Dihybrid Inheritance (AQA A-Level Biology): Revision Notes
Dihybrid Inheritance
Dihybrid inheritance describes how two different characteristics are passed from parents to offspring when these traits are controlled by genes located on separate chromosomes. This builds on monohybrid inheritance by examining what happens when two traits are inherited together.
Understanding dihybrid inheritance
While monohybrid inheritance focuses on a single characteristic, thousands of traits are actually inherited simultaneously. Dihybrid inheritance examines how two different characteristics, controlled by genes on different chromosomes, are passed down through generations.
The key principle is that genes on separate chromosomes behave independently during meiosis, allowing different combinations of alleles to form in gametes.
Mendel's classic pea plant experiment
Gregor Mendel investigated dihybrid inheritance by studying two characteristics of pea plants simultaneously:
- Seed shape: Round shape (dominant) versus wrinkled shape (recessive)
- Seed colour: Yellow seeds (dominant) versus green seeds (recessive)
Worked Example: Mendel's Dihybrid Cross
The experimental setup: Mendel crossed two pure breeding varieties:
- Parent 1: Always produced round-shaped, yellow-coloured seeds (both dominant features)
- Parent 2: Always produced wrinkled-shaped, green-coloured seeds (both recessive features)
Using genetic notation:
- R = allele for round seeds, r = allele for wrinkled seeds
- G = allele for yellow seeds, g = allele for green seeds
The parental cross was:
Results and genetic explanation
F1 generation
All F1 offspring produced round-shaped, yellow-coloured seeds, showing only the dominant characteristics. The genotype of all F1 plants was RrGg, but the phenotype was uniform due to dominance.
F2 generation
When F1 plants were crossed with each other (), the F2 generation showed four different phenotypes in the following numbers:
| Phenotype | Numbers observed |
|---|---|
| Round, yellow | 315 |
| Round, green | 108 |
| Wrinkled, yellow | 101 |
| Wrinkled, green | 32 |
This gave an approximate 9:3:3:1 ratio, which matched theoretical predictions.
How independent assortment works
The F1 plants (RrGg) can produce four different types of gametes: RG, Rg, rG, and rg. This happens because:
The mechanism of independent assortment:
- During meiosis, chromosomes arrange randomly at the cell centre
- The gene for seed colour and gene for seed shape are on separate chromosomes
- Any allele for seed colour (G or g) can combine with any allele for seed shape (R or r)
- Fertilisation occurs randomly between any male and female gametes
When these four gamete types combine randomly during fertilisation, they produce the characteristic 9:3:3:1 phenotypic ratio.
Mendel's law of independent assortment
Based on these observations, Mendel formulated a fundamental principle of genetics.
Mendel's law of independent assortment states that: Each member of a pair of alleles may combine randomly with either allele of another pair.
This law explains why dihybrid crosses produce the 9:3:3:1 ratio rather than the 3:1 ratio seen in monohybrid crosses.
Links to meiosis
Dihybrid inheritance demonstrates the cellular mechanism of meiosis. During meiosis:
- Homologous chromosomes separate independently
- Genes on different chromosomes assort randomly into gametes
- This creates genetic variation in offspring
Understanding chromosome behaviour during meiosis is essential for explaining why alleles on different chromosomes inherit independently. The random arrangement of chromosome pairs during meiosis I directly causes the independent assortment observed in dihybrid crosses.
Applications and significance
Mendel's work on dihybrid inheritance was groundbreaking because:
- It demonstrated that inheritance follows predictable mathematical patterns
- It showed genes behave as discrete units
- It predicted the existence of chromosomes before they were discovered
- It laid the foundation for modern genetics
The principles apply to any two characteristics controlled by genes on separate chromosomes, making dihybrid inheritance relevant across all sexually reproducing organisms.
Key Points to Remember:
- Dihybrid inheritance involves two different characteristics controlled by genes on separate chromosomes
- F1 generation from pure breeding parents shows only dominant traits (RrGg genotype)
- F2 generation produces a 9:3:3:1 phenotypic ratio when F1 individuals are crossed
- Independent assortment occurs because alleles on different chromosomes separate randomly during meiosis
- Mendel's law of independent assortment states that pairs of alleles combine randomly with other pairs during gamete formation