Biology TEACHER
EQ: Do I carry deadly genes?
Enduring Understanding
Nucleic acids transfer genetic information from generation to
generation.
Broad Brush Knowledge
Mendelian genetics, probability
Dihybrid Genetics
Problems
Targeted Skills
analysis, probability
Target
3. Apply Punnett squares to determine the probability of two
factor crosses
Concepts Important to Know and Understand
Heredity
Core Objectives
7. Interpret the role of genetics in determining heredity as it applies to
biotechnology.
TEKS
(6D) compare genetic variations observed in plants and animals
TEACHER MANAGEMENT
Estimated Time: 30 – 40 minutes (option - may start in class and finish for homework)
Materials: handouts
Teacher Prep: make copies of handouts
A dihybrid cross involves inheritance patterns for organisms differing in two traits. The dihybrid cross was
invented by Mendel to determine if different traits of pea plants were inherited independently or as a unit.
The discovery that round peas could be yellow or green led to Mendel’s Law of Independent Assortment
which states that the inheritance of one trait does not influence the inheritance of a second trait. The
genotype alleles are shown by using two sets of letters, one set to represent each trait being studied. Each
parent’s gametes must have two of each letter representing the traits, four letters in all.
Parent:
AaBb
AaBb
AaBb
AaBb
Gametes:
AB
Ab
aB
aa
A a B b
AB Ab
aB ab
KEY
PRACTICE:
1. a. List the possible gametes from Parent A with DDMM. DM, DM, DM, DM (all DM)
b. List the possible gametes from Parent B with DdMM. DM, DM, dM, dM
c. List the possible gametes from Parent C with Ddmm. Dm, Dm, dm, dm

If Parent A mates with Parent B, list the possible offspring genotypes. DDMM, DdMM

If Parent B mates with Parent C, list the possible offspring. DDMm, DdMm, ddMm
2. In Cyclops, one eye (E) is dominant to two eyes (e) and curly hair (H) is dominant to straight hair (h).
a. Write the cross between a homozygous one-eyed, curly-haired Cyclops and a homozygous
two-eyed, straight-haired Cyclops. EEHH vs. eehh
b. List the possible gametes from the homozygous one-eyed, curly-haired Cyclops. EH
Dihybrid Genetics Problems- Biology - TEACHER (Revised June 26, 2008)
(printed 3/9/2016) p. 1
c. List the possible gametes from the homozygous two-eyed, straight-haired Cyclops. eh
d. Write the genotype of all individual produced in the above cross. EeHh
e. Write the phenotype of all individuals produced in the above cross. One-eyed, Curlyhaired
3. In humans, two abnormal conditions, cataracts of the eyes and excessive fragility of the bones are
carried by dominant genes. Below is the Punnett square showing their possible offspring.
a. What are the genotypes of each parent? CCnn and ccNn
b. What are the phenotypes of each parent?
Cataracts and normal bones
No cataracts and normal bones
Cn
Cn
Cn
Cn
cN
CcNn
CcNn
CcNn
CcNn
cn
cN
Ccnn
Ccnn
Ccnn
Ccnn
CcNn
CcNn
CcNn
CcNn
cn
Ccnn
Ccnn
Ccnn
Ccnn
4. In werewolves, sharp fangs are dominant (F) and round fangs are recessive (f). Long hair is
dominant (H) and short hair is recessive (h). Cross a homozygous sharp fanged, heterozygous
longhaired werewolf with a round fanged, shorthaired werewolf.
FH
a. Phenotypic ratio: 8:8:0:0
FH
Fh
Fh
fh
FfHh
FfHh
Ffhh
Ffhh
fh
FfHh
FfHh
Ffhh
Ffhh
fh
FfHh
FfHh
Ffhh
Ffhh
fh
FfHh
FfHh
Ffhh
Ffhh
b. What is the probability of having a werewolf born with sharp fangs and long hair?
8/16 = ½ = .50 = 50%
c. What is the probability of having a werewolf born with sharp fangs and short hair?
8/16 = ½ = .50 = 50%
5. In beauty queens, long legs (L) are dominant to short legs (l) and high cheekbones (B) are dominant
to average cheek bones (b). A man with short legs and average cheekbones marries a woman who
is heterozygous for both traits.
lb
lb
lb
lb
a. Phenotypic ratio: 4:4:4:4
b. Based on known beauty queen
characteristics, what is the probability
of this couple having a future
LB
LlBb
LlBb
LlBb
LlBb
Lb
Llbb
Llbb
Llbb
Llbb
lB
llBb
llBb
llBb
llBb
llbb
llbb
llbb
llbb
lb
Dihybrid Genetics Problems- Biology - TEACHER (Revised June 26, 2008)
(printed 3/9/2016) p. 2
beauty queen?
4/16 = 1/4 = .25 = 25%
6. Pea plants can produce round (R) or wrinkled (r) peas as well as yellow (Y) or green (y) pea pods.
A pea plant with the genotype RrYY is crossed with a pea plant with the genotype rrYy.
a. How many different genotypes can be expressed in the offspring? List them below.
Gametes A: RY and rY
(1) RrYY
(2) RrYy
(3) rrYY
(4) rrYy
Gametes B: rY and ry
b. How many different phenotypes can be expressed in the offspring? List them below.
(1) Round and yellow
(2) Wrinkled and yellow
Dihybrid Genetics Problems- Biology - TEACHER (Revised June 26, 2008)
(printed 3/9/2016) p. 3