Episode 66 is going to make all your puppy dreams come true! Beginning with a word nerd moment before completing a Labrador dihybrid cross.
Episode 66 is going to make all your puppy dreams come true! Beginning with a word nerd moment before completing a Labrador dihybrid cross (1:15). Epistatic interactions don’t always have to be in opposition, as explained in a wheat color dihybrid cross (5:07). Melanie concludes with unit and exam connections (6:13).
The Question of the Day asks (7:08) What famous mouse study connects epigenetics, diet and obesity?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Epistasis
Zoom out:
Unit 5 - Heredity
Topic - 5.4
Big idea - Information Storage and Transmission
Let’s say that you’ve been begging your parents for a dog for years and they finally agree! You’ve had your heart set on a yellow lab and hear that your next door neighbors are expecting a litter any day! Excited, you and your mom go visit the expecting puppy parents. “Aw no” says your mom, “I’m sorry - both parents are black labs. We can try another litter.” But you’re not discouraged. You remember what you learned in AP Bio and know that if these parents are heterozygous, there is 25% chance of yellow lab puppies due to epistasis.
Let’s Zoom in:
Time for a word nerd moment. Epistasis - from the Greek. Epi, meaning upon, and stasis, a stopping or standing. Sooooo epistasis. Something on top of something else which stops it? Pretty much. Epistasis occurs when the phenotypic expression of one gene affects the expression of another gene. Sometimes it depends on the other gene for expression while other times, it masks or covers up a genes expression. You won’t find the term epistasis in the 224 page AP Bio course exam description, or CED. However, it provides another example of non-Mendelian genetics at work.
Consider a dihybrid cross between the two black labs - each are heterozygous for the B allele and E allele with a genotype of capital B lowercase b capital E lowercase e. Foiling for gametes to represent independent assortment, each parent produces capital B capital E, capital B lowercase e, lowercase b capital E and lowercase b lowercase e. Since there are four unique gametes for each parent, this dihybrid cross completes a true 4 by 4 16 box Punnett square.
Ok that was easy so far, seems like a pretty normal genetics problem to me. What makes it epistatic? It all comes down to interpretation. If this were normal dominance Mendelian genetics, we would expect a 9:3:3:1 phenotypic ratio (remember round or wrinkled green and yellow peas? - Mendel’s experiments). But we don’t see that at all. There are only three phenotypic expressions in the offspring. 9 black labs, 3 chocolate labs, and 4 yellow labs. What chocolate? Where did that come from? I thought we were hoping for yellow. Let me explain.
The genotypes of these dogs are unique with some variance having no effect on phenotype. In instances where variance exists, I will express that as a dash. OK - the genotype of all of the black labs is capital B dash capital E dash. That’s right, so long as the offspring has at least one dominant allele for each, they will express the black phenotype. The genotype of the chocolate labs is lowercase b lowercase b capital E dash. The genotype of the yellow labs is dash dash lowercase e lowercase e. The inheritance of the B allele cannot be expressed at all if recessive for e. The dog can be homozygous or heterozygous, dominant or recessive for B, and still - homozygous recessive for e prevails. Yellow lab phenotype.
Epistatic interactions don’t always have to be in opposition. Consider a dihybrid heterozygous cross between two wheat plants. Wheat kernel color is determined by the actions of two genes. Both gene A and gene B can produces the precursor molecule for pigmentation. So the presence of either or both gene will results in pigmented kernels. Only a double homozygous recessive, lowercase aa lowercase bb will lack pigment. The resulting phenotypic ratio in the dihybrid cross is therefore 15:1.
When you approach epistatic problems, remember that a two-gene interaction typically has a phenotypic ratio that totals 16. In these instances, we are assuming the interacting genes are not linked and are still sorting independently into gametes. The interaction between alleles and their affect on phenotype will likely involve protein expression in a pathway. This level of understanding would require more information from the genetics prompt.
Time for unit connections. Don’t forget your chromosome structure and DNA organization from Unit 1. Unit 4 discusses cell communication, which may be intertwined with epistatic protein pathways. This also connects to Unit 6 with transcription and translation. Alright - what about the exam? Any use of epistasis on the exam will likely be part of a larger FRQ prompt or multiple choice question. You will not be expected to recall specific examples, but the best way to prepare is to get comfortable being uncomfortable when phenotypic ratios look “wonky.”
To recap……
Epistasis is a type of non-Mendelian inheritance where one gene interferes with the expression of another gene. The resulting phenotypic ratios vary greatly as traits are partially masked, hidden, or expressed.
Coming up next on the Apsolute RecAP Biology Edition: Codons
Today’s question of the day is about experiments.
Question of the day: What famous mouse study connects epigenetics, diet and obesity?