Lethal Alleles: Definition & Examples

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  • 0:01 Agouti Mice
  • 1:58 Lethal Alleles
  • 4:33 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Some alleles are recessive, some are dominant, and some are just plain bad. In this lesson, we'll learn about how rare lethal alleles can affect the survival of an organism. Specifically, we'll talk about how something like coat color can be lethal in agouti mice.

Agouti Mice

There once was a scientist studying the genetics of coat color in mice who came across a fascinating genetic anomaly. I know what you're thinking. How could studying a common house mouse lead to a scientific discovery that's relevant to humans?

We had a mouse running amuck in our kitchen not too long ago, and my immediate thought was not to use it to study genetics. My first thought was related to lethality, which is related to this lesson, but not exactly in the same way. I have to admit, scientists have learned a lot from these mice. I'll hold you in suspense for a few minutes while we talk about something our scientist noted that was very, very odd.

Agouti is a gene that contributes to coat color in many animals, including mice. In mice, the wild type allele codes for yellow and black striped hair, which looks grey from far away. We can represent this wild type allele by a big 'A.' There are more than 14 identified mutant alleles of this agouti gene. For one of those alleles, the mutation is recessive and causes the hair to be black.

There is another allele of agouti that causes the mice to have a light yellowish color because the black stripes are missing from the hairs. This allele is dominant to the wild type allele. For this lesson, we'll name this allele A^y.

Our scientist was studying these yellow mice. He had two mice that he assumed were heterozygous for this yellow allele and the wild type allele, A. Therefore, we'll give these yellow mice a genotype of A^yA. Because A^y is dominant, our scientist expected that he'd see a 3:1 phenotypic ratio of yellow to grey mice. If you were to create a Punnett square for this cross, you can see why he'd expect that. You would too, right? This appears to be a simple, monohybrid cross.

So, here's where the oddity came in. Our scientist never, ever saw that. In fact, what he saw, without fail, was a 2:1 ratio of yellow and grey mice. I imagine that he must have done a lot of crosses and produced cages full of these mice, trying to get the numbers to work out. But they never did.

Lethal Alleles

Whenever your actual results from a genetic cross differ from your expected results, it indicates something additional is going on. What do you think was happening here? It turns out, that if you look at the genotype of these yellow mice resulting from this cross, they are A^yA. You will never find mice with an A^yA^y genotype. The reason: this genotype is lethal.

Although the A^y allele is dominant to A when it comes to coat color, so that a yellow phenotype is dominant to dark grey, this allele is recessive to A when it comes to lethality. This is known as homozygous lethal. The allele A^y is a recessive lethal allele because it is an allele that affects the survival of an organism homozygous for this allele. If we go back to our Punnett square, we can see why there is a 2:1 ratio of yellow to dark grey, rather than a 3:1 ratio of yellow to dark grey mice because the homozygous A^y genotype is never born.

There are a few characteristics of lethal alleles that we can use to help us judge if an allele might be lethal, in addition to unexpected ratios from a cross. First, because it is lethal, there will be no pure-breeding homozygous individuals for the trait if the individuals do not survive to birth. Second, there might be smaller family sizes and more miscarriages in families where both sides carry a recessive lethal allele because zygotes or fetuses with these genotypes will often not survive to birth.

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