Question

Work out separate monohybrid crosses up to $F_2$ generation between two pea plants and two Antirrhinum plants, both having contrasting traits with respect to the colour of the flower. Comment on the patterns of inheritance in the crosses carried out in such two cases.

Hint:

Remember that in incomplete dominance, the phenotypic and genotypic ratios are identical ($1:2:1$). This serves as a primary point of differentiation from classic Mendelian monohybrid inheritance.

Answer 1

Concept: Inheritance patterns can vary depending on the nature of the alleles involved:
• Complete Dominance (Pea Plant): In a classic Mendelian monohybrid cross, one allele completely masks the expression of the alternative recessive allele in the heterozygous state. The phenotypic ratio in the $F_2$ generation is characteristically $3:1$.
• Incomplete Dominance (Antirrhinum majus): In some organisms, the dominant allele does not completely mask the recessive allele. Instead, the heterozygous phenotype emerges as a fine blend or an intermediate state between the two homozygous phenotypes. Consequently, both the genotypic and phenotypic ratios in the $F_2$ generation align perfectly as $1:2:1$.

Step 1: Monohybrid Cross in Pea Plants (Pisum sativum) - Complete Dominance
In pea plants, flower colour shows complete dominance. Let the allele for dominant Violet flowers be represented by '$V$' and the allele for recessive White flowers be represented by '$v$'.
• Parents ($P_1$): Homozygous Violet Flower ($VV$) $\times$ Homozygous White Flower ($vv$)
• Gametes: $V$ (from violet parent) and $v$ (from white parent)
• First Filial Generation ($F_1$): All offspring are heterozygous ($Vv$) exhibiting a Violet flower phenotype because $V$ is completely dominant over $v$. Now, self-pollinating the $F_1$ generation ($Vv \times Vv$) to obtain the $F_2$ generation:
• Gametes from $F_1$: $V$ and $v$ $F_2$ Generation Punnett Square: {|c|c|c|} Gametes & V & v
V & $VV$ (Violet) & $Vv$ (Violet)
v & $Vv$ (Violet) & $vv$ (White)
Ratios for Pea Plant Cross:
• Genotypic Ratio: $1 \, VV : 2 \, Vv : 1 \, vv$ (i.e., $1:2:1$)
• Phenotypic Ratio: $3$ Violet : $1$ White (i.e., $3:1$)

Step 2: Monohybrid Cross in Snapdragon (Antirrhinum majus) - Incomplete Dominance
In snapdragon plants, flower colour displays incomplete dominance. Let the allele for Red flowers be represented by '$R$' and the allele for White flowers be represented by '$r$'.
• Parents ($P_2$): Homozygous Red Flower ($RR$) $\times$ Homozygous White Flower ($rr$)
• Gametes: $R$ (from red parent) and $r$ (from white parent)
• First Filial Generation ($F_1$): All offspring are heterozygous ($Rr$). Instead of showing a dominant trait, they exhibit an intermediate Pink flower phenotype due to partial expression. Self-pollinating the $F_1$ generation ($Rr \times Rr$) to obtain the $F_2$ generation:
• Gametes from $F_1$: $R$ and $r$ $F_2$ Generation Punnett Square: {|c|c|c|} Gametes & R & r
R & $RR$ (Red) & $Rr$ (Pink)
r & $Rr$ (Pink) & $rr$ (White)
Ratios for Antirrhinum Cross:
• Genotypic Ratio: $1 \, RR : 2 \, Rr : 1 \, rr$ (i.e., $1:2:1$)
• Phenotypic Ratio: $1$ Red : $2$ Pink : $1$ White (i.e., $1:2:1$)

Step 3: Commentary on Patterns of Inheritance

• In Pea Plants: The inheritance pattern confirms the Law of Dominance. The $F_1$ hybrid resembles one parent entirely, and the recessive trait is hidden, only to reappear unaltered in a $3:1$ ratio in the $F_2$ stage.
• In Antirrhinum: The inheritance pattern deviates from Mendel's traditional law and demonstrates Incomplete Dominance. The $F_1$ phenotype does not resemble either of the homozygous parental phenotypes but is a complete blend (pink). In the $F_2$ generation, the phenotypic ratio changes from $3:1$ to $1:2:1$, becoming completely identical to its genotypic ratio.