Download Mark Windschitl

Mark Windschitl
TLP 586
“Using the model of a Punnett Square to predict offspring ratios”
7th/8th grade life science
Objectives:
Objective 1: Students will know that chromosomes have genes that control the phenotype
(appearance) of organisms
Objective 2: Students will calculate the percentages of various types of potential offspring
based on a simple dominance model and using a single trait.
Objective 3. Students will extend the model of the Punnett Square in order to apply the
concept of simple dominance to new traits.
Setup before class: Write on board, “Will your kids be able to roll their tongue?”
Safety considerations for this activity? None necessary
Opening script: (Refer to board)
We have been talking about the idea of sperm and egg getting together to create a new
life. We know that this happens in both plants and animals. In animals, the male
contributes the sperm, and the female contributes the egg. In plants, the male part of the
plant contributes pollen, which acts like the sperm, and in the female part of the plant is
the egg.
Yesterday we talked about how we look like our mothers or our fathers, but we didn’t
take the conversation very far.
____________________
Put up overhead-There are some mysteries involved in how you all have inherited traits from your fathers
and mothers.
• If we look at brothers or sisters who all come from the same parents, do these
brothers and sisters all look exactly the same? So are we not clones of our parents?
• What are some differences between brothers or sisters who come from the same
parents?
Another question:
• If you have a mother who is 5 feet 5 inches tall and a father who is 6 feet tall,
and they have four children, how tall do you think they should be? All the same?
• Is it possible for a child to end up shorter than both parents? Or taller than both
parents?
One more question, have any of you seen a person who looks like they have the traits of
one of their grandparents but not the parent, like the traits skipped a generation? Do any
of you have traits more similar to your grandmothers or grandfathers than your parents?
________________________
To begin to understand how these kinds of traits can be inherited, we have to go back to
the idea of the sperm and the egg contributing to who we are.
We have mentioned that in order to create a human being, you need sperm from a father
and an egg from the mother (show the 46 chromosomes OH). These paired chromosomes
are made up of thousands of sections called genes. Each gene controls some kind of trait,
like color of hair, or color of eyes, or contribute to height. There are about 100,000 genes
that make up the human body.
We have talked about the 23 chromosomes that are in each sperm cell and the 23
chromosomes that are in each egg cell. These chromosomes look something like this
(show male chromosomes OH).
So, when humans are first formed from one sperm and one egg cell, we get two of each of
the 23 chromosomes, and, with those 23 chromosome from each parent comes all the
genes on each chromosome. So you have one gene for eye color from your father and one
from your mother. You have one gene for the shape of your earlobes from your mother
and one from your father. You have two of each.
Stop for questions—
What is interesting is that we can tell a little about what kind of genes we have by
observing various traits in ourselves. I’d like you to take a little data now with your
classmates (show earlobes and tongue OH and talk about which is dominant). There is a
gene for whether or not your earlobes are attached and a gene for whether you can roll
your tongue. I’d like half of you in pairs, to record the number of people that have
attached versus free earlobes. The other half will do another chart for the tongue.
Assign in pairs, half the pairs to the tongue and half do the earlobes.
There seems to be a larger number of people who can roll their tongue. Let’s see if we can
determine why and see if we can answer the essential question for the day.
The reason is because the gene for the ability to roll your tongue or to have free earlobes
is called dominant. The genes that tell your body to have attached earlobes, or that do not
let you roll your tongue are recessive. Here is how it works:
Explain Punnett Squares: YOU FILL IN AS I GO! STEP 1, 2, 3, 4,
F (free) a (attached)
F (free) a (attached)
These are the combinations that are possible. Only possible! They are all equally possible.
We said that free earlobes were dominant, that means kids with just one will show it. So
which of these combinations of kinds will show free earlobes? Go through each cell, what
will they look like?
Hand out two empty Punnett Squares each.
The only way to have attached earlobes is to have two recessive genes, one from the
father and one from the mother. What fraction of offspring will have free and what
fraction will have attached?
Let’s do another together:
FF
aa check for right combinations and ratios
If I look at you and see that you have free earlobes, does that mean you have two genes
for free earlobes? Nooooo. If I see that you have attached earlobes,
Let’s do another for tongue. There are 2 genes for tongue-rolling, one for and one against.
Which do you think is dominant?.
Try
RR
nn
Independent work time/assessment
1. If you can roll your tongue, what do you know about the two genes that you have
inherited? Explain.
2. If you cannot roll your tongue, what do you know about the two genes you have
inherited? Explain.
3. Using a Punnett Square, can you show if it is possible to have another and a father who
can roll their tongues, but have children that cannot roll their tongues? Explain.
4. Using a Punnett Square, can you show if it is possible to have another and a father who
cannot roll their tongues, but have children that can roll their tongues? Explain.
5. Extra challenge: Create a Punnett Square that incorporates not one, but two traits
(tongue rolling AND earlobes). Hint: You will need a total of 16 cells in your Punnett
Square. Tell about the different kind of offspring that are possible.
Where are we going? To study the particular diseases that can be inherited and what those
patterns of inheritance are.
Note to self!
Dominant traits in humans (these are involved in simple dominance):
Hemophilia (sex linked though)
Dimples/no dimples
mid-digital hair/no hair
rolling the tongue/no rolling
widows peak
free earlobe/attached earlobes
Mendel’s story—Austrian Monk in the mid 1800’s who was a high school teacher. And he came upon a puzzling thing
himself—he liked to examine pea plants. He looked at several traits of pea plants—things like their height, their flower
color, the color of the seeds, the shape of the seeds, whether they had flowers on the ends of the stems or on the sides
of their stems. He was fascinated with how these plants inherited characteristics in the same way that animals inherit
their characteristics form their parents. He conducted an interesting experiment to see if he could figure out if there was
some pattern to this inheritance.
In co-dominance, both alleles are expressed; in incomplete dominance, one allele is incompletely expressed while the
other is expressed.
Resources:
Overheads of:
Chromosomes 46 and 23
Handouts of empty Punnett Squares
Tasks for the day
Helpful Website: http://fig.cox.miami.edu/~cmallery/150/mendel/heredity.htm