LESSON IV:
THE FUNNET SQUARE
To Address NYS Standards:
2 (Organisms inherit genetic information in a variety of ways that result in continuity of
structure and function between parents and offspring) and 4 (The continuity of life is
sustained through reproduction and development).
Behavioural Objective:
The students will able to construct a Punnet square given a set of genotypes. The
students will also be able to successfully use the Punnet square to determine the
genotypic ratios of the progeny, as well as their phenotypes.
Explanation of Lesson Plan:
This lesson will teach students how to use the “tool” of a Punnet square to more easily
determine the outcomes of various crosses. Students will become familiar with both
monohybrid and dihybrid crosses.
Hook: (3 minutes)
“You are now going to learn how to construct and use one of the most powerful tools in
the entire scientific toolbox, the funnet square. Many people think that it’s pronounced
“Punnet,” but it’s not. It’s funnet. Silent P, invisible F. This tool will allow you to
quickly and easily discover what the result of some pretty complex mating might be.”
Test of Prior Learning: (3 minutes)
1. What do you already know about what you can do with a funnet square?
2. Why is the square useful? Is it simply just a nuisance?
3. Reginald Punnet invented the square as a tool to figure out more complex crosses.
New Learning: (30 minutes)
1. What you need to keep in mind is that the distribution of many genes into the gametes
is random.
2. Which sperm fertilized which egg is also completely random – remember the spheres
– O – probability.
3. So we can use the square to figure out what the probability of getting certain
genotypes in the next generation is when we know the genotypes of the parents.
4. KEEP IN MIND THAT THIS DOES NOT PREDICT THE OUTCOME OF THE
CROSS. It only gives you the probabilities involved. If a couple has four children, ¾ will
not be affected by a certain disease simply because the square says so. It gives you the
CHANCES that it will be that way.
5. We also have to consider Mendel’s Law of Independent Assortment:
- Genes for different traits are separated and distributed independently and
randomly to gametes. Almost….. (linked genes, etc. to be discussed later)
6. We’ll look at a simple cross first
- demonstrate cross between homozygous dominant and homozygous recessive.
The example will be for seed shape, with round being dominant over wrinkled.
7. What are the chances that the dominant trait will be expressed? What are the
genotypes and phenotypes expressed?
8. Now we can get a little more complicated. Let’s do two heterozygotes.
- demonstrate monohybrid cross. The example will be for wet earwax and dry
earwax, with dry earwax dominant over wet earwax.
8. What are the chances that the trait will be expressed? What are the genotypes and
phenotypes observed?
9. We can now get even fancier. We can use the square as a tool to figure out the
genotype of one organism, if we already know the genotype of another.
10. A test cross is just that. One takes a homozygous recessive organism and crosses it
with an unknown. How do you know what the genotype of the other organism was?
- demonstrate a test cross, using a homozygous recessive white flower and a
heterozygous purple flower, where purple is dominant to white.
11. What if you had two traits, though? Say Tallness and Seed color? We could do a
dihybrid cross.
- Explain how to use the FOIL method to get the genotypes for the top and side
of the square, and do the cross. Tallness is dominant to shortness, and yellow seeds are
dominant to green.
12. Note that the ratios of the phenotypes are 9:3:3:1, that is, 9 AA, 3 Aa, 3 aA, and 1 aa.
The ratios of individual traits are still 3:1, as expected from a hybrid cross.
12. There are always, of course, exceptions to the rules. Incomplete dominance is one of
them. This is when there is not a clear dominant trait, and the resulting phenotype is a
blend of the two traits.
- Do cross with Japanise four o’clock plant. Show that the resulting genotypes
indicate a blend of colors from red to pink to white.
13. Codominance is also another exception. This is where both traits are expressed.
Such as in roan coats, or in human blood types.
14. Finally, not all genes have just two alleles or versions. Blood type has three
versions. RH factors are another example.
15. The blood type funnet square can be done the same way, but the results can be
slightly different.
- demonstrate a cross between a homozygous A type and a heterozygous B type
person. IAIA x IBi
16. Note that some of the progeny can be AB, where codominance causes the expression
of both genes, as neither is clearly recessive.
17. The thing that we need to keep in mind, however, is that not all traits are controlled
by simply one gene. Dog coat color, for example, or duck feather arrangement is
controlled by many genes.
18. Dog coat color is controlled by at least 7 different genes, many of which have more
than two alleles.
- Show table from Pierce, Pg. 113
Test of New learning: (5 minutes)
1. Students will do a dihybrid cross of a homozygous tall, heterozygous green
Nifflenork, and a heterozygous tall, homozygous red Nifflenork. Tall is dominant to
short, and green is dominant to red. TTGg x Ttgg
Assignment: “Single and Double Trait Crosses” from the blue book, pg. 95