4.8.7Additional lessons
Dihybrid Crosses and Unlinked Genes — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 74 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Unlinked Genes and Independent Assortment
Unlinked genes
- Unlinked genes Genes found on different chromosomes, or very far apart on the same chromosome, so that they are not inherited as a unit.
Segregation
- Segregation The two alleles of a gene separate during meiosis, so each gamete receives only one allele of each gene.
Independent assortment
- Independent assortment Homologous pairs line up at random in metaphase I,
- so the allele of one gene entering a gamete does not influence the allele of another gene.
Gamete types
- Gamete types A heterozygote for two unlinked genes, RrYy, makes four types of gamete (RY, Ry, rY, ry) in equal numbers.
Part 2 of 5Setting Out a Dihybrid Cross
Treat the two genes carefully and keep the letters in the same order.
1
Choose a letter for each gene and state which allele is dominant, for example R round and r wrinkled, Y yellow and y green.
2
Write the genotype of each parent, with both genes for one parent written in the same order as for the other.
3
Work out the gametes by pairing one allele of the first gene with one allele of the second gene in every possible way.
4
Place the gametes in a Punnett grid (4 × 4 for two double heterozygotes), fill in the boxes, and count each phenotype.
5
Give the ratio of phenotypes in its simplest whole-number form.
Part 3 of 5Key Dihybrid Ratios
Key Dihybrid Ratios
Double heterozygote cross
- Double heterozygote cross RrYy × RrYy gives 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
Test cross
- Test cross RrYy × rryy gives 1 : 1 : 1 : 1 for the four phenotypes, because the double recessive makes only ry gametes.
Why 9 : 3 : 3 : 1
- Why 9 : 3 : 3 : 1 It is the product of two independent 3 : 1 monohybrid ratios, since (3 + 1) × (3 + 1) = 9 + 3 + 3 + 1.
Mendel's law
- Mendel's law The law of independent assortment is not universal, as linked genes break it, but it holds well for unlinked genes.
| Cross | Gametes from each parent | Phenotype ratio |
|---|---|---|
| RrYy × RrYy | 4 types × 4 types | 9 : 3 : 3 : 1 |
| RrYy × rryy | 4 types × 1 type | 1 : 1 : 1 : 1 |
Part 4 of 5Worked Example
Question
- Two pea plants heterozygous for seed shape (R round, r wrinkled) and colour (Y yellow, y green) are crossed.
- Out of 160 seeds, how many of each phenotype are expected?
- Each parent is RrYy and makes four gamete types in equal numbers: RY, Ry, rY and ry.
- The 16 boxes in the Punnett grid give a ratio of 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
- Divide 160 by 16 to give 10 seeds per box.
- Multiply9 × 10 = 90, 3 × 10 = 30, 3 × 10 = 30, 1 × 10 = 10.
Answer Expected: 90 round yellow, 30 round green, 30 wrinkled yellow and 10 wrinkled green.
Part 5 of 5Exam Checklist & Tips
4.8.7 – Key Points for the Exam
- Unlinked genes Genes on different chromosomes that assort independently.
- Independent assortment Random orientation of homologous pairs in meiosis so allele combinations in gametes are random.
- Dihybrid cross A cross that follows the inheritance of two genes at once.
- Test cross A cross between an individual of unknown genotype and a homozygous recessive individual.
Common Mistakes
- Writing gametes with two alleles of the same gene, such as Rr, instead of one allele of each gene.
- Forgetting that 9 : 3 : 3 : 1 applies only to unlinked genes and double heterozygous parents.
- Leaving the ratio unsimplified, such as 36 : 12 : 12 : 4.
- Counting only 8 boxes of the Punnett grid and missing that two heterozygotes give 16 boxes.
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