 old family photos this is where people say hey you got your eyes from your mom or something like your mouth is like your dad yeah over here I don't see any resemblance but you know what I'm talking about right that's me by the way so why do we resemble our parents well you might know the answer to this you might say hey it's the jeans man jeans are what we call the unit of inheritance basically we say that jeans gets passed along from parents to their kids and that's why we look similar to them but what exactly are these jeans let's find out to answer this question we need to look deep into a cell so let's say this is one of the cells that compose my body now the cells have a lot of stuff inside of it right it also has a nucleus that's what we are interested in if we're to look deep into the nucleus then you will find some squiggly thread-like structures you know what these thread-like structures are called these are called the DNA DNA and if I were to zoom into a small portion of that DNA then you might be seeing something which is familiar right you may have seen these things in movies and cartoons and everything you may be wondering what exactly is this DNA we make a big deal about it right now without getting into too much of detail DNA can be thought of as an instruction manual which is present to build me if this is my DNA it contains the instructions to build everything about me from the tip of my hair through the guts to the tip of the toes every single detail of how to build me is present in this DNA so it's a long instruction manual of how to build Mahesh similarly your DNA is an instruction manual to how to build you the DNA of a chimpanzee is an instruction manual to build a chimpanzee and did you know that about 99% of its DNA matches with human DNA that would not be all that surprising because you already know you might have heard of that we are very close relatives of chimps right but what may be surprising is to learn that about 50% of a fruit flies DNA also matches with ours 50% the fact that our DNA is so common to other animals actually is an evidence for the theory of evolution but of course we'll talk more about evolution in future videos so anyways this DNA is super important but where do you think this instruction manual came from any guesses our parents that's what gets passed along from our parents to the kids and if you're wondering wait a second shouldn't we have two copies of this one from the mom and one from dad well then you are absolutely right we have two copies two of these DNA set one we got from the mom and one we got from the dad in fact and this might sound a little weird but just before my dad's sperm fertilized my mom's egg this DNA was present in that sperm and this mom's DNA was present in her egg and then when fertilization happened the two DNA came into one cell and then that single cell started dividing and every time it divided a copy of this DNA was made which makes sense right you don't want to lose things like these and so as the cell kept on dividing and dividing and dividing I started growing and growing and growing and using this very instruction manual I was built and this might sound silly but this is the reason why human babies grow into humans and dog babies grow into dogs anyways before we talk about genes let me tell you one thing over here whenever a cell gets divided I said that the DNA needs to get copied right it turns out copying DNA like this when the DNA is in this form is very difficult so when the cell is ready to divide the DNA's start getting coiled up in a very tightly condensed condensed coil okay and this coil of DNA tight coil of DNA and a bunch of stuff gets attached to it to make this coil happen but anyways this coil of DNA is what we call chromosomes chromosomes okay so chromosomes are think of them as basically coiled up versions of the DNA and this is why most of the time we use the word chromosomes and DNA interchangeably okay anyways now let's talk about genes what exactly are genes genes are portions of DNA not the entire DNA but these are portions of DNA which carry instructions to make a specific action basically it carries instruction to make a specific protein for example what I mean is let's say this section of DNA contains the recipe or contains the instructions to build a protein which eventually makes the eye color then we will say this is the eye color gene this is the eye color gene and let's say some other portion of the DNA this portion of DNA contains the instructions to make a protein which say makes up our hair so then we'll call this as the hair shape gene now I'm making these names up okay in reality the names of the genes are pretty boring and so you see genes are basically portions of DNA that contain instruction to build a specific protein it's these proteins eventually that end up building you remember your body is mostly made of water fat and proteins and the instruction to build these proteins are given by these genes I like to think of this as a robot building kit which might include a long instruction manual to tell you how to build this robot exactly then this instruction manual represents the DNA but if you open it let's say each page gives you specific instructions of how to build a specific part of that robot maybe one page tells you how to build the lens of the eye that makes up the robot another page might tell you what you know how to build a particular screw something like that then each page we will call as a gene so the whole book is like the DNA the pages which contain the instructions for specific things are like the genes anyways remember that genes contain instructions to build proteins and we have two copies of them one we have dad's copy and one we have mom's copy so this might bring another question if we have two set of instructions then what does our body do well this is where the concept of dominance and recessiveness comes into the picture that we may have learned off from Mendel's experiment so for example let's say the dad's copy of the gene makes up a protein which gives us a brown color of the eye and let's say mom's copy would give us blue color of the eye then what color will our eye get brown or blue well it turns out that brown is the dominant version of that gene and so it'll be brown but I always wonder how does this dominance and recessiveness work well I'm going to oversimplify it over here okay just to get the idea so let's say that dad's eye color dad's gene gives a protein which makes our eyes brown brown pigment then mom's gene doesn't give blue pigment what does mom's gene do you know it creates a protein that doesn't do anything think of them as a broken protein it doesn't give any pigment to the eye and then your eyes receive no pigment surprisingly they look blue and if you're wondering why why don't they just look transparent why do they look blue if they have no pigment that's a little bit of physics actually it's called scattering of light it's the same reason why sky which is made of no color looks blue okay but anyways anyways that's some physics anyways so if your eyes receive no pigments it'll look blue now think about this your dad's gene is gonna make this protein and your mom's gene is gonna make these proteins both the proteins do get built but because brown color goes to the eye your eye will end up looking brown and so now do you see that if you just have one of this gene which gives you brown pigment then regardless of what the other is your eyes will end up looking brown that's the reason we say brown is a dominant version of that gene and so this means if you want your eyes to look blue in color then both the version of the genes must be like this they must make broken proteins which not give you any pigment only then they will look blue but remember I have definitely oversimplified things over here okay for example one thing I didn't tell you earlier is that all your instructions are not found on one single strand of DNA in fact in humans it turns out that your instructions to build your body is spread out over 23 such different strands and so your cells contain 23 DNA strands from your dad 23 from your mom giving your total 46 DNA strands or you can also say you will have 46 chromosomes different animals can have different numbers okay secondly I've drawn these threads to be pretty short over here but in reality if you're to take this entire DNA strand and lay them out turns out it can be about two to three meters long containing about 20 to 25,000 genes so yeah they are highly coiled up inside our cells another thing is in most cases a single gene is not responsible for how any part of you looks for example when it comes to eye color it's decided by about 16 genes which are spread over different different chromosomes or different different DNA strands so yeah things are really pretty complicated but at least it gives us some sense of why we resemble our parents now right because the instruction manual the DNA to build our body directly is a copy of our parents DNA but then why don't we look exactly like any of our parents well that's because we have two set of them so some part of our body is built from the instructions of dad's genes dominant ones and some other parts of our body are built from the instructions of our mom's dominant genes and so it's this combo that makes us resemble our parents but not exactly like them we are unique so we can thank sexual reproduction for this uniqueness I mean think about it in asexual reproduction there's only one parent and so the kid will just get one copy of this DNA so the kid will look exactly like its parent so sexual reproduction makes us unique alright so that's pretty much it okay so let's see if we can summarize and recall what we learned so here are some questions for you can you recall what a DNA is and where is it found exactly inside a cell what are chromosomes how many chromosomes do human cells have what are genes and finally what is the main advantage of sexual reproduction over asexual reproduction if you have difficulty in answering any of these don't worry just go back and revisit the video