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Biology: Independent Assortment


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About this Lesson

  • Type: Video Tutorial
  • Length: 10:21
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 112 MB
  • Posted: 02/10/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology: Final Exam Test Prep and Review (42 lessons, $59.40)
Biology Review (19 lessons, $27.72)
Biology: Cell Reproduction - Mitosis and Meiosis (16 lessons, $23.76)
Biology: Understanding Meiosis (3 lessons, $4.95)

This lesson covers the concept of independent assortment. This is a critical idea for the tracking of genes and heredity with the help of the idea of meiosis. Tracking genes on chromosomes through meiosis can tell us something about genetics. Homologous chromosomes are chromosome pairs that contain genes that control the same traits. Homologous chromosomes can assort independently of other pairs of homologous chromosomes. This concept is called 'independent assortment' and it leads to many possible combinations of chromosomes in gametes and offspring.

The lesson will also explain how disjunction (the separation of homologous chromosomes) further contributes to the number of different chromosomal combinations or outcome possibilities. The combination of synapsis and disjunction occur and create independent assortment, which allows for drastically different trait combinations. The more pairs of chromosomes an animal has, the more possibilities there are for possible combinations of traits and chromosomes in offspring.

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 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.

About this Author

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Recent Reviews

Independent Assortment
~ Rocio

Too messy. It is difficult for my high school students to understand lesson. It needs to be more "spicy". Not worth it at all.

Clear, concise
~ jfilip

This guy knows his stuff. While he isn't the most interesting person in the world, he is engaging and most importantly, he helped me figure out what I wasn't quite getting about independent assortment.

Independent Assortment
~ Rocio

Too messy. It is difficult for my high school students to understand lesson. It needs to be more "spicy". Not worth it at all.

Clear, concise
~ jfilip

This guy knows his stuff. While he isn't the most interesting person in the world, he is engaging and most importantly, he helped me figure out what I wasn't quite getting about independent assortment.

Cell Reproduction
Understanding Meiosis
Independent Assortment

Can you stand just one more thing about meiosis? You know, I want to do meiosis, but I want to look at it now from a
different perspective. Because, you know, if we’re trying to track genes and we’re trying to track heredity, it seems
like the idea of meiosis might really help us understand some things, because that’s the way you make what?
This may look familiar to you guys. I want to start out with this cell, and this cell is going to have four chromosomes,
and we’re going to go through the process of meiosis with these four chromosomes, but we’re not going to look at it
with the colors now. We’re going to look at it with genes, and I am going to make up some letters, and don’t get all
neurotic about these letters, we are going to be okay with these, but I need to call them something. And so I am going
to draw my, and we are not going to go through all the complex phases, I want to follow genes, here’s what we’re
going to do, ready?
Okay, I am going to give – like I did here, I am going to line up my homologs, okay. So, there’s my homologs right
there, I am going to do that, but now we’re going to talk genes. I’m just going to worry about genes on each of these
homologous chromosomes. Now remember, since they are homologous, they have genes that control the same trait.
The genes don’t have to be identical, but they are going to control the same trait. So, for example, let’s just say – so
let me line up my homologs, there is my chromosome, there we go, and there is one homologous pair. And on this
homologous pair, remember it’s doubled and these two are going to be identical, so I am going to call that T. Why I
picked that, just let me live with my T’s, and there is a t and there is t. So, there is a homologous pair.
Now remember, this particular guy had four chromosomes, so I am going to set up another pair. So, I am going to
put, right below that, on the metaphase plate, I am going to do two more chromosomes. I’m not changing colors
because I don’t want you to concentrating on colors here, I want you to concentrate on letters. On this particular case,
I’m going to give them R’s. All right, so we are going to say those two are identical, remember because duplicated
DNA, and over here I am going to put r’s. Again, don’t worry about why. I just need letters and I need to differentiate.
And, I want you to understand; I’m trying to get a concept across to you here, that homologous chromosomes don’t
need to be identical as long as they control the same trait.
Well, remember what’s going to happen is, these things are going to separate into two cells, and now I can give you
this word, because it is an important word, a process called “disjunction” is going to occur, and so the homologs, the
tetrads, are going to separate. So this one is going to go that way and that way, and this one is going to go that way
and that way, and I’m going to end up with two cells that’s going to divide. I’m going to move these cells down here,
because they are easier to draw. So, let’s see where are going to go down and we’re going to do two cells. And this
cell and this cell are going to have – I’m splitting it, I’m putting it in color, but just to show you division, see it splits that
way. So this cell is going to have this chromosome with it’s TT in it and this is also going to have its chromosome with
its RR, okay. All right, hopefully you can see that clearly. All right, and now, I’m going to go to this side and I’m going
to have my tt chromosomes and my rr chromosomes, so far so good? Now we’re going to go through a second
division, remember that is what we do in meiosis, come on don’t forget, see.
So we’re going to do a second division now and get down to the haploid number. And so, let’s do four cells one, two,
three, four. Remember, we split those chromosomes up, okay. So this one, remember they line up on the equator, I’ll
show you, well we don’t have it here, but they are going to line up on the equator, the T above the R, and so this one
is going to get a T and R and it’s going to get T and R and this is one is going to get t and r and, of course, these are
identical, and t and r.
And you’re saying to me, “Well so what’s the big deal here, this is just meiosis again?” But I have a question for you.
Could you see another way this may have end up? Could I have sorted these chromosomes independently? Why did
they end up with TR in these and tr in these? Let’s trace backwards. It comes up here. So if you look at the way I
first set these up, I put my T and T here and my t and my t there, but then I put my R on this side. Why didn’t I put my
r on this side, and my R here? You want to know why I didn’t do that? Because I didn’t feel like it, however, could I
have? Absolutely. Could the cell? Absolutely. Does the cell care whose above it? Absolutely not. Does it care who
is next to it? Absolutely, homologs, but it doesn’t matter how I put these things, so we are going to sort these now,
completely independently of each other. Think about that.
Well, I’m going to skip through all this, because you know what is going to happen now. You know that this is going to
split like that, and so I am going to go down to the end here, and let’s see the differences I can make. I’m going to

end up with four cells – one, two, three, four. But, now, instead of those four cells being the same as these to my left,
they will have assorted slightly differently. So on this cell I am going to have my T and my r, and since this is the
identical cell too, we will put it here, Tr. But this one is going to be what? Well, take a look. It’s going to be tR and tR.
You guys look, if these are sperm, would a TR sperm give me a different offspring than tR sperm? Of course it would,
because if we start realizing that these are genes, and these genes, although they control the same trait, may be
slightly different from each other, you start to realize now, the incredible diversity of numbers we can get from,
because, of the fact that synapsis and disjunction occur, creating independent assortment.
Just one last example. Suppose I was to add a third chromosome on to there, I promise I won’t take you through this
whole thing, but suppose I were to go, all right Tt, Rr, Aa, well, you do the numbers, you have instead of four different
possible combinations, like I did before, eight, you see. And if we’re going to cross that, that’s the male making
sperm, with the Tt, Rr, Aa, with their little chromosomes and everything else, that is another eight. Think of the
possible combinations.
Now here is a little math problem for you. If with three we have eight possible combinations, in other words, three
pairs of chromosomes, eight possible. How many would we have with 23 pairs of chromosomes? You know there is
a myth out there – “somewhere in the world there is an identical twin to me.” The numbers don’t work out. The
numbers don’t make sense. Think about it. 23 pairs, just me alone, how many possible gametes can I make with 23
pairs? 223 – 2x2x2x2x2x2x2. Okay, how many is that? That is an awful lot, okay. That is over a million possible
combinations. Now, that being said, that’s my possible combinations. What about the female end of the gametic
pair? She has the same number of possible combinations. That’s why you don’t find twins within the same family, it
is virtually statistically impossible.
And so, I just want you to start thinking about this. The idea, that when chromosomes sort, they sort independently of
each other and the only thing that matters, is getting those homologs together and allowing disjunction to occur,
making sure that half those cells get one set of the parental homologs and the other cell gets the other half. That’s
what meiosis is all about.

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