BiologyChapter 12: Inheritance
Monohybrid Cross
One gene, two alleles, complete dominance.
Learning objectives
- Explain monohybrid cross in clear biological language.
- Distinguish 1. punnett habit from 2. test cross.
- Use the key terms to answer a short Federal Board exam question.
Definition
One gene, two alleles, complete dominance. Parental TT x tt -> all Tt (tall). F1 x F1: Tt x Tt. Punnett: four boxes, T and t from each. Offspring: 1 TT : 2 Tt : 1 tt. Phenotype 3 tall : 1 dwarf if T is tall. Ratios need large numbers; a family of four can miss the 3:1 by chance. Monohybrid crosses involve one gene with two alleles. The 3:1 phenotype ratio and 1:2:1 genotype ratio appear when both parents are heterozygous with complete dominance. Always show parent gametes on the Punnett grid edges before filling boxes.
Important terms
- MONOHYBRID CROSS
- One gene, two alleles, complete dominance.
- 1. PUNNETT HABIT
- Write parent gametes on the edges.
- 2. TEST CROSS
- Tall unknown x tt.
Overview
Boxes, then the ratio:
Punnett Habit
Write parent gametes on the edges. Fill every box. Circle phenotypes if asked. Do not mix two genes on one small grid. Draw grid with gametes on top and side. Fill four (or sixteen) boxes. State genotype ratio AND phenotype ratio separately. Large samples needed for ratios to appear.
Examples Each gamete is n · Each box is 2n · Equal chance school
Test Cross
Tall unknown x tt. All tall -> parent was TT. Half dwarf -> parent was Tt. This reveals the hidden allele. If all offspring show dominant phenotype → parent was homozygous dominant. If half show recessive → parent was heterozygous. Essential for identifying hidden recessive alleles.
Examples TT x tt -> all tall · Tt x tt -> 1 : 1 · Used in the exam a lot
WHAT — MONOHYBRID CROSS
One gene, two alleles, complete dominance. Parental TT x tt -> all Tt (tall). F1 x F1: Tt x Tt. Punnett: four boxes, T and t from each. Offspring: 1 TT : 2 Tt : 1 tt. Phenotype 3 tall : 1 dwarf if T is tall. Ratios need large numbers; a family of four can miss the 3:1 by chance. Monohybrid crosses involve one gene with two alleles. The 3:1 phenotype ratio and 1:2:1 genotype ratio appear when both parents are heterozygous with complete dominance. Always show parent gametes on the Punnett grid edges before filling boxes.
WHY — Why this topic matters
MONOHYBRID CROSS is an independent FBISE learning unit because students must connect its structures and processes with their biological functions and examples.
HOW — How it works
PUNNETT HABIT: Write parent gametes on the edges. Fill every box. Circle phenotypes if asked. Do not mix two genes on one small grid. Draw grid with gametes on top and side. Fill four (or sixteen) boxes. State genotype ratio AND phenotype ratio separately. Large samples needed for ratios to appear. Examples: Each gamete is n; Each box is 2n; Equal chance school. TEST CROSS: Tall unknown x tt. All tall -> parent was TT. Half dwarf -> parent was Tt. This reveals the hidden allele. If all offspring show dominant phenotype → parent was homozygous dominant. If half show recessive → parent was heterozygous. Essential for identifying hidden recessive alleles. Examples: TT x tt -> all tall; Tt x tt -> 1 : 1; Used in the exam a lot. Worked Tt x Tt (T tall). Gametes: T or t from each. Boxes: TT, Tt, Tt, tt. Genotype 1 : 2 : 1. Phenotype 3 tall : 1 dwarf.
Worked example
- Worked Tt x Tt (T tall).
- Gametes: T or t from each.
- Boxes: TT, Tt, Tt, tt.
- Genotype 1 : 2 : 1.
- Phenotype 3 tall : 1 dwarf.
In simple words
One gene. Chance, not a guarantee in four children.
Key points
- 3 : 1 phenotype
- 1 : 2 : 1 genotype
- gametes on the rim
- cross with recessive
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Monohybrid Cross
Sticky Notes · Quick Revision
Core Idea
- One gene, two alleles, complete dominance. Parental TT x tt -> all Tt (tall). F1 x F1: Tt x Tt. Punnett: four boxes, T and t from each. Offspring: 1 TT : 2 Tt : 1 tt. Phenotype 3 tall : 1 dwarf if T is tall. Ratios need large numbers; a family of four can miss the 3:1 by chance. Monohybrid crosses involve one gene with two alleles. The 3:1 phenotype ratio and 1:2:1 genotype ratio appear when both parents are heterozygous with complete dominance. Always show parent gametes on the Punnett grid edges before filling boxes.
1. PUNNETT HABIT
- Write parent gametes on the edges. Fill every box. Circle phenotypes if asked. Do not mix two genes on one small grid. Draw grid with gametes on top and side. Fill four (or sixteen) boxes. State genotype ratio AND phenotype ratio separately. Large samples needed for ratios to appear.
2. TEST CROSS
- Tall unknown x tt. All tall -> parent was TT. Half dwarf -> parent was Tt. This reveals the hidden allele. If all offspring show dominant phenotype → parent was homozygous dominant. If half show recessive → parent was heterozygous. Essential for identifying hidden recessive alleles.
Exam Example
- Worked Tt x Tt (T tall).
- Gametes: T or t from each.
- Boxes: TT, Tt, Tt, tt.
- Genotype 1 : 2 : 1.
- Phenotype 3 tall : 1 dwarf.
Common Misconception
- Do not treat 1. PUNNETT HABIT and 2. TEST CROSS as the same idea; each explains a different part of the topic.
FBISE Exam Check
- Define monohybrid cross, explain the two key parts, and use the correct biological terms.
- Exam reminder: Worked Tt x Tt (T tall).
60-second revision
- One gene, two alleles, complete dominance.
- 1. PUNNETT HABIT: Write parent gametes on the edges.
- 2. TEST CROSS: Tall unknown x tt.
- Exam reminder: Worked Tt x Tt (T tall).
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