Download Punnett PP

Do Now 2.5
Objectives:
1. Define homozygous and heterozygous.
2. Use a Punnett square to predict the outcome of a
genetic cross for an autosomal dominant trait,
Task:
In Fnords having 3 eyes is dominant to having 2.
2. In a cross between a pure-breeding 3-eyed fnord
and a pure-breeding 2-eyed fnord, what % of the
offspring will have 3 eyes?
3. If two of the fnord babies are crossed, about what
% will have 2 eyes?

Phenotype is the characteristic an organism has
because of the alleles it has.
◦ Examples: brown fur, white flowers, dwarfism, blue eyes,
etc…

Genotype is the different alleles an organism has,
represented by letters
◦ Examples: Bb, pp, Dd, bb, etc…

What is the phenotype of the fnords in the do now?

What is it’s genotype?


The genotype of an organism can be
heterozygous or homozygous
Heterozygous individuals have two different
alleles for a gene – like the rat in the do now.
(Hetero = different)
◦ Example: Bb = one brown eye allele, one blue eye
allele

Homozygous individuals have two of the
same allele. (Homo = same)
◦ Example: BB, bb, PP, pp, etc…

Since a homozygous individual has two identical
alleles, their phenotype will be whatever those alleles
are
◦ Genotype = BB; Phenotype = brown eyes
◦ Genotype = bb; Phenotype = blue eyes


A heterozygous individual will have the dominant trait.
A dominant allele is one that masks or hides the
presence of a recessive one.
◦ Genotype = Bb; Phenotype = brown eyes
◦ Brown eye allele (B) is dominant to blue eye allele (b)

If a gene is autosomal dominant:
◦ The gene is located on an autosome, so all diploids
have 2 copies.
◦ One allele is dominant to the recessive version,
giving homozygous dominant and heterozygous
individuals the same phenotype
◦ Example: tongue rolling in humans
 TT = tongue roller
 Tt = tongue roller
 tt = no tongue roller

In cows, having spots (S) is dominant to no
spots (s)
◦
◦
◦
◦
◦
What is the phenotype of a cow that has spots?
What is the genotype of a heterozygous cow?
Will a heterozygous cow have spots?
What is the genotype of a cow with no spots?
If a cow has spots, what 2 genotypes might it have?

In pea plants, yellow seed color (Y) is
dominant to green seed color (y).
◦
◦
◦
◦
◦
What is the genotype of a plant with green seeds?
What is the phenotype of a Yy plant?
Is a YY plant homozygous or heterozygous?
Is a yy plant homozygous or heterozygous?
If a YY plant self-fertilizes, what color will the seeds
of its offspring be?

A diagram that helps predict the outcome of a
genetic cross.

The offspring produced by a genetic cross are
described in two ways:
◦ Phenotype ratio: the ratio of one phenotype to the other.
◦ Example: 2 black rats, 2 white rats = 1:1 phenotype ratio
◦ Genotype ratio: the ratio of all of the different genotypes
of the offspring
◦ Example: 1 BB, 2 Bb, 1 bb = 1:2:1 genotype ratio





Step 1: draw a 2 x 2 square
Step 2: put the possible alleles from parent
number one on top of the two squares
Step 3: put the possible alleles from parent
number 2 on the left
Step 4: Write the alleles at top down the
column, and the alleles at left across the row
Step 5: Check
Genetic Crosses
Yy
YY
Yy
Y y
yy
x
Yy
3:1
1:2:1



The example problems on your worksheet
will let you know if you’ve picked up the skills
you need to be a future geneticist!
Work on them now, so that I can help before
you go.
Add “Punnett square worksheet” to your
syllabus for homework.