Download Chapter 10

Write everything that is
underlined
A. Dominant vs. Recessive
Dominant traits: the ones you can
see
Recessive traits: are hidden by
dominant
Rule of Dominance: When a
dominant trait is
present the recessive
trait is hidden
Phenotype:
The way an organism looks.
No matter what genes are
present, phenotype of a tall
pea plant is tall and a short
pea plant is short
Genotype:
The gene combination of an
organism
It consists of 2 alleles
 For example:
 Pure dominant, 2 dominant genes
 Pure recessive, 2 recessive genes
 Hybrid, 1 dominant and
gene
1 recessive
Homozygous:
When the 2 alleles are the
same
2 dominant genes =
Homozygous dominant
2 recessive genes = Homozygous
recessive
Heterozygous:
When the 2 alleles for
a trait are different
It’s a hybrid
1 dominant gene and 1
recessive gene
Punnett Squares
Punnett squares allow you to
predict the ratios or percentages
of offspring possible in a cross.
Punnett SquareLettering
 Dominant Genes
 Always get capital letters.
 The letters will be the 1st letter of the trait.
 Recessive Genes
 Always get lower case letters.
 The letter used is the same as the one used for the
dominant gene.
Punnett Squares
All problems will consist of 5 steps:
1. Labeling dominant and recessive letters
2. The Cross- Who are the parents
3. The Square- draw and fill in the square
4. Genotype ratios or percentages
5. Phenotype ratios or percentages
Example Problem:
Red flowers are dominant over purple
flowers in a certain type of plant. What
will the possible genotypes and
phenotypes of the offspring of a hybrid
red flower and a purple flower?
There are five steps to this problem
Step 1:
Label the dominant
and recessive traits
R = Red flower
r = Purple flower
Step 2:
Write the genotypes of each
parent to be crossed
Purple
Hybrid Red
(means heterozygous)
Rr
(since it is recessive it
has to be homozygous)
Rr x rr
rr
Step 3:
Set up the punnett square.
Each parent will give up one gene
or the other so there will be one
gene per box for each parent
R
r Rr
r
rr
r Rr
rr
Step 4:
Analyze the genotypes
R
r Rr
r Rr
r
rr
rr
2 of the 4 (50%)
offspring will be
pure recessive
2 of the 4 (50%)
will be hybrids
Step 5:
Analyze the phenotypes
R
r Rr
r
rr
r Rr
rr
50% of the
offspring
will be red
50% of the
offspring
will be purple
Now try one on your own
Tall plants are dominant over short
plants in a tomato plant. What will
the possible genotypes and
phenotypes of the offspring of a
Homozygous Tall plant and a
Homozygous short plant?
Step 1:
Label the dominant
and recessive traits
T = Tall plant
t = short plant
Remember, it’s NOT T and S
Step 2:
Write the genotypes of each
parent to be crossed
Homozygous Tall
(homo means same)
TT
Homozygous Short
(homo means same)
TT x tt
tt
Step 3:
Set up the punnett square.
Write the parents first
T
t Tt
T
Tt
Tt
Tt
t
Step 4:
Analyze the genotypes
T
t Tt
T
Tt
Tt
Tt
t
4 of the 4 (100%)
offspring will be
heterozygous (or
hybrid) for plant
height
Step 5:
Analyze the phenotypes
T
t Tt
T
Tt
Tt
Tt
t
100% of the
offspring
will be Tall
because having at
least one T makes
it tall because T is
dominant over
short (t)