It is not practical to perform a Punnett square analysis on all possible combinations of all members of a population.
It is not practical to perform a Punnett square analysis on all possible combinations of all members of a population (1:24). Enter the Hardy-Weinberg equation! The Hardy-Weinberg equation is a tool biologists use not only to make predictions about a population but also to show whether or not the population is evolving. There are 5 conditions that must be met for this to be true. (2:14) Melanie break’s down the H-W equation variables and walks you through an example problem. (5:25)
The Question of the Day asks (7:25) What type of genetic drift occurs when a small group of individuals gets separated from a large population?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Hardy-Weinberg Equilibrium
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Unit 7 - Natural Selection
Topic 7.5
Big idea - Evolution
Did you ever have one of those math problems, that goes: You have 3 shirts, two pairs of pants and two pairs of shoes. How many different outfits can you make? Not too challenging to calculate or conceptualize. But, when it comes to mating in the natural world, there are populations with hundreds or even millions of organisms. It is difficult, most likely even impossible, to think of all the potential mating combinations. Add in a few generations, and the allele shuffle continues to be more randomized. So, how can we make predictions about the characteristics of a population?
Let’s Zoom in:
You may remember from studying genetics, that Punnett squares provide an easy way to predict the possible genotypes for an offspring. However, it is not practical to perform a Punnett square analysis on all possible combinations of all members of a population to predict what the population might look like in the future. For that we must turn to math and statistical analysis. Whats a population you ask? A population is a group of organisms of the same species in the same geographic area. It's important that they be in the same location, because this means they are more likely to combine gametes and make offspring. And after all, surviving long enough to pass on those genes is the cornerstone of evolution.
In order for a population to not be evolving, the group of alleles available within the population must be very stable from generation to generation. There are 5 conditions that must be met for this to be true. 1 - the population is very large 2. Mating is random 3. Everybody, meaning every genotype, has to have an equal chance or reproduction (which is essentially the opposite of natural selection) 4. Everybody sticks around and nobody enters, i.e. no immigration or emigration and 5. No mutations. These conditions are rarely met in the natural world, but they do provide a valuable null hypothesis.
Math helps biologists predict the outcome of the population when randomization occurs. The Hardy-Weinberg equation is a tool biologists use not only to make predictions about a population but also to show whether or not the population is evolving. The equations we need (and are found on your AP Biology equation sheet - top left of the first page, right under the laws or probability) were first developed by Godfred Harold Hardy and Wilhelm Weinberg. Hardy was an English mathematician whereas Weinberg was a German obstetrician. Unlike most hyphenated scientific breakthroughs, each independently came to the same conclusion. Thus, Hardy hyphen Weinberg Equilibrium. The equations are p + q = 1 and p2 + 2pq +q2 = 1. Lowercase p represents the frequency of allele 1 in a population whereas lowercase q represents the frequency of allele 2. By convention, p is the dominant allele while q is recessive. This means that p squared is the frequency of homozygous dominant, q squared is the frequency of homozygous recessive, and 2pq is the frequency of heterozygous. These frequencies are fractions of a whole population, represented as decimals, which collectively add to equal 1.
Lets practice. Imagine a mouse population of 100, where 20 represent the homozygous recessive, white mouse. What is the frequency of the dominant allele? Well, the 20 white mice represent q squared, so 20 divided by 100 is 0.2, and the square root of 0.2 is 0.45. Therefore, the frequency of the recessive allele, q is 0.45. And since p + q = 1, then p must equal 0.55! You could then continue your calculations to determine the expected frequency of p2 (homozygous dominant) and 2pq (heterozygous) mice in this population. If the frequency of genotypes in a population matches that predicted by the Hardy- Weinberg equation, then the population is said to be in Hardy-Weinberg equilibrium. If not, then the population is evolving.
Time for unit connections. The derivation of the Hardy-Weinberg equation has origins in the Punnett square and genetic probabilities - so strong connection to Unit 5: Heredity. The concept of population dynamics will repeat in Unit 8: Ecology.
Alright - what about the exam? You can use chi-square analysis to compare observed phenotypic ratios to those predicted by the p and q allele frequencies. It's important that you are careful to distinguish between allele and genotypic frequencies when making predictions about a change in a population and justifying the reasoning for your predictions.
To recap……
Allele frequencies stay constant when a population is in Hardy-Weinberg equilibrium. This occurs with five assumptions, no mutations, random mating, no gene flow, big population, and no natural selection. p equals the dominant allele frequency and q is the recessive.
Coming up next on the Apsolute RecAP Biology Edition: Phylogeny
Today’s question of the day is about genetic drift
Question of the day: What type of genetic drift occurs when a small group of individuals gets separated from a large population?