Question

Explain Incomplete Dominance and Co-dominance with suitable examples.

Hint:

Incomplete dominance produces an intermediate phenotype (e.g., red × white flowers = pink), whereas in co-dominance both alleles express equally in the heterozygote (e.g., AB blood group in humans).

Answer 1

Concept: In classical Mendelian genetics, one allele completely dominates over the other. However, in some cases the dominant allele does not completely mask the recessive allele. Such patterns of inheritance are called non-Mendelian inheritance. Two important examples of this are Incomplete Dominance and Co-dominance. These patterns explain how traits may show intermediate or combined expressions in heterozygous individuals.
Step 1:Incomplete Dominance.
Incomplete dominance is a pattern of inheritance in which neither allele is completely dominant over the other. As a result, the heterozygous individual shows an intermediate phenotype between the two parental traits. A classical example is seen in the flower colour of the \textit{Mirabilis jalapa} (four-o'clock plant).

• When a plant with red flowers (RR) is crossed with a plant having white flowers (rr), the F\(_1\) generation produces pink flowers (Rr).
• The pink colour appears because the red allele does not completely dominate the white allele.

When two pink plants (Rr × Rr) are crossed: \[ RR : Rr : rr = 1 : 2 : 1 \] Thus, the phenotypic ratio in the F\(_2\) generation becomes: \[ 1 \text{ Red} : 2 \text{ Pink} : 1 \text{ White} \] This is a characteristic feature of incomplete dominance where the phenotypic ratio matches the genotypic ratio.
Step 2:Co-dominance.
Co-dominance is a pattern of inheritance in which both alleles express themselves fully and independently in the heterozygous condition. Neither allele masks the effect of the other. A well-known example is the AB blood group in humans.

• The ABO blood group system is controlled by three alleles: \(I^A\), \(I^B\), and \(i\).
• When a person inherits \(I^A\) from one parent and \(I^B\) from the other, the genotype becomes \(I^A I^B\).
• In this case, both A and B antigens are expressed on the surface of red blood cells.

Thus, the individual shows AB blood group, where both alleles are expressed equally without blending.
Step 3:Difference between Incomplete Dominance and Co-dominance.

• Expression of alleles: In incomplete dominance, the heterozygote shows an intermediate phenotype, whereas in co-dominance both alleles are expressed simultaneously.
• Nature of phenotype: In incomplete dominance, parental traits blend to produce a new phenotype, while in co-dominance both parental traits appear together.
• Example: Flower colour in \textit{Mirabilis jalapa} shows incomplete dominance, whereas the ABO blood group system in humans shows co-dominance.

Thus, incomplete dominance and co-dominance are important exceptions to Mendel's principle of dominance and help explain variations in genetic inheritance.