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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.

In one line: Test cross tests the hidden genotype; incomplete dominance makes the heterozygote visibly different.

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Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1If 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.
  2. 2A TT parent would produce all tall offspring.

Method you can reuse

  1. 1Choose symbols for alleles.
  2. 2Write parental genotypes.
  3. 3List gametes.
  4. 4Fill the Punnett square.
  5. 5Count genotype and phenotype ratios.
  6. 6For test cross, infer the unknown genotype from offspring distribution.

Sketch it in your answer

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
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.

Common wrong answer

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

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    A monohybrid cross follows one pair of contrasting traits.

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