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AP®︎/College Biology
Course: AP®︎/College Biology > Unit 5
Lesson 2: Mendelian genetics- Introduction to heredity
- Fertilization terminology: gametes, zygotes, haploid, diploid
- Alleles and genes
- Worked example: Punnett squares
- Mendel and his peas
- The law of segregation
- The law of independent assortment
- Probabilities in genetics
- Pedigrees
- Mendelian genetics
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Introduction to heredity
Heredity and Classical Genetics. Dominant and recessive traits. Heterozygous and homozygous genotypes. Created by Sal Khan.
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- Why is the darker eye color usually dominant?(136 votes)
- A new hypothesis is that all Homo Sapiens originally had Dark Eyes (Brown) and that one individual about 6,000 to 10,000 years ago had a mutation which partially turned off the gene for Dark Eyes which reduced the amount of Melanin in the eyes. It is further believed that all people with blue eyes are descended from that individual.
http://en.wikipedia.org/wiki/Eye_color(151 votes)
- is it possible that one day people will all have the same eye color and skin color(73 votes)
- If everyone has the dominant allele for eye and skin color by some chance, then yes I think it is POSSIBLE (trying to stress it), but highly improbable since there are now about 7 billion people in the world. By using punnet squares, even if everyone has the dominant allele from one parent, their children would have the possibility of having recessive alleles from both parents. It would all depend on the parent's genotypes. So in practical terms, it is not possible for everyone to have the same(81 votes)
- What eye colors are the most dominant in descending order?(20 votes)
- You must remember that dominance of an allele and how common that allele is in the population (its frequency) are not the same thing. Most of the answers here are actually about frequency, not dominance. Also remember that eye color is not determined by a single gene. Any least 5 separate genes affect human eye color, which helps explain the many different colors possible. In general, alleles for darker eyes (more pigment) are dominant to those for lighter eyes (less pigment), but more precision than that is way beyond the scope of these lessons.(16 votes)
- This may be a silly question, but are the 46 chromosones in one cell different to the 46 in another cell or are they the same in each cell??(18 votes)
- All of your cells have the same chromosomes, in which 23 came from your father and 23 from your mother. Just remember, only human diploid cells have 46 chromosomes. The human haploid gametes have 23 chromosomes. When 2 gametes (male and female) meet, they fuse creating a zygote, that contains 46 chromosomes.(19 votes)
- Why is it that darker colors are dominant over lighter colors?
Is the probability actually correct, that if the punnet square goes as such in the video that 3/4 chance of brown eyes, and 1/4 chance of blue eyes?(9 votes)- If Brown is dominant and Blue is recessive: B=brown and b=blue, the chances of brown are dependent on the parental traits. If the parents had Bb and Bb, the chance of brown and blue would be 75% brown and 25% blue. But if the parents were Bb and bb, it's 50% for both. If it's BB and BB, you get 100% brown, and if you get bb and bb, you get 100% blue. Hope this helped!(21 votes)
- Atdoes the order in which you write the heterozygous trait matter? Like does the dominant allele always appear first? 15:09(8 votes)
- It doesn't matter in a Punnett square, you can put whichever genotype you prefer first. Personally, I prefer to put the dominant allele on the top. Either way, you will end up with the same genotypes for possible offspring. Hope this helped!(5 votes)
- How are there so many eye colors if we all came from 2 people. If the two people had different colored eyes, like blue and brown, than they will most likely become brown. How can so many people have different colored eyes?(6 votes)
- You pose a good question: how could all of this variation derive from just two people? We have people with red hair and black hair, brown, and blonde. Eyes of a huge variety, as well as differences in facial structure and skin color. And those are just the external differences.
Scientists see this, and many other factors (such as fossils), as evidence that our human history is much older than we originally thought (~1 million years) and that there's no way that it started with two people (commonly called Adam and Eve).(10 votes)
- can a hetrozygote have more than 2 alleles for eye or skin color?(4 votes)
- Note: My original answer was a bit sloppy — this is an edited version.
First, many genes may influence a single phenotype. Eye color is an excellent example of this with 16 known genes that can have some influence on this trait§. My answer below assumes that we are talking about a single gene.
----------------------------------------------------
Yes, but only under circumstances that are not addressed at this level of study.
For example in organisms with more than two sets of chromosomes (e.g. a tetraploid organism has four copies of each chromosome and thus could have four different alleles for each gene).
Another exception occurs when organisms are genetic mosaics aka. chimeras — i.e they contain cells that are not genetically identical! This is actually much more common than you might expect — for example any female animal that has been pregnant is likely to have at least a few cells from each fetus she carried growing somewhere in her body!
So, the answer is "no", but only if you rephrase the question as "Can a diploid heterozygous cell contain more than two alleles for a single gene?"
Does that help?
§Note: Of the 16 genes involved in eye color two have major effects, and four more show relatively significant effects.
This section of the wikipedia article on eye color is worth checking out:
https://en.wikipedia.org/wiki/Eye_color#Genetic_determination(13 votes)
- Is anyone else totally lost on what homozygous recessive means? I keep looking it up but I still don't really understand...(3 votes)
- Let's start from the beginning:
Our physical characteristics (phenotypes) are coded by our genes - they are the reason why we appear the way we are.
A pair of genes which are genetic variants (alleles) can code for a certain phenotype/physical feature such as eye colour or a disease.
I will use an example:
cystic fibrosis is caused by a recessive allele
F=dominant allele (no cystic fibrosis)
f=recessive allele (cystic fibrosis)
A human's gene exist in pairs, so the combinations we have are the following: FF, Ff, ff (no fF, this is the same as Ff, but we always write it as Ff [dominant allele first])
FF is homozygous dominant
Ff is heterozygous
ff is homozygous recessive
for your question, homozygous recessive is when there are two recessive alleles.
A dominant allele's phenotype will be shown if there is at least one dominant allele. As cystic fibr is caused by a recessive allele (hence a dominant allele's phenotype would be no cystic fibr), a person won't have it if he has a dominant allele. So as long as he is homozygous dominant or heterozygous (FF or Ff), the person will not have cystic fibrosis.
However, if the person is homozygous recessive (ff), he will have it. The recessive allele will only be shown if there is no dominant allele.
TL;DR
Homozygous recessive is when there are two genes (alleles) which are both recessive, hence they will show a recessive characteristic(7 votes)
- why are there only brown hazel and blue eyes.
Why couldnt their be red, orange, pink, purple?(2 votes)- There are basically 2 genes responsible for the production of the protein used in the brow pigment in the eye.
Blue eyes are actually a lack of pigment and the blue color is caused by light scattering similar to what makes the sky blue.
Hazel is a malfunctioning the production of the brown pigment so there is a minimal amount brown pigment so that you get a combination of brown that ends up looking more yellow and blue making it look green. Depending on the amount of the brown pigment will cause the various shades of hazel between brown and blue.
Since there is only one pigment involved in eye color there only one set of color variations for eye color.(7 votes)
Video transcript
Well, before we even knew what
DNA was, much less how it was structured or it was replicated
or even before we could look in and see meiosis
happening in cells, we had the general sense that offspring
were the products of some traits that their parents had. That if I had a guy with blue
eyes-- let me say this is the blue-eyed guy right here --and
then if he were to marry a brown-eyed girl-- Let's say this
is the brown-eyed girl. Maybe make it a little
bit more like a girl. If he were to marry the
brown-eyed girl there, that most of the time, or maybe in
all cases where we're dealing with the brown-eyed girl,
maybe their kids are brown-eyed. Let me do this so they have a
little brown-eyed baby here. And this is just something--
I mean, there's obviously thousands of generations of
human beings, and we've observed this. We've observed that kids look
like their parents, that they inherit some traits, and that
some traits seem to dominate other traits. One example of that tends to
be a darker pigmentation in maybe the hair or the eyes. Even if the other parent has
light pigmentation, the darker one seems to dominate, or
sometimes, it actually ends up being a mix, and we've seen
that all around us. Now, this study of what gets
passed on and how it gets passed on, it's much older than
the study of DNA, which was really kind of discovered
or became a big deal in the middle of the 20th century. This was studied a long time. And kind of the father of
classical genetics and heredity is Gregor Mendel. He was actually a monk, and he
would mess around with plants and cross them and see which
traits got passed and which traits didn't get passed and
tried to get an understanding of how traits are passed from
one generation to another. So when we do this, when we
study this classical genetics, I'm going to make a bunch of
simplifying assumptions because we know that most of
these don't hold for most of our genes, but it'll give us a
little bit of sense of how to predict what might happen
in future generations. So the first simplifying
assumption I'll make is that some traits have kind of this
all or nothing property. And we know that a lot
of traits don't. Let's say that there are in
the world-- and this is a gross oversimplification --let's
say for eye color, let's say that there
are two alleles. Now remember what
an allele was. An allele is a specific
version of a gene. So let's say that you could
have blue eye color or you could have brown eye color. That we live in a universe where
someone could only have one of these two versions
of the eye color gene. We know that eye color is far
more complex than that, so this is just a simplification. And let me just make
up another one. Let me say that, I don't know,
maybe for tooth size, that's a trait you won't see in any
traditional biology textbook, and let's say that there's one
trait for big teeth and there's another allele
for small teeth. And I want to make very clear
this distinction between a gene and an allele. I talked about Gregor Mendel,
and he was doing this in the 1850s well before we knew what
DNA was or what even chromosomes were and how DNA was
passed on, et cetera, but let's go into the microbiology
of it to understand the difference. So I have a chromosome. Let's say on some chromosome--
let me pick some chromosome here. Let's say this is
some chromosome. Let's say I got that
from my dad. And on this chromosome, there's
some location here-- we could call that the locus on
this chromosome where the eye color gene is --that's
the location of the eye color gene. Now, I have two chromosomes,
one from my father and one from my mother, so let's say
that this is the chromosome from my mother. We know that when they're
normally in the cell, they aren't nice and neatly organized
like this in the chromosome, but this is just to
kind of show you the idea. Let's say these are homologous
chromosomes so they code for the same genes. So on this gene from my mother
on that same location or locus, there's also the
eye color gene. Now, I might have the same
version of the gene and I'm saying that there's only
two versions of this gene in the world. Now, if I have the same version
of the gene-- I'm going to make a little
shorthand notation. I'm going to write big B--
Actually, let me do it the other way. I'm going to write little b
for blue and I'm going to write big B for brown. There's a situation where this
could be a little b and this could be a big B. And then I could write that my
genotype-- I have the allele, I have one big B from my
mom and I have one small b from my dad. Each of these instances, or
ways that this gene is expressed, is an allele. So these are two different
alleles-- let me write that --or versions of
the same gene. And when I have two different
versions like this, one version from my mom, one version
from my dad, I'm called a heterozygote, or
sometimes it's called a heterozygous genotype. And the genotype is the exact
version of the alleles I have. Let's say I had the
lowercase b. I had the blue-eyed gene
from both parents. So let's say that I was
lowercase b, lowercase b, then I would have two identical
alleles. Both of my parents gave me the
same version of the gene. And this case, this genotype
is homozygous, or this is a homozygous genotype, or I'm a
homozygote for this trait. Now, you might say,
Sal, this is fine. These are the traits that you
have. I have a brown from maybe my mom and a
blue from my dad. In this case, I have a blue
from both my mom and dad. How do we know whether my eyes
are going to be brown or blue? And the reality is it's
very complex. It's a whole mixture
of things. But Mendel, he studied
things that showed what we'll call dominance. And this is the idea that
one of these traits dominates the other. So a lot of people originally
thought that eye color, especially blue eyes,
was always dominated by the other traits. We'll assume that here,
but that's a gross oversimplification. So let's say that brown
eyes are dominant and blue are recessive. I wanted to do that in blue. Blue eyes are recessive. If this is the case, and this
is a-- As I've said repeatedly, this is a gross
oversimplification. But if that is the case, then
if I were to inherit this genotype, because brown eyes
are dominant-- remember, I said the big B here represents
brown eye and the lowercase b is recessive --all you're going
to see for the person with this genotype
is brown eyes. So let me do this here. Let me write this here. So genotype, and then I'll
write phenotype. Genotype is the actual versions
of the gene you have and then the phenotypes
are what's expressed or what do you see. So if I get a brown-eyed gene
from my dad-- And I want to do it in a big-- I want
to do it in brown. Let me do it in brown so
you don't get confused. So if I've have a brown-eyed
gene from my dad and a blue-eyed gene from my mom,
because the brown eye is recessive, the brown-eyed allele
is recessive-- And I just said a brown-eyed gene, but
what I should say is the brown-eyed version of the
gene, which is the brown allele, or the blue-eyed version
of the gene from my mom, which is the blue allele. Since the brown allele is
dominant-- I wrote that up here --what's going to be
expressed are brown eyes. Now, let's say I had
it the other way. Let's say I got a blue-eyed
allele from my dad and I get a brown-eyed allele for my mom. Same thing. The phenotype is going
to be brown eyes. Now, what if I get a brown-eyed
allele from both my mom and my dad? Let me see, I keep changing
the shade of brown, but they're all supposed
to be the same. So let's say I get two dominant
brown-eyed alleles from my mom and my dad. Then what are you
going to see? Well, you could guess that. I'm still going to
see brown eyes. So there's only one last
combination because these are the only two types of alleles
we might see in our population, although for
most genes, there's more than two types. For example, there's
blood types. There's four types of blood. But let's say that I get two
blue, one blue allele from each of my parents, one from
my dad, one from my mom. Then all of a sudden, this is a
recessive trait, but there's nothing to dominate it. So, all of a sudden, the
phenotype will be blue eyes. And I want to repeat again, this
isn't necessarily how the alleles for eye color work, but
it's a nice simplification to maybe understand how
heredity works. There are some traits that can
be studied in this simple way. But what I wanted to do here
is to show you that many different genotypes-- so these
are all different genotypes --they all coded for
the same phenotype. So just by looking at someone's
eye color, you didn't know exactly whether
they were homozygous dominant-- this would be
homozygous dominant --or whether they were
heterozygotes. This is heterozygous
right here. These two right here
are heterozygotes. These are also sometimes called
hybrids, but the word hybrid is kind of overloaded. It's used a lot, but in this
context, it means that you got different versions of the
allele for that gene. So let's think a little bit
about what's actually happening when my mom and
my dad reproduced. Well, let's think of a couple
of different scenarios. Let's say that they're
both hybrids. My dad has the brown-eyed
dominant allele and he also has the blue-eyed recessive
allele. Let's say my mom has the same
thing, so brown-eyed dominant, and she also has the blue-eyed
recessive allele. Now let's think about if these
two people, before you see what my eye color is, if you
said, look, I'm giving you what these two people's
genotypes are. Let me label them. Let me make this the mom. I think this is the standard
convention. And let's make this right
here, this is the dad. What are the different genotypes
that their children could have? So let's say they reproduce. I'm going to draw a
little grid here. So let me draw a grid. So we know from our study of
meiosis that, look, my mom has this gene on-- Let me draw
the genes again. So there's a homologous
pair, right? This is one chromosome
right here. That's another chromosome
right there. On this chromosome in the
homologous pair, there might be-- at the eye color locus
--there's the brown-eyed gene. And at this one, at the eye
color locus, there's a blue-eyed gene. And similarly from my dad, when
you look at that same chromosome in his cells-- Let
me do them like this. So this is one chromosome there
and this is the other chromosome here. When you look at that locus
on this chromosome or that location, it has the brown-eyed
allele for that gene, and on this one,
it has the blue-eyed allele on this gene. And we learn from meiosis when
the chromosomes-- Well, they replicate first, and so you have
these two chromatids on a chromosome. But they line up in meiosis
I during the metaphase. And we don't know which
way they line up. For example, my dad might give
me this chromosome or might give me that chromosome. Or my mom might give me that
chromosome or might give me that chromosome. So I could have any of
these combinations. So, for example, if I get this
chromosome from my mom and this chromosome from my dad,
what is the genotype going to be for eye color? Well, it's going to be capital
B and capital B. If I get this chromosome from
my mom and this chromosome from my dad, what's
it going to be? Well, I'm going to get the big
B from my dad and then I'm going to get the lowercase
b from my mom. So this is another
possibility. Now, this is another possibility
here where I get the brown-eyed allele from my
mom and I get the blue eye allele from my dad. And then there's a possibility
that I get this chromosome from my dad and this chromosome
from my mom, so it's this situation. Now, what are the phenotypes
going to be? Well, we've already seen that
this one right here is going to be brown, that one's going to
be brown, this one's going to be brown, but this one
is going to be blue. I already showed you this. But if I were to tell you ahead
of time that, look, I have two people. They're both hybrids, or they're
both heterozygotes for eye color, and eye
color has this recessive dominant situation. And they're both heterozygotes
where they each have one brown allele and one blue allele, and
they're going to have a child, what's the probability
that the child has brown eyes? What's the probability? Well, each of these scenarios
are equally likely, right? There's four equal scenarios. So let's put that in
the denominator. Four equal scenarios. And how many of those
scenarios end up with brown eyes? Well, it's one, two, three. So the probability is 3/4, or
it's a 75% probability. Same logic, what's the
probability that these parents produce an offspring
with blue eyes? Well, that's only one of
the four equally likely possibilities, so blue
eyes is only 25%. Now, what is the probability
that they produce a heterozygote? So what is the probability
that they produce a heterozygous offspring? So now we're not looking at
the phenotype anymore. We're looking at the genotype. So of these combinations,
which are heterozygous? Well, this one is, because
it has a mix. It's a hybrid. It has a mix of the
two alleles. And so is this one. So what's the probability? Well, there's four different
combinations. All of those are equally likely,
and two of them result in a heterozygote. So it's 2/4 or 1/2 or 50%. So using this Punnett square,
and, of course, we had to make a lot of assumptions about the
genes and whether one's dominant or one's a recessive,
we can start to make predictions about the
probabilities of different outcomes. And as we'll see in future
videos, you can actually even go backwards. You can say, hey, given that
this couple had five kids with brown eyes, what's the
probability that they're both heterozygotes, or something
like that. So it's a really interesting
area, even though it is a bit of oversimplification. But many traits, especially some
of the things that Gregor Mendel studied, can be
studied in this way.