Mendel's Second Law: The Law of Independent Assortment

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  • 0:20 Ear Size
  • 1:10 Dihybrid Cross
  • 3:16 Second Law
  • 6:16 Summary
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Lesson Transcript
Instructor: Greg Chin
Understanding how Mendel's law of independent assortment describes inheritance of genes is as easy as flipping a coin. Grab a few coins, cue up the video and see how.

Genetics of Flying Hamster Ear Size

Adrian's flying hamster research is going really well. He's figured out that hamster coat color is determined by a single gene and that the brown coat phenotype is dominant over the white coat phenotype.

Now, he's going to turn his attention to the ear size phenotype, because he's noticed that some have small ears and some have large Dumbo ears. Based on his previous results, Adrian hypothesizes that a single gene is also determining this phenotype and that he can verify this if he can find a 3:1 ratio between the two phenotypes.

Recall that he used big B and little b for coat color, so he decides he's going to use big E and little e for ear size. He picks big E to represent small ears and little e to represent large ears. He decides to perform a genetic cross again to confirm his hypothesis.

Dihybrid Cross

First, he has to establish true breeding strains. He takes brown hamsters with small ears and white hamsters with large ears, and he establishes true breeding strains of each. Adrian takes his brown hamsters with small ears and mates them with his white hamsters with large ears, and that's going to produce double heterozygotes at the F1 generation. So, he's going to have BbEe for every single progeny in this F1 generation.

Since we're talking about a cross with double heterozygotes, what we're monitoring here in the F1 generation is called a dihybrid cross. That is a cross between individuals that are heterozygous at two different loci. Adrian expects that his F1 cross will produce a 3:1 ratio between the dominant and recessive traits. However, he's perplexed to observe that instead he sees a 9:3:3:1 ratio among four different phenotypes.

What's going on here? Were the conclusions to his first experiment just flawed, or is there some other explanation that could possibly reconcile both sets of data? Well, let's see if we can help Adrian figure out what's going on.

Adrian recalls that genes are located on specific chromosomes. He also remembers that homologous chromosomes segregate into separate gametes during meiosis. However, there's really no rhyme or reason to predict which homolog a gamete receives. Now, what does this mean for genes that are located on separate chromosomes? Well, since they're located on separate chromosomes, it also means that the ear size genes are segregating independently. Adrian's experiment has demonstrated Mendel's second law.

Genes Sort Independently

Mendel's second law is also known as the law of independent assortment. It states that the alleles of one gene sort into the gametes independently of the alleles of another gene. To better understand the law of independent assortment, let's consider some coin tossing. Okay, do me a favor and pause the video while you obtain a coin. Don't worry - I'll wait for you.

Flipping a coin can simulate the randomness of chromosome segregation into gametes.
The Law of Independent Assortment

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