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How do traits get passed down in our DNA? And what do genes have to do with cat fur? In this episode of Crash Course Biology, we’ll untangle the simplest patterns of inheritance, and reassure our redheaded friends—you’re not going anywhere.

Are Redheads Going Extinct? 00:00
Alleles & Traits 1:32
Patterns of Inheritance 4:50
Autosomal Dominance 5:56
Incomplete Dominance 7:51
Codominance 8:47
Review & Credits 10:20

This series was produced in collaboration with HHMI BioInteractive, committed to empowering educators and inspiring students with engaging, accessible, and quality classroom resources. Visit for more information.

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Watch this series in Spanish on our Crash Course en Español channel here:

Are we in the middle of a redhead mass extinction?

Headlines like this tend to surface every few   years, fretting that the rare  ginger genes are getting rarer.  But are we really watching the final  embers of the fiery-haired flame?  Well, hold onto your Ed Sheerans, rumors of  their demise have been greatly exaggerated.  Sure, if you could pull a hundred  random strangers into a room,   it’s likely only a few of them will have red hair. And true, red hair mostly happens only when a baby   gets two copies of the red-hair gene  variant — which, yeah, is pretty rare.  But good news: The gene variant also  exists in people without red hair.  In fact, it can pass on, hidden, for generations,  until the stars align, literal ginger syzygy, and   it meets up with a second red-hair gene variant.

That’s why, sometimes, two non-redheads can   have a baby with a shock of flaming hair. And unless every single person with the   gene variant vanishes, the redheads  of the world aren’t going anywhere.  We know this thanks to the study of how  genes get passed down across generations,   and a healthy understanding of how this happens  can help us avoid these kinds of sensationalized   stories—that’s right, today we’re breaking out  our Punnett squares and talking about genetics.  Hi, I’m Dr. Sammy, your friendly neighborhood  entomologist, and this is Crash Course Biology.  Ehm, Ed, Reba, Flo, y’all gonna  hit us with that funky theme music?  [THEME MUSIC]

So, let’s talk about genes – sorry, not  those jeans, these genes – individualized   sections of DNA on the chromosomes of cells.

Chromosomes are the long strands of genetic   material in our cell nuclei—typically, humans  have 46 in total, or 23 pairs of two—but genes   are the segments along those chromosomes  that are used to carry instructions.  Humans have over twenty thousand  spread across our 46 chromosomes.  Every living thing has them,  and some way more than others.  Like the nearly microscopic freshwater flea, which  clocks its gene count at over thirty thousand.  Most genes carry specific instructions  for making molecules called proteins.  When those instructions get read — in  a process called gene expression — they   shape how an organism looks and functions. You see, each gene comes in different versions,   called alleles, which determine an  organism's different traits — like,   the shape of a seed or a person’s blood type. Whenever we talk about versions, or variants,   of genes, we’re talking about alleles; scientists  just tend to use the phrases interchangeably.  And the reason we don’t all look like clones  of each other is that we all have different   versions of the same genes, these alleles.

We call an organism’s complete collection of   alleles its genotype, which accounts  for its entire genetic makeup.  Meanwhile, the phenotype describes all the  traits that we can observe in an organism.  So your genotype might include the alleles  for both a long big toe and a short big toe,   but you present with a short big toe so  that’s the one that’s part of your phenotype.  Most traits are determined by more than one gene. Like, there is no single gene out there that   determines the shape of your  nose structure or your height.  Those traits arise from several different  alleles interacting with each other,   like a bunch of protein-making puppeteers—which  I guess sort of makes us all elaborate muppets.  Which, I’m cool with by the way. [SINGS] Can you  tell me how to to get to Sesame street?  So it’s not exactly like that,  my desire to meet Elmo aside.  But, it is true that not every  gene gets expressed in every cell.  Cells are always collaborating  and communicating, making sure   that the right genes turn on in the right place. And while the stuff going on inside is important,   factors outside the body, like the environment,  can also affect an organism's traits.  Take the buckeye butterfly for example.

Buckeyes that hatch during long summer   days have light tan wings as part of their  phenotype, but those that hatch during   shorter fall days have darker red wings. See, the darker wings of autumn buckeyes   absorb heat, raising their body temperature  so that they can still fly, while lighter   wings help the summer babies stay cool. A buckeye’s alleles contain the potential for   both shades, but their environment—specifically  the temperature and day-length when they’re   born—shapes how those genes are expressed.

Now, because humans and a lot of other species   reproduce sexually, the offspring’s genetic  code ends up a unique mix of their parents’.  That’s what happens when an egg and a sperm  come together, each containing one set of all   of their parents’ genes—one allele for each, so  that they end up with two alleles for each gene.  Sorry if that ruined any illusions  of stork-based delivery for ya.  The offspring inherits a unique   combination of alleles from both parents. Same genes, but different versions,   and different chromosomes of a matching pair. And the interactions of those alleles—and in   many cases the organism's environment—affect  the traits that ultimately get expressed.  We call the way traits pass from generation  to generation patterns of inheritance.  Most traits come together almost  like mosaics — from multiple   genes interacting with the environment.

But some are determined by simpler patterns.  Take for example the length of a cat’s fur. It’s determined by a single gene called   “fibroblast growth factor 5,” or FGF5. There are a number of allele variants of FGF5,   including four different long-hair alleles.

But to keep it simple today,   we’ll lump them together as one. Think of this big L here as a short-hair   allele, and it’s what we call dominant. When the two alleles meet, the dominant   allele is the loudest, so it drowns out the little  L here, which is the recessive long-hair allele.  It only takes one dominant allele  for a cat to have short hair.  But for long hair, a cat needs  two quieter, recessive copies.  When one allele completely overrides   the other, we call that autosomal dominance.

You might also hear this called Mendelian   inheritance — named after Gregor  Mendel, a monk who first described   this pattern in pea plants over 150 years ago. So, let’s take a look at how this pattern of   inheritance actually works. Meet Mortimer.  He has short hair, white markings like a  tuxedo, and loves long naps in the sun.  A real prince, this guy.

And this is Bagel.  She’s got long orange fur, razor-sharp claws,  and a penchant for spilling coffee on keyboards.  You wouldn’t believe how many electronics  she’s destroyed, how much blood we’ve lost,   or how much we love her. Anyway, opposites attract.  And we happen to know Mortimer has one dominant  short hair allele, masking one long hair allele.  That makes him heterozygous;  which means he carries two   different alleles for the hair length gene. But fluffy Bagel has two recessive alleles,   so she’s homozygous, genetics-speak for  samesies: having two matching alleles,   either both recessive or both dominant.

And because we know our cats’ genotypes,   we can predict the genotypic and phenotypic  ratios of their hypothetical kittens.  That’s their probability of inheriting different  allele combinations, and therefore having   long locks versus sleek fur. We can map out the odds with   this chart called a Punnett square. There’s a fifty percent chance that   Mortimer and Bagel’s kitten will inherit one copy  of each allele, making it heterozygous dominant.  The dominant big “L” talks over the recessive  little “l,” giving it a short hair phenotype.  You wouldn’t even know the kitten has  the long-hair allele by looking at it.  But there’s also a fifty percent chance  a kitten turns out homozygous recessive,   inheriting a little “l” from Bagel  and a little “l” from Mortimer.  Putting a one hundred percent  chance of floof in the forecast.  But if Mortimer has two copies of the dominant  allele, then there’s no way around it:   this kitten will be bound for short hair. [CHAPTER 5 - INCOMPLETE DOMINANCE]  Sometimes, one allele doesn’t completely  overshadow the other, and instead they blend   together in what we call incomplete dominance.

This can lead to an expressed trait that’s a   mix of the traits associated with each allele. Like, let’s say that the white in Mortimer’s   little tuxedo and Bagel’s socks are determined by  an incompletely dominant allele for white spots.  A cat with big “S” alleles will  be mostly covered in white.  But in a heterozygous cat  with just one copy, the white   patches will cover less than half of its body. And since Mortimer and Bagel are both heterozygous   for spots, their kitten has a twenty-five percent  chance of inheriting two dominant alleles and   being mostly white—and the same odds of inheriting  two recessive alleles and being spotless.  But if I were a gambler, I’d put my bet  on it being heterozygous like its parents.  There’s a fifty percent chance of that, and a  one hundred percent chance of it being adorable.

Sometimes, instead of mixing together, traits linked to two alleles show  up separately in what’s called codominance:  Kind of like wearing a Hawaiian  shirt and a sequined jumpsuit.  They’re both jostling for attention. Calico cats — with their blotches of   orange and black fur — owe their  looks in part to codominance.  But they’ve got another pattern  of inheritance going on at the   same time, called sex-linked inheritance. That’s where a gene is carried on only one of the   chromosomes that influences an organism’s sex.

For most mammals, sex chromosomes   come in two versions, X or Y. But a cool and important thing to know:   XY is /not/ the only system out there. And some organisms don’t have sexes at all.  Anyway, in the XY system, an organism with  two X chromosomes usually develops as female,   while an organism with an X and  a Y usually develops as male.  So when a gene shows up on the X chromosome  and not the Y, its traits will show up at   different rates in females and males.

Now back to those calico kitties.  A color gene on cats’ X chromosome comes  in two alleles: an orange version (XB) and   a black version (Xb). If a kitten inherits   both alleles (XBXb), each cell  only expresses one at a time.  So the fur ends up looking like  Joseph’s Technicolor Dreamcoat.  It’s more straightforward for the XY  carrier, who gets only one X chromosome.  Inherit one allele, and it’s orange  fur; inherit the other, and it’s black.  That’s why calico cats are almost always XX. And while codominance isn’t always dependent on   sex, it is in the case of calico kitties.

There is so much we can  learn by studying genetics.  It helps us understand how traits are passed from  parent to offspring, why some cats are orange and   others are black, and why we don’t have to worry  about running out of redheads anytime soon.  But it also helps us on a much larger  scale, from creating more sustainable   agriculture to designing better medicine. Our genes are the first gift our parents ever   gave us, instructions that made you, you. And we can trace some basic patterns   of inheritance by understanding how  dominant and recessive alleles interact.  But it goes even deeper than that.

Most traits aren’t just a matter   of one allele talking over the other. They’re the result of an ongoing conversation   between the stuff in your chromosomes and  the stuff around them — multiple alleles,   multiple genes, and your whole environment. But we’ll learn more about that next time.  Peace!

This series was produced in   collaboration with HHMI BioInteractive. If you’re an educator, visit for classroom  resources and professional development related   to the topics covered in this course. Thanks for watching this episode of   Crash Course Biology, which was filmed at our  studio in Indianapolis, Indiana, and was made   with the help of all these nice people.

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