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AP®︎/College Biology
Course: AP®︎/College Biology > Unit 7
Lesson 4: Hardy-Weinberg equilibriumAllele frequency
Allele frequency describes how often an allele (a variant of a gene) appears in a population. In this video, eye color is used as an example, with brown (B) eyes being dominant and blue (b) eyes being recessive. Allele frequency (or genotype frequency) can also differ from phenotype frequency. Created by Sal Khan.
Want to join the conversation?
- Is autism a genetic disorder? If so, which chromosome determines the mutation? What about autism spectrum disorders such as Asperger's syndrome?(5 votes)
- Autism spectrum includes a wide range of Autistic-like disorders that present with similar traits. Significantly, many of these disorders are caused by Copy Number Variants (CNV), or duplications or deletions of chromosome regions. Recent research in this area has focused on SNP's (Single Nucleotide Permutations) in genes whose dysfunction have correlations with ASD (Autism Spectrum Disorder).
Affected genes include MECP2, FOXP1, Neuroligins and Neurexins - often only one of these is the cause.
The reason that Autism has such a complex etiology (origin) is due to two factors: 1) The source of dysfunction can arise from a single change among many genes, and 2) the nature of the disorder (or where the patient falls on the "spectrum") relies on when the affected gene is expressed.
Simply put, Autism results from the brain requesting a gene during development, and getting a result that is too big or too small. A good (advanced) reference is an article by Ebert and Greenberg, published in Nature, 2013.(17 votes)
- I don't know the difference of Allele Frequency (ex. of Bb and bb) and the possibility of the offspring having the dominant gene.(6 votes)
- Think of allele's as small sub sections of gene's. Chromosomes have genes and within genes are allele's. Allele Frequency is the possibility the a specific allele will be transferred. Same concept as finding the possibility of the dominant gene but at a much smaller and more detailed scale.(9 votes)
- Is brown dominant to other eye color alleles in the real world?(4 votes)
- Yes, brown eyes are dominant and brown hair is dominant(3 votes)
- Isn't a gene essentially a strand of DNA that has a specific number of base pairs, and that strand "codes" for a specific trait (such as eye colour, hair colour, and teeth size)?
And just going a bit further, are alleles just variations of a gene (or strand of DNA), that code for the same trait but with physical differences (for ex. the gene is eye colour, but one allele codes for blue eyes, and another brown)?(4 votes)- Yes and no.
Genes are found on strands of DNA, they are not themselves a strand of DNA.
A gene is a sequence of base pairs that code for a specific protein, not a specific phenotypic trait as such. The various proteins interact in very complex ways to form traits. So, genes do not, strictly speaking, code for phenotypic traits. However, a specific trait might be reliant on the protein produced by one gene for some key aspect of its nature. So while we might often say the gene for this or that trait, what is more accurate is saying the gene for the protein is involved in this or that trait.
Genes, in fact, tend to be involved in multiple phenotypic traits because the protein they code for can be used in a variety of ways.
Alleles are variations of genes, yes. But they may or may not produce observable differences in some phenotypic trait. The reason is that proteins tend to be very large molecules and so a few minor changes here or there in their structure may or may not have much of an effect on their functions. So, some alleles produce proteins that produce pretty much identical phenotypic traits. Also, remember that some amino acids can be coded for by more than one codon, so some alleles merely contain different ways of coding for the same protein.
But, obviously, some alleles, even with minor changes, can produce proteins that lead to significantly different traits. Eye color is not a good example because there are many genes involved in that (perhaps as many as 16 genes).(11 votes)
- What causes some people to be born albino?(4 votes)
- A missing or malfunctioning gene that is required to make pigment.(6 votes)
- so to be clear punnet squares so the percentage of phenotype for brown, and allele frequency is just focusing on one allele?(4 votes)
- The percentages for the phenotypes are based the offspring's appearance/expressed trait, which results from the genotype (the combination of different alleles). Punnet squares allow you to find the the possible combinations of alleles of that offspring can have, and the probability that a combination will occur. In Sal's example, 50% of the population will have brown eyes (Bb, which is the genotype). You are correct in that allele frequency is the percentage of specific allele in a population :) So even though the dominant trait is expressed in 50% of the population, only 25% of the possible alleles are B.(3 votes)
- At the end of the videowhen he says p+q = 1, isn't the problem giving you p^2 and q^2 so to find the allele frequency you would have to find p and q by itself and to do so you would have to square root? 7:00(3 votes)
- No, you don't need to square anything. It's more simple than that. One in four alleles is the dominant one therefore p=25% or 0.25. Three in four alleles is the recessive one so q=75% or 0.75. The total is 100% or 1, therefore for a gene with two alleles, p+q=1 always.(3 votes)
- What makes certain alleles dominant, and others recessive?(3 votes)
- That is predetermined and written in DNA.
Scientists do not know for sure 'what' si it, but once they act together, certain alleles 'mask' others (dominant mask recessive).(2 votes)
- How can it be three that code for blue eyes when there is only one pair of alleles that codes for blue eyes so wouldn't that be 25%?(3 votes)
- No. You are thinking wrong way. Follow me.
In Bb x bb
How many b you see?
Once there is only one b, can't you see that we are crossing heterozygous and homozygous for b giving us 3? 3/4 is 0.75.(1 vote)
- Are hazel eyes special ?(2 votes)
Video transcript
Voiceover: What I want
to do with this video is explore the idea of allele frequency. Allele frequency. Just as a reminder, an allele
is a variant of a gene. You get a variant of a
gene from your mother, and you get another variant of the gene from the father. So, when we're talking about
the allele, we're talking about that specific
variant that you got from your mother or your father. We've seen this before, but now let's dig a little bit deeper. To help us get our heads
around this, we'll start with a fairly common model for this. We're going to think about eye color. Obviously, this is a very
large simplification, but let's just assume that we
have a population where there's only two variants
of an eye color gene. Let's first assume there
is an eye color gene. Let's assume there's two variants. One variant, one allele for
eye color, we'll use the shorthand, capital B. Let's say that's the allele for brown. Brown eye color. We're going to assume
that this one is dominant. It's dominant over the other allele. Now the other allele, we're
going to assume is for blue eye color, and we'll
represent that with a lower case B. So that is blue eye color,
and we're going to assume that this is recessive. Once again, this is review. Someone who has one of
the big B alleles, the brown alleles, it doesn't
matter what their other allele is going to be, because
it's either going to be another brown or it's
going to be a blue, they're going to show brown eyes. This is going to be brown
eyes, and this is going to be brown eyes, because the capital B is dominant. The only way to get blue
eyes is to be a homozygote for the recessive allele. All of that, of course, is review. We've seen that before. Now let's think about allele frequency. To think about that, I'll
set up a very artificially small population. Let's say our population has
exactly two people in it. Population has exactly
two people in it, Person 1 and Person 2, and let's
say we're able to look into their DNA and figure
out their genotypes. Person 1, say, has a capital
B allele, has a brown allele and a blue allele,
while Person 2 has two blue, two blue alleles. Given that we know the
genotypes in this artificially small population, we
could start thinking about the allele frequencies. Or the frequencies of
the different alleles. What do you think is
going to be the frequency the frequency of the brown
allele in this population? I encourage you to pause
this video and think about this on your own. I'm assuming you've had a
go at it, so you might be tempted to say, "Looks
like one out of two people "have it, maybe it's 50%." But that wouldn't be the
right way to think about allele frequencies. In allele frequencies, you
want to dig a little bit deeper and look at the individual alleles. When you look at that,
you say, "Okay, there's "four individual alleles in
this population, or there's "four variants, or there's
literally four chromosomes "that are carrying that
gene in this population." Out of them, one of them
carry, one of them is the capital B allele, so we could
say that that is going to be zero point two five, or 25%. Once again, 25% of the
genes for eye color have the capital B allele, have the brown allele. Now we can do the same,
ask ourselves the same question for the lower case B allele. What fraction of the genes
in this population are code for or represent
the lower case B, the blue allele? Once again, I encourage
you to pause the video and think about it. Well, very similar idea. There's four genes in
the population that are coding for eye color. Of them, one, two, three
code for or are the lower case blue allele. So that's zero point seven five or 75%. 75% of the genes code for
the lower case blue allele, while 25 are the brown allele. I really want to hit this
point home, how this is different than, say,
the phenotype frequency. If I asked you, in the
population, if I asked you the percent of brown-eyed
people, so now I'm talking about phenotype, what would that be? Well, there's two people
in the population. One of them is exhibiting
brown eyes, so that's going to be one-half. Similarly, if I were to ask
you what is the percentage of people who are
blue-eyed that, too, would be one-half. This person is one of
the two people, they're exhibiting blue eyes. But allele frequency,
we're digging deeper, we're looking at the genotypes. We're saying out of the
four genes here, one of them is the big B allele,
so that's 25% of the gene population codes for the brown allele and 75% is the blue allele. This is really important to internalize. Because once we internalize
this, then as we'll see, that the ideas in the
Hardy-Weinberg principle start to make a lot of sense. I'll do a little bit of foreshadowing. We can denote this, this
is just a convention that's often used, by the lower
case letter P, and we can use lower case Q to denote the frequency. So lower case P is the
frequency of the dominant allele, lower case Q the frequency of the recessive allele. What's true here? What's true of P, what's
going to be true of P plus Q, what's P plus
Q going to be equal to? I encourage you to pause the video again and think about that. What is this going to be equal to? Well, when we started
off, we said that there's only two potential, that's
one of the assumptions we assumed, we assumed
there's only two alleles in this population, in kind of the allele population for this gene
population for this trait. The frequency of the
dominant ones plus the frequency of recessive
ones, well everyone's going to have one of those two,
so if you add those two frequencies, it's going
to have to add to 100%. We see that there. One-fourth plus three-fourths
is one, or 100%. And 25% plus 75% is also 100%. So we could say P plus Q
is equal to 100%, or we could say that P plus Q is equal to one. Is equal to one. So, in the next video, we're
going to start from the seemingly fairly simple
idea, to get to a more richer and fairly neat idea
that's expressed in the Hardy-Weinberg equation.