Back To CourseBiology 101: Intro to Biology
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For a while now, we've been studying all kinds of interesting flying hamster traits with Adrian: coat color, ear size and tail color, to name a few. But, now Adrian's probably found the coolest flying hamster yet. He is really excited to discover a white, fire-breathing hamster. After capturing his latest prize, he rushes back to the lab to study the new trait.
He first decides to mate the white, fire-breathing hamster with a true-breeding brown hamster. At the F1 generation, as expected, he gets all brown, non-fire-breathing hamsters.
After mating F1 brown hamsters together, he only produced white, fire-breathing hamsters and brown ones that can't breathe fire. Based on previous experiments, Adrian figured he would have seen both brown and white fire-breathing hamsters. In fact, we know from earlier studies that if the two traits were sorting independently, he should have seen a 9:3:3:1 phenotypic ratio among the four different phenotypic classes in the population.
What could possibly be going on here?
Well, remember that a chromosome is like a cookbook with a variety of recipes. That means that it is a distinct physical storehouse of genetic information.
However, I may own a particular cookbook because I really like one of the specific recipes. But, every time I move and bring the recipe with me (because the cookbook is a single physical entity) I'm also bringing every other recipe in that book with me at the same time.
Similarly, a single chromosome contains many genes which make many different gene products that participate in different biological processes. We observed a 3:1 ratio between brown hamsters that can't breathe fire and white ones which can. That means the coat color and fire-breathing traits are segregating together as if we are studying only one trait. In other words, it appears as if the genes may be located on the same chromosome. When two genes are located on the same chromosome, they are said to be linked.
Chromosome linkage is another example of an exception to Mendel's law of independent assortment. Although both homologs possess the coat color and the fire-breathing genes, the allele which allows the hamster to breath fire is located on the same homolog which possesses the white coat color allele. Since the two alleles are linked, we'd expect to see those two recessive traits together in the same hamster.
Adrian was happy with the conclusion that linked traits segregate together, until he tried to mate a bunch of his F1 hamsters with white, fire-breathing hamsters. After analyzing this larger data set, he is perplexed to find a few brown, fire-breathing hamsters as well as some white hamsters that can't breathe fire.
So, let's consider everything we know about chromosomes and genetics to see if we can help Adrian figure this one out.
Well, recall that during meiosis I, homologous chromosomes pair and form a structure known as a tetrad.
But, what if this exchange is responsible for Adrian's unusual results?
The crossover event helped orient the tetrads during meiosis, but the fact that pieces from each homolog are exchanged in the process can have a significant effect on genotypes and phenotypes which can be produced by a mating.
Let's reconsider crossing over again in more detail with genetics in mind this time. Suppose our original hypothesis is correct, and the coat color gene and the fire-breathing gene are on the same chromosome.
Based on our research, that means they are both located on chromosome three. Let's use B again to represent coat color and use F to represent the fire-breathing trait.
And, for argument's sake, let's place the fire-breathing gene between the coat color gene and the centromere on chromosome three and examine a cross over event in a heterozygote.
Let's say a crossover event occurs between the two genes. How will that affect the genotype of the resulting gametes?
To start with, there are four chromatids in the tetrad, consisting of exactly two copies of each homolog. Note that the dominant F and B alleles are located on the same arm of one homolog and the recessive f and b alleles are located on the same arm of the other homolog.
A crossover event between the two genes would swap the information between the two chromatids.
Notice there are now four unique combinations of alleles. The two original chromatids F, B and f, b are referred to as parental chromosomes because that's the way the information was organized in the parent's cells. The two new chromatids F, b and f, B are referred to as recombinant chromosomes since they were produced by recombination.
Now, let's see how crossing over alters the expected distribution of phenotypes among the offspring. In Adrian's crosses, he mated a heterozygote with a white fire-breathing hamster. The white, fire-breathing hamster can only contribute f, b gametes, so if crossing over didn't occur, we would expect to see only brown hamsters that can't breathe fire and white hamsters that can. However, crossing over in the heterozygote makes it possible for this mating to also produce white hamsters that can't breathe fire and brown hamsters that can.
So far, it seems like the four phenotypes should occur at the same frequency, but that's not what we observed in Adrian's experiment. The reason for this is that we considered a scenario where the crossover event occurred between the two genes we were studying. However, we just arbitrarily considered an example where the crossover event occurred between the two genes in question. That location is just one of many possible locations where the crossover event could occur.
If the crossover event occurs outside this region, the linkage is not altered, and no recombinant phenotypes are possible. Thus, the number of recombinant progeny you expect to see will vary depending on how far apart the genes are located on the chromosome arm. The farther apart they are, the more likely it is that a crossover event will occur between the genes. The average number of crossovers that occur on a chromosome arm also varies from species to species.
In summary, when two genes are located on the same chromosome they are said to be linked. If two alleles of different genes are linked, they are inherited together more often not.
Although linked genes are generally inherited together, a crossover event can alter the linkage of alleles on homologous chromosomes. Whether or not the linkage between alleles is altered is determined by the location of the crossover event during meiosis.
Creating new combinations of alleles makes new combinations of phenotypes possible and increases the genetic diversity of the population.
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Back To CourseBiology 101: Intro to Biology
21 chapters | 137 lessons