If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:59
AP.BIO:
SYI‑3 (EU)
,
SYI‑3.C (LO)
,
SYI‑3.C.1 (EK)
,
SYI‑3.C.2 (EK)

Video transcript

let's give ourselves a reminder of how important Gregor Mendel's work was in 1866 that he published in 1866 and it's important to realize it wasn't like immediately in 1866 or 1867 the whole world changed and everyone said Oh Gregor Mendel figured it all out like a lot of times in science the big discoveries the ones that really change people think it change people's thinking's aren't really taken that seriously at first and actually Mendel's work a lot of people either didn't take it seriously or kind of ignored it in 1866 and it wasn't until the early 1900's that people rediscovered his work and realized wait wait there's something very very powerful here and we might be able to connect it to things that we are actually observing inside of cells but let's just remind ourselves about Mendel's work so for most of human history we've recognized probably that okay looks like the animals and we're not just animals any type of living creature seems to pass on traits to their offspring I could look at you and I'd say well you know your hair is kind of like your dad's and your eyes are kinda like your mom's maybe your nose looks like something in between you walk a little bit like your uncle so it we've always recognized that that we pass on traits to our offspring but we didn't really have a rigorous way of thinking about it and we definitely didn't have any way to make predictions that were testable based on those traits and that's what Mendel gave us he said well look I'm observing and he did this with pea plants I observed these heritable factors and there might be heritable factors on let's say height if we're talking about plants it would be the height of a plant there might be heritable factors on let's say flour color so flour flour color and you recognize that there are different versions of those factors and so a given plant might have one of the tall versions so they might have a tall version for the height factor and they might have a short version or they might have two tall's or they might have two shorts or they might have a red factor and they have a pink factor or they could have two Reds two Reds it would look like that or two pinks would look like that but the important realization was that there was these there was these versions of the factor today we call these factors you say hey there's a gene for height if there is one or that there is a gene for flower colors and those variations of the genes today we call these alleles so we'd say you have the variation you have one copy of the tall allele and one copy of the short let me just write it this way let me just say these are all these are all alleles right here so you have one tall one short allele and what Mendel did is he realized well look these things are the art we didn't know how but these things are the things that get passed on from one and from-from a parent to their offspring and he started described about how they got passed on he observed that even if you have two of these that they tend to segregate when you go to the next generation and what we what do we mean by segregation or I guess we could say the law of segregation law of segregation well that means if this was if I'm a pea plant and these are these are the versions that I have to my offspring I might pass on an a a capital A the tall one or I could pass on the lowercase a I might pass on the tall or I might pass on the red version of the flower color factor or I might pass on or I might pass on the pink one and he also realized that whether or not I pass on the capital A or the lowercase a it's independent of whether I pass on the capital B or the lowercase B so they independently assort how this one of sorts is independent of how this one of sorts so independent assortment in the pendent independent assortment assortment law law of independent assortment and he also observed that some of these versions the other one so if an offspring has a tall version and a short one if the tall one is dominant the the observed trait will still look tall and that the only way they look short is if they have two versions of the short one and so that one he described as his law of dominance law of dominance and if all of this is completely new to you I encourage you to watch watch the videos on Mendelian genetics on Khan Academy but this is just going to appreciation a little bit of a historical appreciation but as big as Handel has as big of a deal as Mendel's work was it's also important to realize what he didn't know he had no idea of how this was actually happening at a molecular level or at a cellular level and it wasn't until the early 1900s that people started to have fairly robust theories of how this happens and so in 1902 and 1903 so 1902 1903 these two gentlemen independently start coming up with the chromosome theory of inheritance and it's called the very Sutton chromosome theory of inheritance cuz right around the same time they both started to realize that maybe chromosomes were the actual molecular mechanism the cellular mechanism by which these factors segregate and independently assort and so this is let me write this down this is the bow very well very sutton and sometimes called the sutton bow very but very sutton chromosome theory chromosome theory and even even though they're starting to say maybe chromosomes have something to do with it they still don't know exactly what is inside the chromosomes that are allowing the somehow this information to be encoded and we'll get to that and we will get to that in a little bit but let me just underline this beware the very Sutton chromosome theory and so what was their key insight well they started a look inside of cells meiosis was observed actually after Mendel published his his laws of inheritance and then chromosomes or how chromosomes behave in meiosis were discovered after that and then these guys they independently studied different organisms Walter Sutton he studied grasshoppers Theodor Boveri he studied sea urchins but they looked at meiosis and they looked at the reproduction and the fertilization during these processes and they saw that the chromosomes seemed to do things that were very similar to these laws of segregation laws of independent assortment laws of dominance and actually the law of dominance we'll talk more about in future videos but he saw that let's say that you had a an organism here and in this a particular organism I just did it for simplification it has two pairs of homologous chromosomes so what does homologous chromosomes mean well these these two are different chromosomes but they seem to be very similar it seems like they're kind of the same length same size same shape so that's one pair of homologous chromosomes that's another pair of homologous chromosomes so notice homologous chromosomes two things that are kind of looking the same but maybe they're they're a little bit different we're not sure well maybe maybe this is this is this is this is what fits what's going on right over here with these factors maybe just maybe maybe one of these chromosomes somehow has on at some place what encodes for the capital a capital A and maybe the other chromosome in a in the similar part of the chromosome in a similar part of the chromosome has what encodes a similar part of the chromosome has what encodes for a lowercase a now this this is starting to make sense because they would be homologous chromosomes similar they're those chromosomes look like they code for the same thing for the same factors for the same genes but there might be some variation between these chromosomes and these guys weren't you know they weren't able to somehow sequence the the chromosomes so they didn't know they don't even know that the the DNA was what was important in the chromosomes but they said well it looks like these two things as we look through the process of meiosis it seems like they segregate from each other for example this capital A one it'll replicate so you have capital A but then and then the the lowercase a one right over here you might have some crossover and we'll talk about that when you you could review meiosis if if that looks unfamiliar but then they segregate you could have your capital a ones right over here and then these sister chromatids split apart so capital a capital A and then you have lowercase a these lowercase a ones they segregate and they independently assort from the other chromosomes so this one right over here might be the capital B this might be the lowercase B and whether or not this gets a whether or not this gets a capital B or a lowercase B is independent of whether it got a capital A or a lowercase a so it seems like these chromosomes independently assort and so they came up with this chromosomal theory that it looks like maybe chromosomes are what contain these heritable factors that Mendel was talking about because it seems like chromosomes behaved very similar to those heritable factors maybe chromosomes code for multiple of these heritable factors that segregate and independently assort and as we know now they were right so this was a very very big deal but it's important to realize that they weren't sure they established the theory they were able to make some of observations with the grasshoppers and the sea urchins and they saw the patterns between what Mendel was describing in the way chromosomes behaved during meiosis and then they know that each of these each of these products of meiosis each of these each of these gametes will then go and form with other gametes to form the next organism so you say look parents will contribute either a capital A or a lowercase a either a capital B or a lowercase B so it says this is very similar to what Mendel was describing so they laid the foundry but they still weren't sure they didn't definitively prove it and we'll take a few more another decade or so until it's definitively proven and even then no one was really sure exactly how these different traits were encoded and for that we would have to wait a little bit longer
Biology is brought to you with support from the Amgen Foundation