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Video transcript

so I have this brother John you may have heard of him hi there and as it happens John and I have the exact same pair 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 Hank and 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 that's very scientific John I will remind you that I am doing you a favor [Music] 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 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 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 semen was like highly purified menstrual blood which is why we still refer to bloodlines when we're 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 reading in a modern way was this Austrian monk named Gregor Mendel and Mendel demonstrated that inheritance followed particular patterns the mid-1800s Mendel spent sort of an unhealthy amount of time grubbing around in his garden with a bunch of P plans 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 Mendel's ideas about how traits get passed along from parents to children ganna 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 in 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 and 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 Playa truck 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 Mendel studied when he discovered all of this stuff and when that happens in Mendel's honor we call that a Mendelian trait there are a couple examples of Mendelian traits in humans one being the relative wetness or dryness of your earwax so there is just one gene that determines the consistency of your earwax 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 allele that says the wax is going to be wet and there's another allele 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 earwax but if it's arginine it's dry now comes the question of how you get what you get from your parents and most animals basically any cell in the body that isn't a sperm or an egg these are called somatic cells are diploid meaning that there are two sets of chromosomes one inherited from each of your parents so you get one earwax determining allele from your mom and one from your dad and I should mention that the reason for this is that gametes or sex cells senior sperm and Madam egg are haploid cells meaning that they only have one set of chromosomes again for emphasis non sex cells are called somatic cells in their diploid sex cells are called gametes and they are haploid this makes a lot of sense because a sperm or an egg has a very specific motivation there seriously hoping to score and if they do they plan to join with a complementary haploid 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 polyploid cells which means that they have more than two sets of chromosomes in each cell which is it better or anything it's just how they do but anyway the point of this all that we inherit one version of the earwax gene from each of our parents so back to earwax so let's just say that your mom gives you a wet earwax allele and your dad gives you a dry earwax allele good lord your dad has horribly ugly ears anyway since your parents have two alleles each for one gene inherited from each of their parents the one passed along to you is entirely random so a lot of what Mendel discovered is that when there are two alleles that decide the outcome of a specific trait one of these alleles could be dominant and the other one recessive dominance is the relationship between alleles in which one allele masks or totally suppresses the expression of another allele so back to earwax cuz I know we all love talking about it so much it turns out that mom's wet earwax allele is dominant which is why she gets a big w and dad's dry earwax allele is recessive which is why he has to be a little w go mom your back yeah you sound surprised anyway mom's allele is dominant and that settles it right we're gonna have wet earwax something about the way that you said that tells me it's not that easy wow you are so much smarter than you look it is indeed not that easy so just because an allele is recessive doesn't mean that it's less common in all of your genetic material than the dominant allele 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 alleles from their parents mom got one from nanny and one from pop on let's just say that mom got a little W from nanny and a Big W allele 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 as a homozygote let me guess that means he had two of the same allele 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 earwax phenotype is and phenotype is what's expressed physically or in this case what you'd see if you looked into our ears alright so we're gonna do like a punnett square anything this is why I do History if we're doing punnett squares I'm leaving but I was just gonna start to talk about people again so Reginald see punnett who was a total Gregor Mendel fanboy invented the punnett square as a way to diagram the outcome of a particular crossbreeding experiment and a really simple one looks like this so let's put mom on side here and give her a big w in a little W and let's put down at the top and he gets two little W so if you fill this in looks like there's a 50-50 chance that any child of this meeting will be homozygous or heterozygous and as for our phenotype shakes up the same way John and I both have a 50% chance of having wet earwax a 50% chance of having dry earwax so I just had to go and call John because now he's not participating because he doesn't like punnett squares and it turns out that he has wet earwax I also have wet earwax which you know is not that unlikely considering that our parents were homozygous and heterozygous this may explain the odor of our bathroom when we were growing up because it turns out that there's a correlation between wet earwax and body odor because earwax 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 pleiotropic gene because the gene affects both how wet your earwax is and how much you stink one more thing you might find interesting sex-linked inheritance so we've got 23 chromosomes 22 pairs are autosomes or non sex chromosomes and one pair 23rd pair to be exact is a sex chromosome at that 23rd pair women have two full-length 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 chromosome since dudes don't have two full chromosomes on pair 23 there may be recessive alleles 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 allele 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 allele located in a gene on the X chromosome so it's rare that women get two recessive alleles but men need just one recessive allele and don't Baldy bald and that allele is on their X chromosome which they got for Mom but was Mambo probably not and where did mom get that allele on her X 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 per so 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 swap out your ears actually they say that you really shouldn't do that because you have earwax for a reason and you might poke your brain or something okay that's the last time I'm mentioning earwax
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