Heredity: Crash Course Biology #9

Key Takeaways

so I have this brother John you may have heard of him hi there and and as it happens John and I have the exact same parents yes Mom and Dad green and since we have the same parents it's to be expected that John and I would have similar physical characteristics because the source of our DNA is exactly the same Han can I share some genes but no one knew anything about chromosomes or DNA until like the middle of the 20th century and people have been noticing that Brothers tend to look alike since like people started noticing stuff or whatever it was very scientific John I will remind you that I am doing you a [Music] favor heredity it's basically just the passing on of genetic traits from parents to offspring and like John said the study of heredity is ancient although the first ideas about how the goods were passed on from parents to kids were really really really really really really wrong for instance the concept that people were working with for nearly 2,000 years came from ER Aristotle who suggested that we are each a mixture of our parents traits with the father kind of supplying the Life Force to the new human and the mother supplying the building blocks to put it all together Aristotle also thought that Seaman was like highly purified menstrual blood which is why we still refer to Bloodlines when we were talking about heredity anyway since nobody had a better idea and since nobody really wanted to tangle with Aristotle for hundreds of years everybody just assumed that our parents traits just sort of Blended together in us like if a black squirrel and a white squirrel fell in love and decided to start a family together their offspring would be gray the first person to really start studying and thinking about heredity in a modern way was this Austrian monk named Gregor mendle and mendle demonstrated that inheritance followed particular patterns in the mid 1800s mendle spent sort of an unhealthy amount of time grubbing around in his garden with a bunch of pea plants and through a series of experiments crossing the pea plants and seeing which traits got passed on and which didn't he came up with a framework for understanding how traits actually get passed from one generation to another so to talk about classical genetics which includes mle's ideas about how traits get passed along from parents to children kind of have to simplify the crap out of genetics I hope you don't mind so we've all got chromosomes which are the form that our DNA takes in order to get passed on from parent to child human cells have 23 pairs of chromosomes now a gene is a section of DNA and a specific location on a chromosome that contains information that determines a trait of course the vast majority of the time a physical trait is a reflection of a bunch of different genes working together which makes this all very confusing when this happens it's called a polygenic trait polygenic many genes and then again sometimes a single Gene can influence how multiple traits are going to be expressed and these genes are called petrop however some very few but some single traits are decided by a single Gene like the color of pea flowers for example which is what mendle studied when he discovered all of this stuff and when that happens in mle's honor we call that a mendelian trait there are a couple examples of mandelian traits in humans one of them being the relative wetness or dryness of your ear wax so there is is just one gene that determines the consistency of your ear wax and that Gene is located at the very same spot on each person's chromosome right here chromosome 16 however there's one version of this gene or alil that says the wax is going to be wet and there's another alil that says the wax is going to be dry you may be asking yourself what the difference is between these two things and I'm glad you asked because we actually know the answer to that question among the many amino acids that make up this particular Gene sequence there is one exact slot where they are different if the amino acid is glycine in that slot you're going to have wet ear wax but if it's Arginine it's dry now comes the question of how you get what you get from your parents in most animals basically any cell in the body that isn't a sperm or an egg these are called sematic cells are diploid meaning that there are two sets of chromosomes one inherited from each of your parents so you get one ear wax determining a Le from your mom and one from your dad and I should mention that the reason for this is that gametes or sex cells seor sperm and damn egg are haid cells meaning that they only have one set of chromosomes again for emphasis non-sex cells are called sematic cells and they are diploid sex cells are called gametes and they are haid this makes a lot of sense because a sperm or an egg has very specific motivation they're seriously hoping to score and if they do they plan to join with a complimentary haid cell that has the other pair of chromosomes they're going to need to make a new human or Buffalo or squid or whatever also just so you know some plants have polypoid cells which means that they have more than two sets of chromosomes in each cell which isn't better or anything it's just how they do but anyway the point of this all is that we inherit one version of the ear wax Jee from each of our parents so back to ear wax so let's just say that your mom gives you a wet ear wax alal and your Dad gives you a dry ear wax Al good lord your dad has horribly ugly ears anyway since your parents have two Al each for one gene inherited from each of their parents the one past along to you is entirely random so a lot of what mle discovered is that when there are two alal that decide the outcome of a specific trait one of these alal could be dominant and the other one recessive dominance is the relationship between alals in which one alil masks or totally suppresses the expression of another alil so back to ear wax cuz I know we all love talking about it so much it turns out that Mom's wet ear wax alal is dominant which is why she gets a big W and Dad's dry ear wax alal is recessive which is why he has to be a little W go Mom oh you're back yeah you sound surprised anyway mom's Al is dominant and that settles it right we're going to have wet ear wax uh something about the way that you said that tells me it's not that easy you are so much smarter than you look it is indeed not that easy so just because an alil is recessive doesn't mean that it's less common in all of your genetic material than the dominant alil which leads us to the Assumption the correct assumption that there's something else going on here I'm definitely getting that vibe from you so it has to do with mom and dad's parents because everybody inherits two alal from their parents mom got one from Nanny and one from Papa and let's just say that mom got a little W from Nanny and a Big W from Papa that means that Mom's genotype or genetic makeup when it comes to that single trait is heterozygous which means that she inherited two different versions of the same gene from each of her parents dad on the other hand is a homozygote let me guess that means he had two of the same Al either a little W or a Big W inherited from both Grandma and Grandpa right and in order for this to all work out the way that I want it to let's just say that both Grandma and Grandpa would have passed little W's down to Dad making his genotype homozygous recessive for this Gene okay so I'm keeping score in my head right now and according to my calculations mom is a Big W Little W and dad is a little W Little W and now we're going to try to figure out what our ear wax phenotype is and phenotype is what's expressed physically or in this case what you'd see if you looked into our ears oh so we're going to do like a punet square or anything this is why I do history if we're doing punet squares I'm leaving but I was just going to start to talk about people again so Reginald C punet who was a total Gregor mendal Fanboy invented the punet square as a way to diagram the outcome of a particular cross breeding experiment and a really simple one looks like this so let's put mom on side here and give her a big W and a little W and let's put down at the top and he gets two little W's so if you fill this in looks like there's a 50-50 chance that any child of this mating will be homozygous or heterozygous and as for our phenotype shakes out the same way John and I both have a 50% chance of having wet ear wax a 50% chance of having dry ear wax so I just had to go and call John because now he's not participating because he doesn't like punet squares and it turns out that he has wet ear wax I also have wet ear wax which you know is not that unlikely considering that our parents were homozygous and heterozygous this uh may explain the odor of our bathroom when we were growing up because it turns out that there's a correlation between wet ear wax and body odor because ear wax and armpit sweat are produced by the same type of gland because this one gene has an effect on multiple traits or phenotypes it's an example of a pleotropic gene because the gene affects both how wet your ear wax is and how much you stink one more thing you might find interesting sexlink inheritance so we've got 23 chromosomes 22 pairs are autosomes or non-sex chromosomes and one pair the 23rd pair to be exact is a sex chromosome at that 23rd pair women have two fulllength chromosomes or xx and men have One X chromosome that they inherited from their mom and this one little short puny shriveled chromosome that we call Y which is why men are XY so certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes since dudes don't have two full chromosomes on pair 23 there may be recessive alals on the X that they inherited from their mom that will get expressed since there's not any information on the Y chromosome to provide the possibility for a dominant deal counteracting that specific trait take for instance balding women rarely go bald in their youth like some men do because it's caused by a recessive alal located in a gene on the X chromosome so it's rare that women get two recessive alals but men need just one recessive Al and dope baldy bald that alil is on their ex chromosome which they got from Mom but was mom bald probably not and where did Mom get that alil on her ex chromosome either from her dad or her mom so if you're bald you can go ahead and blame it on your maternal grandmother or your maternal maternal great-grandfather or your maternal maternal maternal great great grandfather who probably went bald before he was 30 so genetics you guys resistance is futile thanks to my brother John for sharing his personal genetic information with us and also his face and voice and all that stuff that was very nice and think of us next time you swab out your ears actually they say that you really shouldn't do that because you have ear wax for a reason and you might poke your brain or something okay that's the last time I'm mentioning ear wax review click on any of these things back to that section of the video if you have any questions please ask them in the comments