Properties of Alleles

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  • 0:29 Homozygous and Heterozygous
  • 1:20 Translating Genotype…
  • 2:14 Dominant and Recessive Genes
  • 4:34 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
What is a dominant phenotype and how will it affect Adrian's flying hamster research? Tune in as he studies homozygous and heterozygous genotypes and the phenotypes they produce.

Let's check back with our intrepid scientist, Adrian. When we last saw Adrian he was trying to determine the genetic basis for flying hamster coat color. To understand his experiment though, we need to learn a little bit more about how the genetic makeup of these hamsters impacts the coat color.

Homozygous and Heterozygous

In a diploid organism, we know we have two copies of every gene. We also know that different versions - or alleles - of the gene may exist. Let's say we represent the coat color gene with the letter 'B'. That means I can represent two different alleles with either the upper case or lower case 'B'. Because the flying hamster is diploid, it also means that it can have two copies of the same allele; I could have 'BB' or I could have 'bb' in the same organism. I could also have a scenario where I have one of each kind of allele in a given hamster ('Bb').

We refer to a locus that has two different alleles as heterozygous and one that has two copies of the same allele as homozygous.

Translating Genotype into Phenotype

Let's remember that Adrian hypothesized that the coat color of his hamsters was determined by a single locus.

So let's apply our new knowledge of alleles so that we can refine this hypothesis a little bit. Let's say that the 'B' allele represents brown and we'll have the 'b' represent white. Let's see then what's going to happen in the case of the three different genotypes that a hamster could have.

It should be fairly obvious that if I have two brown alleles, I should produce a brown hamster. If I have two white alleles, I should produce a white hamster.

But what happens when I have one of each kind of allele? The heterozygous state is resolved by considering the relationship between alleles.

Dominant and Recessive Genes

Adrian decides to breed a brown hamster with a white one. He finds that the brown and white hamster mating only produces brown hamsters. Adrian has encountered one of the central principles of genetics; while there may be more than one allele associated with a given gene, one trait is sometimes dominant over the other one. Since the brown and white populations of hamsters always produce the same color - so for instance, if I have a brown hamster mating with a brown hamster, we always see brown hamsters. And by the same token, if I have white hamsters mating, they always are producing white hamsters in these specific populations that are isolated from the wild.

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