Key Question: How can you predict the probability of traits
appearing? Initial Thoughts: 4 minutes
Slide 4
Step by Step 1. Write the possible alleles donated by each
parent along the top and left side of the table? TT tt *Each T
along the top represents an allele in the sperm cell produced by
Skye. Each t on the left represents an allele in the egg cell from
Poppy. T T t t Skye Poppy
Slide 5
Step by Step 2. Complete each box of the table by combining one
allele from the top and one allele from the left. TT tt * When you
combine one allele from each parent into a box, you are
representing a sperm cell fertilizing an egg. T T t t Skye
Poppy
Slide 6
Step by Step 3. Now you can use the Punnett square to make some
conclusions. *All of the offspring of Skye and Poppy will have one
allele for blue tail color and one allele for orange tail color.
Their genotypes will be Tt. An organism that has alleles for two
different traits is called heterozygous. Because blue tail color is
dominant over orange, the phenotype of all offspring is blue tails,
as found in the breeding experiment between Skye and Poppy.
Slide 7
The cross between the Generation 2 (Tt) critters Ocean and Lucy
is: Ocean X Lucy Tt X Tt T = allele for blue tail color (dominant)
t = allele for orange tail color (recessive) *Note that while Ocean
and Lucy both have blue tails, they are both heterozygous.
Slide 8
1.Complete this Punnett square for the cross between Ocean and
Lucy? Tt a. Place Oceans and Lucys alleles on the dotted lined in
the Punnett square. b. Complete the Punnett square by filling in
each box with the allele above it and the allele to its left. -----
Ocean Lucy
Slide 9
c.Use either a blue pencil or a regular pencil to shade in the
squares for offspring that will have blue tails in your Punnett
square. d.About what fraction of the offspring of Ocean and Lucy
are predicted to have blue tails, according to the Punnett square?
e.About what fraction are predicted to have orange tails?
Slide 10
Paste the punnett square practice problems into your
notebook.
Slide 11
1.Why is it impossible for offspring to show the recessive
trait if one parent is homozygous for the dominant trait? 2.What is
the probability of having a boy (xy) vs. a girl (xx)? x y x x Mom
Dad
Slide 12
3.A scientist has some purple-flowered pea plants. She wants to
find out if the pea plants are homozygous dominant for the purple
flower color. White is recessive. Use P = purple p = white. a)What
cross will be best to find out if the purple- flowered peas are
homozygous? b)Use Punnett squares to show what will happen if the
plants are crossed with white-flowered plants and i.the
purple-flowered plants do not have an allele for the white trait.
ii.the purple-flowered plants do have an allele for the white
trait.
Slide 13
What did you think about predicting the probability of traits
before? What did you learn from your evidence about predicting the
probability of traits? What are some further thoughts or questions
you have about predicting the probability of traits?
Slide 14
How can you tell if a flower is homozygous dominant? What
experiment can you do?
Slide 15
Punnett squares can be used to predict the probabilities of
certain dominant and recessive traits.