law

Mendel's law of independent assortment

Mendel's law of independent assortment states that genes for different traits assort independently of one another during gamete formation.
11 related articles

Description

After Mendel figured out the inheritance of a single trait, he wondered: what if you track two at once? He took peas with yellow smooth seeds and crossed them with green wrinkled ones. The first generation, as expected, was uniform—all yellow smooth. But in the second generation, besides the expected yellow smooth and green wrinkled, two new types appeared: yellow wrinkled and green smooth. And in strict proportions: 9 yellow smooth, 3 yellow wrinkled, 3 green smooth, and 1 green wrinkled. Mendel realized: genes for different traits are distributed into gametes independently of each other—as if nature were shuffling two separate decks of cards. Today we know this holds true for genes located on different chromosomes or far apart on the same chromosome.

History

Gregor Mendel described the law in the same 1866 paper where he also presented the law of segregation. He conducted dihybrid and trihybrid crosses of peas and showed that traits combine independently. After the rediscovery of Mendelian laws in 1900, the law of independent assortment was confirmed in experiments on various organisms. However, in 1906, William Bateson and Reginald Punnett discovered linked inheritance in sweet peas, which led to understanding the limitations of the law. Thomas Hunt Morgan and his students in the 1910s, using fruit flies, showed that genes are arranged linearly on chromosomes, and independent assortment is characteristic only of genes on different chromosomes or far apart on the same chromosome.

How it works

The law underlies combinative variation—the main source of genetic diversity in sexual reproduction. It allows breeders to combine useful traits from different parental lines in one organism and predict the proportion of offspring with the desired combination. In medical genetics, the law is used to calculate the probability of a child being born with two independent recessive diseases. Limits of applicability: the law holds only for genes located on different chromosomes or at a significant distance from each other on the same chromosome (crossover frequency > 50%). With tight linkage, genes are inherited together, and ratios deviate from Mendelian ones.

In practice

Breeders crossing a wheat variety with large ears but weak resistance to rust and a variety with small ears but high resistance can obtain a plant combining both useful traits. The law explains why a single litter of puppies can have individuals with different combinations of coat color and length.

Fun fact

Mendel tested the law on seven traits of peas, and all of them indeed were inherited independently. Later it turned out he was incredibly lucky: the genes for these traits in peas are located on seven different chromosomes—and peas have exactly seven. If even two genes had been close together on the same chromosome, he would have seen linkage and might not have deduced his law.

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