12.6
Law of Independent Assortment
While Mendel’s Law of Segregation states that the two alleles for one gene are separated into different gametes, a different question of how different…
Gregor Mendel’s first experiments showed that units called genes help pass traits from parents to offspring.
Each organism has two copies of each gene, called alleles, with one inherited from each parent.
Mendel’s next experiment used dihybrid crosses of pea plants that differed in two traits, such as height and flower color, to test whether the inheritance of one trait would affect the inheritance of another.
If the alleles for the two traits were inherited together as a single unit in the F1 generation, the F2 offspring would always show either both dominant or both recessive phenotypes and would never show a mix of the two.
When Mendel crossed the F₁ dihybrid pea plants, he found that among every 16 F₂ offspring, about nine showed both dominant phenotypes—tall plants with purple flowers. Three showed a dominant height trait and a recessive flower color trait—tall plants with white flowers. Another three showed a recessive height trait and a dominant flower color trait—short plants with purple flowers. One showed both recessive phenotypes—short plants with white flowers.
When each trait is considered separately, such as plant height in the F2 generation, the ratio of dominant tall plants to recessive short plants remains 3:1. The same pattern appears for flower color, with a 3:1 ratio of purple flowers to white flowers.
All four phenotype combinations could appear only if the alleles for height assorted independently from the alleles for flower color.
These results formed the basis of Mendel’s Law of Independent Assortment, which states that alleles for genes on different chromosomes, or far apart on the same chromosome, sort into gametes independently of one another.
The 9:3:3:1 phenotypic ratio shows that each dihybrid parent is equally likely to pass on all possible combinations of dominant and recessive alleles: tall with purple flowers, tall with white flowers, short with purple flowers, or short with white flowers.
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Q1: What is the Law of Independent Assortment?
The Law of Independent Assortment states that alleles for different genes are segregated into gametes independently of one another. This means the inheritance of one trait does not influence the inheritance of another trait. Each dihybrid parent is equally likely to pass on all possible combinations of dominant and recessive alleles to offspring.
Q2: Why did Mendel use dihybrid crosses to test independent assortment?
Mendel used dihybrid crosses to examine whether the inheritance of one trait would influence the inheritance of another. By crossing pea plants differing in two traits, such as height and flower color, he could determine if alleles for different genes were inherited together or separately into gametes.
Q3: What does the 9:3:3:1 ratio reveal about gene inheritance?
The 9:3:3:1 phenotypic ratio in F2 offspring indicates that all four combinations of phenotypes occur in predictable proportions. This ratio proves that dominant and recessive alleles for different traits assort independently, producing nine dominant-dominant, three dominant-recessive, three recessive-dominant, and one recessive-recessive offspring.
Q4: How does chromosome behavior during meiosis explain independent assortment?
Independent assortment occurs because chromosomes pair up randomly during meiosis I along the metaphase plate. Genes on different chromosomes sort independently into gametes. However, genes on the same chromosome violate this law and are inherited together, a phenomenon called linkage.
Q5: What is genetic linkage and how does it differ from independent assortment?
Genetic linkage occurs when two genes reside on the same chromosome and are inherited together rather than independently. Linked genes, especially those close together, do not demonstrate the 9:3:3:1 ratio in F2 generations of dihybrid crosses, violating the Law of Independent Assortment.
Q6: How did Mendel's F1 and F2 generations demonstrate independent assortment?
Mendel crossed homozygous parents with different traits, producing F1 offspring heterozygous for both traits. When F1 plants self-fertilized, the F2 generation displayed all four phenotype combinations in a 9:3:3:1 ratio, proving that alleles for height and flower color segregated independently into gametes.
Q7: Why would linked genes not produce a 9:3:3:1 ratio in a dihybrid cross?
Linked genes on the same chromosome are inherited together and do not assort independently. This causes parental phenotype combinations to appear more frequently in offspring than recombinant phenotypes, disrupting the expected 9:3:3:1 ratio characteristic of independent assortment and violating Mendel's law.