Do Now: Anatomy of Punnett
Squares
1. What do the letters
across the top and
left side of the
Punnett square
represent?
2. What do the
combinations of
alleles inside the 4
boxes represent?
Polygenic Inheritance
Polygenic traits are traits that are caused
by more than one gene. For example, in
parakeets, there are two genes for
feather color, resulting in four different
possible feather colors (there are also
more colors possible, but they are caused
by different genes)
There is one gene that codes for a yellow
pigment (C for the dominant yellow allele
and c for the recessive white allele).
There is also a second gene that codes for
a blue pigment (B for the dominant blue
gene, and b for the recessive white gene).
In these birds, having both blue and
yellow pigments gives the feathers a
green color.
Dihybrid Cross
A dihybrid cross is a
4x4 Punnett square
that can be used to
calculate the
probability of offspring
with traits causes by
more than one gene
(polygenic
inheritance) OR we
can use them to look
at two totally different
traits.
Dihybrid Cross Looking at One Trait
Caused by Two Genes
Feather color in
parakeets is
polygenic. There is
one gene that
produces a yellow
color, and one that
produces a blue
color. Different
combinations of
alleles lead to
different feather
colors.
BB or Bb = blue
bb = no color
(white)
CC or Cc = yellow
cc = no color
(white)
Dihybrid Cross Looking at Two Traits
In this dihybrid cross, we are
studying two different traits in
peas (seed smoothness and
seed color). We can use the
dihybrid cross to calculate the
probability of offspring
receiving different
combinations of the traits.
For the wrinkled/smooth trait, R
is dominant for smooth seeds,
and r is recessive for wrinkled.
Y is dominant for yellow seeds,
and y is recessive for green
seeds.
Let’s make a dihybrid cross together
In this dihybrid cross, we will look at two
different genes that code for two
different traits: flower color and height
in pea plants. For height, use T for the
dominant allele (tall) and t for recessive
allele (short). For flower color, use P for
the dominant allele (purple) and p for
the recessive allele (white). We will cross
two plants that are heterozygous for
both traits.
First, lets figure out what the genotypes
of the parents are:
Next, let’s figure out what the possible
gametes are for each parent:
Now, complete the cross!
What are the ratios of genotypes?
What are the ratios of phenotypes?
Now, get out the Meiosis Lab you
started on Friday
At the start of the timer, you will have two
minutes go to your notebook and get out the
Meiosis lab so we can go over the pre-lab
questions 1 and 2. You must have these
questions answered before the lab tomorrow.
Hold on to this for now.
Using a Microscope
We will watch a short video on how to use a
microscope. This will be a review for most of
you. Before we watch it, let’s read the questions
together (they are in your handout).
Using a Microscope
Now, let’s go over the answers.
Microscope Handouts
You have a microscope handout in your notes
with the parts of the microscope on one side
and blanks for you to fill in on the other side.
Right now, label all the parts of the microscope
on the side with blanks. You may work with a
partner, but you may not change seats.
Quiz Corrections
If there is time in class after you finish the
microscope handout, you may start quiz
corrections.
Your homework is to finish the quiz corrections.
You may not re-take the quiz unless you have
completed the corrections.