Download DD - Montville.net

5 Steps to Solving
Monohybrid Crosses
First, analyze the
problem and
highlight important
information
PROBLEM: Dimples are dominant
over no dimples. A father is
homozygous dominant for dimples and
a mother is homozygous recessive for
dimples. What are the chances that
their child will have dimples?
Step 1: Choose a letter to
represent a trait.
Dimples = D
No Dimples = d
Step 2: Write down the genotype
of the parents. Remember that
each parent has 2 alleles.
Mom
Dad
d d
DD
Step 3: Write the genetic cross.
dd
x
DD
Step 4: Set up the Punnett Square.
A Punnett Square is a chart scientists use to
show all the possible combinations of alleles
that can result from a genetic cross.
Remember, the Punnett Square shows all the
possible outcomes that any 1 child may
inherit.
Then next time mom and dad have another
child it is a new roll of the genetic dice.
d
d
D
Dd
Dd
D
Dd
Dd
Step 5: Determine the phenotypes
and genotypes of the offspring.
Remember to label the ratios.
0
0
Genotype Ratio 0 DD : 4 Dd :
dd
express in % 0 % DD : 100 % Dd : % dd
Phenotype Ratio: 4 Dimples : 0 No dimples
express in % 100% Dimples : 0 % No dimples
Independent Practice
PROBLEM: In humans, freckles are
dominant over no freckles. A man
heterozygous for freckles reproduces with a
woman also heterozygous for freckles. What
chance does their child have for no freckles?
**Show your work for Steps 1-5 below:
1. freckles = F
no freckles = f
2. Mom = Ff
Dad = Ff
3. Ff x Ff
F
f
4.
F FF Ff
f Ff
ff
5. Genotypic Ratio
Genotypic Percent
Phenotypic Ratio
Phenotypic Percent
1 FF : 2 Ff : 1 ff
25%FF : 50% Ff : 25% ff
3 freckles : 1 no freckles
75% freckles: 25% no freckles