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.So we know that these parent hamsters came from populations that are referred to as true breeding. What that means is, in terms of the trait we’re talking about, brown hamsters that are mated with brown hamsters always produce more brown hamsters.
White hamsters mating with white hamsters always produce more white hamsters.Since that’s the case, we can infer that the genotype of these animals are ‘BB’ for the brown one and ‘bb’ for the white one. Therefore, all the progeny from this cross should have a ‘Bb’ genotype. The fact that all of these hamsters in the progeny generation are brown indicates that the brown allele is dominant over the white allele.
So a dominant trait is referred to as a trait that is expressed in the heterozygous state.
That means that a recessive trait is a trait that is not expressed in the heterozygous state. Geneticists usually represent the dominant allele with a capital letter and the recessive allele with a lower case letter.So this experiment has provided Adrian with some data that supports his hypothesis that flying hamster coat color is determined by a single locus, and we’ll see how further experimentation can provide more support for this hypothesis.
In this lesson, we’ve learned that an organism homozygous at a given locus possesses two copies of the same allele. For instance, if we’re talking about gene B, ‘BB’ and ‘bb’ are both homozygous genotypes. An organism heterozygous at a given locus possesses two different alleles. For instance, ‘Bb’ would be an example of a heterozygous genotype.
A dominant trait is the trait that is expressed in a heterozygous state. A recessive trait is the trait that is not expressed in a heterozygous state.