Law of Independent Assortment
The Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation, meaning the inheritance of one gene does not influence the inheritance of another gene located on a different chromosome. This law, Mendel's Second Law, explains why a dihybrid cross (RrYy × RrYy) produces a 9:3:3:1 phenotypic ratio. It applies specifically to genes on non-homologous (different) chromosomes; genes on the same chromosome may be linked and do not assort independently.
Worked Example
Problem
Two dihybrid pea plants (RrYy × RrYy) are crossed, where R = round seeds (dominant), r = wrinkled seeds (recessive), Y = yellow (dominant), y = green (recessive). What is the expected phenotypic ratio?
Solution
Step 1: Determine possible gametes from each parent (RrYy): Each parent produces 4 gamete types: RY, Ry, rY, ry — each with probability 1/4. Step 2: Construct a 4×4 Punnett square (16 cells total). Step 3: Count phenotypic classes: R_Y_ (round yellow): 9 cells R_yy (round green): 3 cells rrY_ (wrinkled yellow): 3 cells rryy (wrinkled green): 1 cell Step 4: The ratio is 9:3:3:1.
Answer
Expected phenotypic ratio = 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
Dihybrid Cross Phenotypic Outcomes (RrYy × RrYy)
| Phenotype | Genotype Pattern | Number of Combinations | Fraction |
|---|---|---|---|
| Round Yellow | R_Y_ | 9 | 9/16 |
| Round Green | R_yy | 3 | 3/16 |
| Wrinkled Yellow | rrY_ | 3 | 3/16 |
| Wrinkled Green | rryy | 1 | 1/16 |
| Total | — | 16 | 16/16 |
Interactive Tools
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Related Terms
Mendel's Law of Segregation
Mendel's Law of Segregation states that during the formation of gametes (sex cells), the two alleles for each gene separate (segregate) so that each gamete receives only one allele. This segregation is random, and therefore each allele has an equal probability of ending up in any given gamete. Proposed by Gregor Mendel in 1865 based on his pea plant experiments, this law forms the foundation of modern genetics and explains why offspring inherit one allele from each parent.
Punnett Square
A Punnett square is a diagram used to predict the probability of genotypes and phenotypes in the offspring of two parents, based on the alleles each parent can contribute. Developed by British geneticist Reginald Crundall Punnett in the early 20th century, it arranges parental alleles along the axes of a grid, and each cell represents a possible offspring genotype. It is a foundational tool in Mendelian genetics for calculating ratios such as 3:1 (phenotypic) or 1:2:1 (genotypic) for monohybrid crosses.
Heterozygous
An organism is heterozygous for a particular gene when it carries two different alleles at that genetic locus — one dominant and one recessive (e.g., Aa). The dominant allele is expressed in the phenotype, while the recessive allele is carried but not expressed, making the individual a "carrier" of the recessive trait. Heterozygosity is important in population genetics because it maintains genetic diversity and affects the probability of recessive disorders appearing in offspring.
Named after Gregor Mendel. "Independent" comes from Latin "independens" (not hanging on anything), and "assortment" comes from Old French "assortir" meaning "to match" or "to sort". The term describes the random sorting of alleles from different chromosomes into gametes independently of one another.