seed shape (Round / Wrinkled)
• Seed colour (Yellow / Green)

Each trait is controlled by a separate gene, and both genes are inherited together.

• Explains simultaneous inheritance of traits
• Introduces Law of Independent Assortment
• Explains origin of new trait combinations
• Forms the foundation of modern genetics

Term Meaning Gene Unit of inheritance Allele Alternative form of a gene Character General feature (e.g., seed colour) Trait Specific form (e.g., yellow seed) Dihybrid cross Cross involving two genes Homozygous Same alleles (RR, yy) Heterozygous Different alleles (Rr, Yy) Phenotype Observable characters Genotype Genetic constitution

Mendel conducted his dihybrid experiment on pea plants using two contrasting traits.
Characters Selected
1. Seed Shape o Round (R) → Dominant o Wrinkled (r) → Recessive 2. Seed Colour o Yellow (Y) → Dominant o Green (y) → Recessive

Mendel crossed two pure breeding (homozygous) plants:

• Round Yellow seeds → RRYY
• Wrinkled Green seeds → rryy

These plants differed in both characters.

• RRYY produces only RY gametes
• rryy produces only ry gametes

This is because pure lines form only one type of gamete.

Cross: RRYY × rryy Genotype of F₁: All offspring → RrYy Phenotype of F₁: All plants show Round Yellow seeds Conclusion from F₁ Generation

• Dominant traits express
• Recessive traits remain hidden
• Confirms Law of Dominance

F₁ plants (RrYy) are self-pollinated to produce the F₂ generation.

Due to heterozygosity, each F₁ plant produces four types of gametes:

• RY
• Ry
• rY
• ry

This occurs due to independent assortment of alleles.

When four gametes from one parent combine with four gametes from another, 16 possible combinations are formed. These combinations give rise to four phenotypes.

Phenotype Number Round Yellow 9 Round Green 3 Wrinkled Yellow 3 Wrinkled Green 1 Phenotypic Ratio = 9 : 3 : 3 : 1

Statement Alleles of different genes assort independently during gamete formation. Explanation

• Inheritance of seed shape does not influence seed colour
• Genes segregate independently
• New combinations appear

Phenotypes like:

• Round Green
• Wrinkled Yellow

were absent in parents and appear newly in F₂ generation. These are called recombinant phenotypes.

Cross
RrYy × rryy Phenotypic Ratio
1 : 1 : 1 : 1 Significance
Confirms Law of Independent Assortment

• Genes are located on chromosomes
• During meiosis, homologous chromosomes align randomly
• Independent assortment occurs during Metaphase I

This explanation was given by Sutton and Boveri.

The law applies when:

• Genes are on different chromosomes
• Or located far apart on the same chromosome

• Does not apply to linked genes
• Cannot explain polygenic traits
• Does not cover incomplete dominance or codominance

• Explains genetic variation
• Helps in plant and animal breeding
• Essential for understanding heredity
• Foundation of classical genetics

• Dihybrid cross → Two traits
• F₁ genotype → RrYy
• F₂ phenotypic ratio → 9:3:3:1
• Test cross ratio → 1:1:1:1
• Law involved → Independent Assortment

What is inheritance of two genes?
It is the inheritance of two different characters controlled by two separate genes.
Who discovered dihybrid inheritance?
Gregor Johann Mendel.
Which law is proved by dihybrid cross?
Law of Independent Assortment.

Inheritance of two genes explains how multiple traits are inherited together and how new genetic combinations arise. Mendel’s dihybrid experiment laid the foundation for understanding genetic variation and heredity.