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Inheritance of One Gene: Monohybrid Cross

Inheritance of one gene studies how a single character controlled by one gene pair passes from parents to offspring, producing predictable genotypic and phenotypic ratios.

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Student-friendly explanation

A monohybrid cross follows one pair of contrasting traits. In complete dominance, a heterozygote shows the dominant phenotype and the F2 phenotypic ratio becomes 3:1, while the genotypic ratio is 1:2:1. A test cross uses an unknown dominant phenotype crossed with a homozygous recessive individual to identify genotype. In incomplete dominance, the heterozygote has an intermediate phenotype, as in Mirabilis flower colour, giving a 1:2:1 phenotypic ratio. In codominance, both alleles express in the heterozygote, as in the AB blood group. Exams commonly test ratio interpretation and genotype identification.

How to write this in exams

  1. 1

    Start with the exact idea

    Inheritance of one gene studies how a single character controlled by one gene pair passes from parents to offspring, producing predictable genotypic and phenotypic ratios.

  2. 2

    Then show how to use it

    Choose symbols for alleles. Write parental genotypes. List gametes. Fill the Punnett square. Count genotype and phenotype ratios. For test cross, infer the unknown genotype from offspring distribution.

  3. 3

    Add one concrete example

    In Mirabilis, red flowered plants crossed with white flowered plants produce pink F1 flowers. Selfing pink F1 plants gives red, pink, and white flowers in a 1:2:1 ratio, showing incomplete dominance.

  4. 4

    Avoid this incomplete answer

    Writing IAIB as a blended blood group is wrong; A and B antigens are both expressed, so it is codominance.

Definition

Inheritance of one gene studies how a single character controlled by one gene pair passes from parents to offspring, producing predictable genotypic and phenotypic ratios.

Example

In Mirabilis, red flowered plants crossed with white flowered plants produce pink F1 flowers. Selfing pink F1 plants gives red, pink, and white flowers in a 1:2:1 ratio, showing incomplete dominance.

Rule to remember

Complete dominance: phenotype ratio 3:1 and genotype ratio 1:2:1. Incomplete dominance and codominance: phenotype ratio often matches genotype ratio as 1:2:1 for a single gene pair.

Memory hook

Test cross tests the hidden genotype; incomplete dominance makes the heterozygote visibly different.

Examples and method

Worked example

If a tall pea plant with unknown genotype is crossed with a dwarf plant and the offspring include tall and dwarf in a 1:1 ratio, the tall parent must be heterozygous Tt. A TT parent would produce all tall offspring.

Method to apply

Choose symbols for alleles. Write parental genotypes. List gametes. Fill the Punnett square. Count genotype and phenotype ratios. For test cross, infer the unknown genotype from offspring distribution.

Diagram support

Use a Punnett square with labels for parents, gametes, F1 genotype, and F2 combinations. For ABO inheritance, label IA, IB, and i alleles clearly and show codominant expression in IAIB.

How CBSE asks it

Questions may ask for a test cross result, the difference between incomplete dominance and codominance, ABO blood group genotypes, or the reason for 1:2:1 instead of 3:1.

Avoid common mistakes

Common confusion

A frequent mistake is to call incomplete dominance blending inheritance. The alleles do not blend permanently; they segregate and parental phenotypes reappear in F2.

Common wrong answer

Writing IAIB as a blended blood group is wrong; A and B antigens are both expressed, so it is codominance.

Exam tip

Always write both genotypic and phenotypic ratios. In incomplete dominance and codominance, do not force the 3:1 ratio.

Quick check

How does incomplete dominance change the F2 phenotypic ratio in a monohybrid cross?

Incomplete dominance changes the F2 phenotypic ratio to 1:2:1 because the heterozygote has its own distinct intermediate phenotype. Therefore genotype and phenotype ratios become the same.

Answer writing and exam use

1-mark answer

Name the cross, assign allele symbols, show parents and gametes, draw the Punnett square, write genotype and phenotype ratios, and explain the inheritance pattern in one sentence.

2-mark answer

Name the cross, assign allele symbols, show parents and gametes, draw the Punnett square, write genotype and phenotype ratios, and explain the inheritance pattern in one sentence.

3-mark answer

Name the cross, assign allele symbols, show parents and gametes, draw the Punnett square, write genotype and phenotype ratios, and explain the inheritance pattern in one sentence.
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