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Biology: Bombay Phenotype: Infidelity or Epistasis

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About this Lesson

  • Type: Video Tutorial
  • Length: 9:47
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 105 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Mendelian Genetics and Mutation (36 lessons, $54.45)
Biology: Epistasis (2 lessons, $2.97)

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/biology. The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science. Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel. His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp. He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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You know, one of the great things about epistasis, it can sometimes get you out of a jam. Any of your professors will tell you that when they teach blood types, on occasion, they get into a dilemma because their students will come up to them and they'll say, There's something I don't understand. I'm blood type AB and both of my parents are O, how can that be? Or they'll say something like, Oh, you know, my Dad is AB and my Mom is B and I'm O. How can that be? And that always put us, you guys, in a very difficult position because, you know, you look at it and you say, well, the Bombay phenotype. And the Bombay phenotype is the way to explain a lot of these things. Now on a serious note, I'm not making this up. There really is something called the Bombay phenotype and it is an example of epistasis. What's epistasis? This is so great. This is going to be good because we're going to be able to review blood types now, and continue our discussion of epistasis.
O.K. What is the Bombay phenotype? Well, it originated in Bombay, India, and what happened was, this was clearly not a case of infidelity. It was pretty solid that what I'm about to show you, did, indeed, happen. And there is an explanation for it. Well, it started out and we're going to use pedigrees. Let's use pedigrees. I like pedigrees. It started where a woman and her husband, and let's see. The woman was blood type O and the husband was blood type B, and had a daughter. And the daughter married a guy. Now here's the thing. The daughter was blood type O and the man she married was blood type A. Let's stop for a second. What would you predict from this crossing? Well, if he is a heterozygote, then we know that he is I^Ai, if he's a heterozygote. Or if he's a homozygote, he's I^AI^A. She seems to be ii. So one would expect that we could get from their offspring, we could get a blood type A or a blood type O. And the first child was a baby, bouncing girl and she was blood type O and life was good. And then came their next child, AB. Not O, not A. AB. Well, at first you can well imagine that things were kind of rocky in the home. And because there should not have been a blood type AB, but you know, it was clearly documented that this woman never had the opportunity to go out and be unfaithful. Well, how do we explain that? Well, there is an explanation for this. And the answer--you got it. Epistasis.
Here's the thing. It turns out that blood types are made from a precursor molecule and all blood types start out with the same precursor. So if we have a precursor molecule and remember what I said. Blood types are determined by an antigen, a chemical on the red blood cell, and that chemical is a glycolipid. And it turns out that at the end of that glycolipid is just a little monosaccharide unit and it's a different monosaccharide in A than B. And in O, it's missing completely. So we have a molecule to which is added a different thing to make A, B and nothing at all to make O. Let's draw this out and see if this makes sense.
We have a precursor, a chemical, and that chemical is going to make a substance called H. H is what's going to be converted. So, for example, if you have the gene I^A, you are going to convert H into an antigen. If you have I^B, the gene I^B, you are going to convert H into B antigen. And if this I gene is not functioning, you make neither. You don't add anything onto this H. And we call that O. When we test it, we don't get A or B so we call it O. And you remember we call that i. But what about someone who has a defective gene here. What about someone who I'm going to call--if that's the precursor, I'm going to call this the H gene. I'm going to call this the H gene named after substance H that it makes. Remember that's a protein. Well, if you make H, and the gene works, we'll say it's dominant. It expresses substance H. On the other hand, if you are hh, meaning that both genes are defective, you don't express H. No substance H.
Let's go back to our pedigree, or at least a part of the pedigree and see if we can get this woman out of trouble. So here we go. What have we got? She had a child and the child that was really giving them the dilemma was the one that was AB. We know that she was O and he was A. We know that her parents were O and A.
Now, let's explain this. Let's go to this child right here. This child's pretty easy to explain. I'll write it underneath. I^AI^B. Where did she get this A from? She got it from her father. Oops. There we go. B. Got it. Now so we got I^AI^B. All right. So we got the A from Dad. But where did she get the B from? She couldn't have gotten it from Mom unless Mom got this B from right here. Check this out. Imagine that this woman looks something like this. If she is hhI^b something, would she make that B? hh, can she make substance H? No. And so remember the epistatic interaction here. The precursor to substance H to the antigen. If she doesn't make this substance H, she can't make the antigen. So this woman indeed had a B gene. It's obvious because the B gene ended up coming to her offspring. Where did she get that B gene from? Well, she obviously got the B gene from right here.
Now this parent had to have had at least one H. Right? And we know that this parent also had I^B because the parent was blood type B and that could have been i or whatever, but we don't care about that. What we see that happened was this. She must have gotten her B from this parent and now we can say something else. You see, this is the fun of pedigrees. We know that this parent must have been a carrier, Hh, and this parent had to have had at least h, maybe was ii. We don't know. Because if they were hh, they could have been anything. But what we care about is this. This parent gave her the h and this parent gave her the h. She was hhI^B whatever, but it was the I^B that came down here. It was the I^B that came down here and what happened was that's how she ended up with an AB child.
So if any of you end up biology teachers or biology professors, remember this story. Because some day, you, too, will be asked an awkward question. Remember, the Bombay phenotype.
Mendelian Genetics and Mutation
Epistasis
The Bombay Phenotype: Infidelity or Epistasis? Page [2 of 2]

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