Phenotypes Affected by Environment and Epistasis – Beyond Gregor Mendel

00:00 Another example is that environment could affect the expression of genes. Here we are going to look at our Himalayan Rabbits or Siamese Cats and they have specific fur that is temperature dependent. When the temperature is below 33°C, we have an active form of tyrosinase and active form of tyrosinase ends up being allowing the cat to produce color in that fur or allowing the fur to produce color. We see that extremities, the tips of the ears, tips of the nose, tips of the feet and tail end up having darker color than the body because at, body temperature above 33°C, we see that tyrosinase is inactive and has no pigment production, so that accounts for the lighter fur. You will notice that when you put a Siamese cat outside, it is more of an outdoor cat than an indoor cat, that will end up with much darker body fur because they are experiencing cooler temperatures in general. Of course exclusively they have to live in a colder environment. When they are indoor cats, they tend to be lighter and this blackening tends to also help them stay warmer because the darker color absorbs more heat. It is an adaptation to particularly called environments.

01:29 Let us think about metabolic pathways. So many things are created in a pathway. We have addressed metabolism previously and know that one enzyme changes the substrate into a product and so on and so forth and there are enzymes in line for example in cellular aspiration. There isn't really any independence between those enzymes, some thing at the end of the pathway is completely dependent on earlier enzymes in the pathway. Let us look at this example if we have a precurssor molecular that does not produce any color and that molecule is converted by enzyme A into another molecule that also does not have any color. But it is necessary to have that in order to have enzyme B turn on the pigment or purple color.

02:27 This metabolic pathway exists in corn plants where we can take this individual plant that is homozygous for dominant and recessive genes combined, so we have the A enzyme in the homozygous dominant form, B enzyme in homozygous recessive and the opposite in the other parent and when we cross them we end up having a purple offspring or a purple population of offspring because they have both dominant A and dominant B and you need the dominant B in order to get the purple color and move on through the pathway. In the offspring, we will see some skewed ratios, Remember that we would naturally expect to 9:3:3:1 ratio, but because there is an interplay between gene A and gene B, we see a skewed ratio. This skewing of ratios is called epistasis. Another great example of epistasis or one gene affecting the expression of another is seen in Labrador retrievers. Here we have two heterozygously black labs crossed with each other. But the gene E actually codes for an enzyme that lets color be expressed or not expressed. When we cross these two Labradors together, we see that there are some skewed results and that is because of the interplay of these genes. When we don't see a E as in the bottom corner, you end up with yellow labs that anytime you see Es were allowed to make color and so now we can actually interpret the Bb alleles. B is the dominant allele and if that allele is present, we end up with black labs. In order to get a brown lab, you will have to have the homozygous recessive form, bb. As we look at the results, you'll notice any time there is a B and a E, we end up with a black lab. But in order to get a brown lab, you have to have a little bb as well as a E to permit production of that color and any lab that does not have a E is not able to produce the specified genotype as a phenotype because it cannot make that pigment.

04:54 Again a great example of epistasis, which involves the interaction of genes in metabolic pathways. In this lecture, we have explored a lot of variations on Mendel's predicted outcomes although each of them still exhibit Mendelian inheritance. By now, you should be able to predict the outcome of crosses using the methods of probability. In addition, you will be able to interpret test crosses to determine an unknown genotype and it will explain why not all crosses fit Mendel's predicted phenotypic outcomes. Thank you so much for your attention. I will look forward to seeing you in the next lectures on genetics.