Monohybrid Ratio
The monohybrid ratio is the expected ratio of traits or genotypes when only one character is studied in a cross.
In one line: One trait, one cross, 1:2:1 and 3:1.
Practice This ConceptUnderstand it
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.
Example
Rule to remember
For Tt x Tt, genotype ratio = 1 TT : 2 Tt : 1 tt, and phenotype ratio = 3 dominant : 1 recessive.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
In Tt x Tt, the Punnett square gives TT, Tt, Tt, and tt.
- 1TT and Tt show the dominant trait, so three offspring are tall, and one is short.
- 2That is why the phenotype ratio is 3:1.
Method you can reuse
- 1Write the two parent genotypes.
- 2Separate the alleles of each parent.
- 3Fill the Punnett square.
- 4Count genotype combinations.
- 5Convert them into phenotype ratio if asked.
Visual tip
A Punnett square is the best diagram for a monohybrid cross because it clearly shows all four offspring combinations.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
What phenotype ratio is expected when two heterozygous tall plants are crossed?
Think first, then show answerHide answer
Common wrong answer
A common wrong answer is to write 3:1 for genotype ratio.
Why it loses marks: 3:1 is the phenotype ratio, not the genotype ratio.
Don't mix this up
Exam tip
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
- 1
Enough for 1 mark
The monohybrid ratio is the expected ratio of traits or genotypes when only one character is studied in a cross.
- 2
Add for 2 marks
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.
- 3
Add for 3 marks
In a monohybrid cross, only one pair of contrasting traits is considered, such as tall and short stem height.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
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