TNBGGA: The No Bullshit Guide to Genetic Analysis

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chapter_18 [2024/09/18 17:50] – mikechapter_18 [2026/10/25 01:04] (current) – [The Hardy Weinberg equilibrium] mike
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 <typo fs:x-large> Chapter 18. The Hardy-Weinberg equilibrium</typo> <typo fs:x-large> Chapter 18. The Hardy-Weinberg equilibrium</typo>
  
-Until now, we have been carrying out genetic analysis of individuals, This approach works well when you are studying a model organism, such as Drosophila or mice. It doesn't work well when you are interested in the genetics of organisms that cannot be studied in this way. For the next several chapters, we will consider genetics from the point of view of groups of individuals, or populations. We will treat this subject entirely from the perspective of human population studies where population genetics is used to get the type of information that would ordinarily be obtained by breeding experiments in model organisms.+In earlier chapters, we have been carrying out genetic analysis using controlled crosses where the genotypes are known and we can breed organisms however we want. This approach works well when you are studying a model organism, such as Drosophila or mice. It doesn't work well when you are interested in the genetics of organisms that cannot be studied in this way. For the next several chapters, we will consider genetics from the point of view of populations. We will treat this subject entirely from the perspective of human population studies where population genetics is used to get the type of information that would ordinarily be obtained by breeding experiments in model organisms.
  
 In the next several chapters, we will use a substantial amount of math. To avoid confusion with fractions, we will write genotypes as $A/a$ instead of $\frac{A}{a}$. In the next several chapters, we will use a substantial amount of math. To avoid confusion with fractions, we will write genotypes as $A/a$ instead of $\frac{A}{a}$.
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 </figure> </figure>
  
-Here is an example: $M$ and $N$ are alleles of a gene that specifies different blood antigens. The alleles are codominant, so a simple blood test can distinguish the three possible genotypes of $M/M$, $M/N$, and $N/N$. A survey of the population reveals that 83% of the population has only the $M$ antigen, 1% of the population only has the $N$ antigen, and 16% of the population has both the $M$ and $N$ antigens. In other words:+Here is an example: $M$ and $N$ are alleles of a gene that specifies different blood antigens. The alleles are codominant, so a simple blood test can distinguish between the three possible genotypes of $M/M$, $M/N$, and $N/N$. A survey of the population reveals that 83% of the population has only the $M$ antigen, 1% of the population only has the $N$ antigen, and 16% of the population has both the $M$ and $N$ antigens. In other words:
  
 <figure Fig5> <figure Fig5>
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 ==== Migration of individuals between different populations ==== ==== Migration of individuals between different populations ====
  
-When individuals from populations with different allele frequencies mix, the combined population will be in a Hardy-Weinberg equilibrium after one generation of random mating. The combined population will be out of equilibrium to the extent that mating is assortatative.+When individuals from populations with different allele frequencies mix, the combined population will be in Hardy-Weinberg equilibrium after one generation of random mating. The combined population will be out of equilibrium to the extent that mating is assortatative.
  
 ===== An example of calculating allele frequency in humans: albinism ===== ===== An example of calculating allele frequency in humans: albinism =====
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 Therefore, $f(A/A)=p^2 \approx 1$, $f(A/a)=2pq \approx 2q$, and $f(a/a)=q^2$. Since most genetic diseases are rare, these approximations are valid for many of the population genetics calculations that are of medical importance. Note that by convention, the $q$ symbol is used to represent rare alleles.   Therefore, $f(A/A)=p^2 \approx 1$, $f(A/a)=2pq \approx 2q$, and $f(a/a)=q^2$. Since most genetic diseases are rare, these approximations are valid for many of the population genetics calculations that are of medical importance. Note that by convention, the $q$ symbol is used to represent rare alleles.  
  
-Let's look at a real-life example. Albinism occurs in approximately 1 in 20,000 individuals in humans. Let's say that this condition is due to a recessive allele $a$ of a single gene that is in a Hardy-Weinberg equilibrium. Based on this information, we can derive the allele frequency for $a$:+Let's look at a real-life example. Albinism occurs in approximately 1 in 20,000 individuals in humans. Let's say that this condition is due to a recessive allele $a$ of a single gene that is in Hardy-Weinberg equilibrium. Based on this information, we can derive the allele frequency for $a$:
  
 $$f(a/a) = \frac{1}{20000} = 5\times10^{-5}=q^2\\ $$f(a/a) = \frac{1}{20000} = 5\times10^{-5}=q^2\\
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