# Law of independent assortment

Mendel's second law. Alleles of different genes are sorted into gametes independently, if the genes are on different chromosomes. A dihybrid cross of two double heterozygotes (AaBb x AaBb) gives a 9:3:3:1 ratio.

Biology (university) · Part 07: Genetics and inheritance · Card 3 of 8 · Last updated 2026-10-19

## Analogy

Shuffling two separate decks and drawing one card from each.

## Example

**Dihybrid ratio**

AaBb x AaBb gives:
9 both dominant
3 A only
3 B only
1 both recessive

## In practice

Round yellow pea plants crossed with each other give round yellow, round green, wrinkled yellow and wrinkled green in a 9:3:3:1 ratio.

## Practice

### Flashcards

- **What ratio comes from AaBb x AaBb?** 9 : 3 : 3 : 1 for independently assorting genes.

### Interview questions

**Describe how recombination frequency is used to map genes and why linked genes break Mendel's second law.**

- Linked genes on same chromosome
- Do not assort independently
- Crossing over separates them
- Frequency proportional to distance

## Questions people ask

### Why does independent assortment not always hold?

It holds only for genes on different chromosomes, or far apart on the same one. Genes close together on the same chromosome are linked and tend to be inherited together. Mendel's pea traits happened to be on different chromosomes, or far apart, so his ratios came out clean.

### How do I do a dihybrid Punnett square?

List the four types of gamete from each parent, such as AB, Ab, aB, ab. Write them along each side of a four-by-four grid and fill in the 16 boxes. Count the phenotypes. For two heterozygous parents you get 9:3:3:1.

## Related concepts

- [Linked genes](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-6)
- [Allele](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-1)
- [Law of segregation](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-2)
- [Incomplete dominance and codominance](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-4)

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