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POLYGENIC INHERITANCE

Not all traits show just two distinct forms. Traits like human height and skin colour show continuous variation because they are controlled by multiple genes.

Key Features:
• Controlled by three or more genes.
• Each dominant allele contributes additively to the phenotype.
• More dominant alleles = stronger expression of the trait.
• Environmental factors also influence the final phenotype.

NCERT Example:
Human skin colour is controlled by multiple genes. Different combinations of dominant and recessive alleles produce a wide range of skin tones.

Remember:
Polygenic inheritance results in continuous variation, creating the diversity we observe in nature.

CO-DOMINANCE

In co-dominance, both alleles express themselves equally in a heterozygous condition. As a result, the F₁ generation shows the characters of both parents.

NCERT Example:
ABO blood group system in humans.

• IA and IB are co-dominant alleles.
• Both express their own type of antigen on red blood cells.
• When IA and IB are present together, blood group AB is formed.

Possible Blood Group Phenotypes:
A, B, AB and O

Co-dominance explains how two different traits can appear together without blending.

LAW OF INDEPENDENT ASSORTMENT

According to Mendel’s Law of Independent Assortment, the alleles of different genes assort independently during gamete formation.

NCERT Example:
Mendel crossed pea plants with round yellow seeds (RRYY) and wrinkled green seeds (rryy). All F₁ plants showed round yellow seeds (RrYy). On selfing, different combinations of traits appeared in the F₂ generation.

Phenotypic Ratio in F₂:
9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green

This law explains how different traits are inherited independently of each other.

LAW OF SEGREGATION

According to Mendel’s Law of Segregation, the two alleles of a gene separate during gamete formation so that each gamete receives only one allele.

NCERT Example:
When Mendel crossed pure round seeds (RR) with wrinkled seeds (rr), all F₁ plants showed round seeds (Rr). On selfing, both round and wrinkled seeds appeared in F₂ generation.

Phenotypic Ratio:
3 Round : 1 Wrinkled

Genotypic Ratio:
1 RR : 2 Rr : 1 rr

LAW OF DOMINANCE

According to Mendel’s Law of Dominance, in a heterozygous condition only one allele expresses itself while the other remains masked.

NCERT Example:
Purple flower (PP) × White flower (pp)
→ All F₁ plants show purple flowers (Pp)

The dominant allele expresses itself in the presence of the recessive allele.

In incomplete dominance, neither allele completely dominates the other. The heterozygous condition produces an intermediate phenotype.

NCERT Example:
Red flower (RR) × White flower (rr)
→ Pink flower (Rr)

Phenotypic Ratio in F₂ Generation:
1 Red : 2 Pink : 1 White

Genotypic Ratio in F₂ Generation:
1 RR : 2 Rr : 1 rr

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