Shirley Tilghman: Gregor Mendel's Famous Genetics Experiment • iBiology

00:00:00:04 - In the 19th century
00:00:01:13 there was virtually nothing known
00:00:03:16 about how characters or traits
00:00:06:28 are passed from one generation to the next.
00:00:09:28 Mendel was really the discoverer
00:00:13:08 of the process that we now know as genetics.
00:00:17:17 (soft music)
00:00:18:22 So I've been practicing genetics really for 30 years
00:00:23:12 but what is most beautiful
00:00:24:23 about the story of Mendel
00:00:26:24 is how he went about making those discoveries.
00:00:33:04 Gregor Mendel went to university to study mathematics.
00:00:37:25 He then joined an abbey
00:00:40:22 and began to become very interested
00:00:43:27 in the question of how do traits get passed
00:00:46:23 from one generation to the next.
00:00:50:10 Mendel's original idea was to use mice
00:00:53:18 but he was forbidden to do this
00:00:55:12 by the Abbott who felt that it was inappropriate
00:00:58:04 for a monk to be studying
00:01:00:05 what appeared to be sexual behavior between animals.
00:01:04:22 Mendel then turned around and studied the sex life of peas.
00:01:08:14 (soft music)
00:01:10:22 They had what we now realize are purebred strains
00:01:14:19 strains where no matter how many times
00:01:18:17 you breed that strain to itself,
00:01:20:21 you get the same plant back.
00:01:22:26 And for Mendel this was incredibly important.
00:01:26:06 But the second reason,
00:01:27:17 is that pea plants are self-fertilizing.
00:01:30:29 The pollen, which is the equivalent of the sperm
00:01:34:03 and the ovule which is the equivalent of the egg
00:01:37:10 are basically contained within the head of the flower.
00:01:40:29 And so the only way you get the pollen inside
00:01:44:06 is it's either self fertilized or artificially fertilized
00:01:49:04 by the scientist like Mendel.
00:01:51:05 But it was critically important to his being able to know
00:01:55:02 with every single plant
00:01:57:13 who the original parents were of the cross.
00:02:01:21 (soft music)
00:02:02:16 What had happened
00:02:03:11 in breeding experiments that had been done
00:02:05:08 prior to Mendel,
00:02:06:18 is too many things were varying at the same time.
00:02:10:19 And that made it virtually impossible
00:02:12:20 to interpret the results
00:02:14:21 of the breeding experiments that were done.
00:02:17:10 I think Mendel realized
00:02:18:22 that he had to make things really simple.
00:02:23:03 The common view at the time
00:02:25:08 was that if you take plants
00:02:27:23 that have two different characters,
00:02:30:06 what you will get in the hybrid
00:02:32:09 is sort of a blended average of the two characteristics.
00:02:37:05 So if you had a tall plant and a short plant
00:02:39:13 you'd get an intermediate size plant.
00:02:42:10 That is not what Mendel observed.
00:02:45:03 He observed instead,
00:02:47:24 that all of the plants in the hybrid cross
00:02:51:09 were actually tall.
00:02:53:13 And that told him something very important,
00:02:55:15 which is that the tall character prevailed
00:03:00:12 and the short character
00:03:01:28 wherever that came from had disappeared completely.
00:03:06:05 And so the question that Mendel might've asked
00:03:09:13 at that point is whether the character
00:03:13:10 that determines shortness in a plant
00:03:15:25 had completely disappeared.
00:03:17:12 So he did the next really critical experiment
00:03:21:11 which is he allowed those tall plants
00:03:24:27 to self fertilize themselves.
00:03:27:11 And he asked,
00:03:28:10 "what do the seeds look like
00:03:29:18 from the progeny of that cross?"
00:03:32:26 And the answer was three
00:03:35:04 of the progeny are tall and one of the progeny is short.
00:03:41:17 So in that original tall plant
00:03:43:29 in the first generation
00:03:46:12 the short character had not been destroyed.
00:03:49:09 It had not completely disappeared into the ozone.
00:03:52:26 It had been dominated by the character tall.
00:03:59:07 And in fact, Mendel used that word
00:04:01:17 he said, "tall is dominant over short."
00:04:06:19 And the word he used to describe short is recessive.
00:04:10:24 And what's really remarkable is to this day
00:04:13:15 we use these terms to describe the behavior
00:04:17:16 of genes in crosses, which is amazing to me.
00:04:23:09 So the first group of plants
00:04:26:06 that he did this with didn't produce exactly the ratio
00:04:30:12 of three to one.
00:04:31:18 In fact I think the ratio was something
00:04:33:24 in the neighborhood of 2.84:1
00:04:38:02 It's a probability just as it is with tossing a coin
00:04:42:22 but the more one conducts the coin toss,
00:04:47:15 and just like the more one does the experiments
00:04:50:14 that Mendel conducted, the closer you will come
00:04:54:04 to the ratio, the correct ratio.
00:04:58:08 So he then repeated this exact same experiment
00:05:03:05 using different traits.
00:05:06:08 But when he got to that second generation
00:05:08:29 and asked what had happened to those differences
00:05:11:29 they all approached a ratio of three to one.
00:05:16:22 And it was through the ratio
00:05:19:14 that told him something very important
00:05:22:02 about the fact that traits come as discrete units.
00:05:27:10 He didn't know they were called genes.
00:05:28:26 We didn't know that for another 40 or 50 years
00:05:32:24 but in fact that's what he was looking at.
00:05:35:05 I am sure there was an aha moment, a Eureka moment
00:05:39:01 for Mendel.
00:05:40:24 I don't know whether it happened
00:05:43:01 at 5,000 plants or 10,000 plants or 20,000 plants.
00:05:48:08 But at some moment he must have looked at these numbers
00:05:52:19 and said to himself,
00:05:55:05 "this is really and truly approaching three to one.
00:05:58:24 How can I now go back
00:06:01:05 and understand what that ratio is telling me
00:06:05:21 about characters and how they get passed from one generation
00:06:09:24 to the next?"
00:06:12:07 Once Mendel had completed this monumental study,
00:06:16:14 the next big question for him was,
00:06:18:10 "well, what would happen
00:06:20:10 if actually I had two traits varying in my cross
00:06:26:01 instead of just one trait, do they always go together?
00:06:31:07 Do they flow through generations together?
00:06:34:24 Or can they actually be independently inherited
00:06:39:27 from one another?"
00:06:40:23 So he took a tall plant that has pink flowers
00:06:45:13 and a short plant that has white flowers.
00:06:49:10 And he made the first generation hybrid.
00:06:52:22 And even at this point, Mendel himself
00:06:55:08 I suspect predicted what he would see
00:06:57:16 in the first generation, which is the plants would be tall
00:07:02:19 and they would have pink flowers
00:07:05:04 because he'd already shown
00:07:06:08 those were the dominant characters.
00:07:09:02 But now the key became what happens in the next generation.
00:07:13:23 So he self fertilized these plants
00:07:17:29 and then planted the seeds that resulted.
00:07:21:23 He found that nine of them
00:07:23:29 were the original parental phenotype of tall and pink.
00:07:28:15 Three of them were the hybrid combination
00:07:32:27 of tall and white,
00:07:34:16 and three were the combination of short and pink.
00:07:38:21 And one was the original parental short and white.
00:07:46:01 A nine to three to three to one ratio.
00:07:49:23 And what that demonstrated to him
00:07:53:03 is that tall and pink
00:07:57:09 and white and short do not travel together inevitably.
00:08:02:29 But in fact are behaving
00:08:05:08 as though they are utterly independent
00:08:07:15 of one another in these crosses.
00:08:10:25 To Mendel this was a very important
00:08:14:02 Second Law of Inheritance that he articulated for us.
00:08:20:06 (soft music)
00:08:21:24 I think Mendel really was a genius.
00:08:24:28 He laid out the scientific method
00:08:27:22 in such a methodical and rigorous way
00:08:30:26 that I don't think had ever been done before
00:08:33:14 particularly in this field.
00:08:37:18 That was true genius.
00:08:39:01 And then the other was his ability to interpret his results.
00:08:43:10 I don't think it was obvious.
00:08:45:04 And yet he got it exactly right.
00:08:48:03 And we wouldn't change a thing in his paper even today.
00:08:52:15 (soft music)
00:08:57:07 Now this was the days before the internet
00:08:59:17 and days before social media and Twitter.
00:09:03:10 Had Twitter existed in the 19th century
00:09:05:29 everyone would have known about Mendel's experiments
00:09:08:19 within a day.
00:09:10:21 But instead it was published and promptly forgotten.
00:09:15:08 So he was only rediscovered
00:09:18:02 essentially as the giant that he clearly is
00:09:23:07 well after his death.
00:09:25:09 In reading his paper,
00:09:27:12 what I came to appreciate
00:09:29:00 is that all of the rules
00:09:31:27 that I have learned in the 20th century
00:09:35:16 about how to go about designing a good experiment,
00:09:39:10 how to conduct the experiment, how to interpret the results
00:09:43:13 and then how to write them really clearly
00:09:46:27 so audiences can appreciate what I've done
00:09:51:11 we're all on display by Gregor Mendel in the 19th century.
00:09:58:14 It is perhaps one of the finest examples
00:10:01:09 I know of a perfect experiment.

This Talk

Speaker: Shirley Tilghman

Audience:

General Public
Educators of H. School / Intro Undergrad
Student
Educators of Adv. Undergrad / Grad
Researcher
Educators

Recorded: July 2018

Talk Overview

Gregor Mendel’s experiments with pea plants laid the foundation for our understanding of genetic inheritance. In this video, Shirley Tilghman tells the story of Mendel’s studies and how his data led to an understanding of how the information within genes is passed from generation to generation.

Please head to the Science Communication Lab’s website for more films like this along with educator resources, full video transcript, and most up to date content.

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Speaker Bio

Shirley Tilghman

Shirley Tilghman

Shirley Tilghman is the President Emerita and Professor of Molecular Biology at Princeton University. Dr. Tilghman is a strong visionary and leader in academic research and higher-level education. As President of Princeton University for 13 years (2001-2013), she implemented policies and initiatives that supported better training of Princeton students and increased diversity in the faculty… Continue Reading

Credits

Elliot Kirschner (Executive Producer, Science Communication Lab): Executive Producer Sarah Goodwin (Executive Director, Science Communication Lab): Executive Producer
Nona Griffin: Producer and Editor
Chris George: Animations
Derek Reich: Cinematography
Eric Kornblum: Camera

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