The Hardy-Weinberg Equilibrium
How can we make predictions about the characteristics of a population?
Why?
Punnett squares provide an easy way to predict the possible genotypes for an offspring, but it is not prac-
tical 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 statistics. The Hardy-
Weinberg equation is a tool biologists use to make predictions about a population and to show whether or
not evolution is occurring in that population.
Model 1 – Controlled (Selective) Mating
Males Females Males Females
Bb bb Bb bb bb Bb bb Bb
Bb bb Bb bb bb Bb bb Bb
Bb bb Bb bb bb Bb bb Bb
1. How many mating pairs are illustrated in Model 1?
There are twelve mating pairs.
2. Describe the parents in each mating pair in Model 1. Use terms such as homozygous, heterozy-
gous, dominant, and recessive.
One parent is heterozygous and the other is homozygous recessive.
3. Use two Punnett squares to determine the possible genotypes for offspring from the pairs.
B b b b
b Bb bb B Bb Bb
b Bb bb b bb bb
1
, 4. If each mating pair has one offspring, predict how many of the first generation offspring will
have the following genotypes.
BB Bb bb
0 6 6
5. Imagine the 24 beetles in Model 1 as a population in an aquarium tank.
a. How likely is the pairing scenario in Model 1 to take place during the natural course of things
within that tank?
This pairing scenario is too organized. It is not very likely.
b. Why is Model 1 labeled “Selective Mating”?
Each heterozygous beetle was mated with a homozygous beetle purposefully.
6. List two other pairings that might occur in the population in Model 1 if the beetles were allowed
to mate naturally.
Bb with Bb bb with bb
7. If the population of beetles in Model 1 mated naturally would your prediction for the offspring
in Question 4 still be valid? Explain.
No, the prediction from the Punnett squares would not include the offspring from the other two types of pairings.
8. Discuss in your group the limitations of Punnett square predictions when it comes to large
populations. Summarize the key points of your discussion here.
Answers will vary. Punnett squares only give the predicted offspring ratios for one type of pairing. In large
populations it would be cumbersome to do a Punnett square for every possible pairing scenario. There is no way to
know how many of each type of pairing will occur naturally in a large population.
2
How can we make predictions about the characteristics of a population?
Why?
Punnett squares provide an easy way to predict the possible genotypes for an offspring, but it is not prac-
tical 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 statistics. The Hardy-
Weinberg equation is a tool biologists use to make predictions about a population and to show whether or
not evolution is occurring in that population.
Model 1 – Controlled (Selective) Mating
Males Females Males Females
Bb bb Bb bb bb Bb bb Bb
Bb bb Bb bb bb Bb bb Bb
Bb bb Bb bb bb Bb bb Bb
1. How many mating pairs are illustrated in Model 1?
There are twelve mating pairs.
2. Describe the parents in each mating pair in Model 1. Use terms such as homozygous, heterozy-
gous, dominant, and recessive.
One parent is heterozygous and the other is homozygous recessive.
3. Use two Punnett squares to determine the possible genotypes for offspring from the pairs.
B b b b
b Bb bb B Bb Bb
b Bb bb b bb bb
1
, 4. If each mating pair has one offspring, predict how many of the first generation offspring will
have the following genotypes.
BB Bb bb
0 6 6
5. Imagine the 24 beetles in Model 1 as a population in an aquarium tank.
a. How likely is the pairing scenario in Model 1 to take place during the natural course of things
within that tank?
This pairing scenario is too organized. It is not very likely.
b. Why is Model 1 labeled “Selective Mating”?
Each heterozygous beetle was mated with a homozygous beetle purposefully.
6. List two other pairings that might occur in the population in Model 1 if the beetles were allowed
to mate naturally.
Bb with Bb bb with bb
7. If the population of beetles in Model 1 mated naturally would your prediction for the offspring
in Question 4 still be valid? Explain.
No, the prediction from the Punnett squares would not include the offspring from the other two types of pairings.
8. Discuss in your group the limitations of Punnett square predictions when it comes to large
populations. Summarize the key points of your discussion here.
Answers will vary. Punnett squares only give the predicted offspring ratios for one type of pairing. In large
populations it would be cumbersome to do a Punnett square for every possible pairing scenario. There is no way to
know how many of each type of pairing will occur naturally in a large population.
2
