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Monohybrid Ratio

The monohybrid ratio is the expected ratio of traits or genotypes when only one character is studied in a cross.

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Student-friendly explanation

In a monohybrid cross, only one pair of contrasting traits is considered, such as tall and short stem height. When two heterozygous parents are crossed, the offspring show a standard pattern: the genotype ratio is 1:2:1 and the phenotype ratio is 3:1. This chapter often asks students to identify or use these ratios in simple crosses.

How to write this in exams

  1. 1

    Start with the exact idea

    The monohybrid ratio is the expected ratio of traits or genotypes when only one character is studied in a cross.

  2. 2

    Then show how to use it

    1. Write the two parent genotypes. 2. Separate the alleles of each parent. 3. Fill the Punnett square. 4. Count genotype combinations. 5. Convert them into phenotype ratio if asked.

  3. 3

    Add one concrete example

    If Tt is crossed with Tt, the offspring can be TT, Tt, Tt, or tt. The phenotypic result is three tall plants and one short plant.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is to write 3:1 for genotype ratio. That is incorrect because 3:1 is the phenotype ratio, not the genotype ratio.

Definition

The monohybrid ratio is the expected ratio of traits or genotypes when only one character is studied in a cross.

Example

If Tt is crossed with Tt, the offspring can be TT, Tt, Tt, or tt. The phenotypic result is three tall plants and one short plant.

Rule to remember

For Tt x Tt, genotype ratio = 1 TT : 2 Tt : 1 tt, and phenotype ratio = 3 dominant : 1 recessive.

Memory hook

One trait, one cross, 1:2:1 and 3:1.

Examples and method

Worked example

In Tt x Tt, the Punnett square gives TT, Tt, Tt, and tt. TT and Tt show the dominant trait, so three offspring are tall, and one is short. That is why the phenotype ratio is 3:1.

Method to apply

1. Write the two parent genotypes. 2. Separate the alleles of each parent. 3. Fill the Punnett square. 4. Count genotype combinations. 5. Convert them into phenotype ratio if asked.

Diagram support

A Punnett square is the best diagram for a monohybrid cross because it clearly shows all four offspring combinations.

How CBSE asks it

Find the genotype or phenotype ratio from a monohybrid cross, or predict offspring traits using a Punnett square.

Avoid common mistakes

Common confusion

Students often mix genotype ratio with phenotype ratio. They also forget that the 3:1 result is for visible traits, not for gene combinations.

Common wrong answer

A common wrong answer is to write 3:1 for genotype ratio. That is incorrect because 3:1 is the phenotype ratio, not the genotype ratio.

Exam tip

For a monohybrid cross of two heterozygous parents, remember 1:2:1 for genotype and 3:1 for phenotype.

Quick check

What phenotype ratio is expected when two heterozygous tall plants are crossed?

The expected phenotype ratio is 3 tall plants to 1 short plant. This happens because tallness is dominant and appears in three of the four combinations.

Answer writing and exam use

1-mark answer

The monohybrid ratio is the expected ratio of traits or genotypes when only one character is studied in a cross.

2-mark answer

The monohybrid ratio is the expected ratio of traits or genotypes when only one character is studied in a cross. For Tt x Tt, genotype ratio = 1 TT : 2 Tt : 1 tt, and phenotype ratio = 3 dominant : 1 recessive. If Tt is crossed with Tt, the offspring can be TT, Tt, Tt, or tt. The phenotypic result is three tall plants and one short plant.

3-mark answer

In a monohybrid cross, only one pair of contrasting traits is considered, such as tall and short stem height. When two heterozygous parents are crossed, the offspring show a standard pattern: the genotype ratio is 1:2:1 and the phenotype ratio is 3:1. This chapter often asks students to identify or use these ratios in simple crosses. For Tt x Tt, genotype ratio = 1 TT : 2 Tt : 1 tt, and phenotype ratio = 3 dominant : 1 recessive. In Tt x Tt, the Punnett square gives TT, Tt, Tt, and tt. TT and Tt show the dominant trait, so three offspring are tall, and one is short. That is why the phenotype ratio is 3:1. Find the genotype or phenotype ratio from a monohybrid cross, or predict offspring traits using a Punnett square. A common wrong answer is to write 3:1 for genotype ratio. That is incorrect because 3:1 is the phenotype ratio, not the genotype ratio.
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