Lab 9
Mendelian Genetics
Punnett Squares
MATERIALS
1. Punnett square worksheet
OBJECTIVES
1. Work and interpret a monohybrid cross.
2. Interpret a dihybrid cross.
PROCEDURES
Part I Introduction
Gregor Mendel, sometimes called the “father of genetics”, formulated the basic laws of genetics examined in this
laboratory. He determined that individuals have two alternate forms of a gene (two alleles, in modern terminology)
for each trait in their body cells. Today, we know that alleles are on the chromosomes. An individual can be
homozygous dominant (two dominant alleles, GG), homozygous recessive (two recessive alleles, gg), or
heterozygous (one dominant and one recessive, Gg). Genotype refers to an individual’s genes, while phenotype
refers to an individual’s appearance. Homozygous dominant and heterozygous individuals show the dominant
phenotype; homozygous recessive individuals show the recessive phenotype.
Use the key T=tall and t= short to complete the following:
1. What is the genotype for a homozygous tall plant?
2. What is the phenotype for Tt?
Part II One-Trait Crosses
1. Using the sets of parents described below, determine what gene combinations would be possible for
their children.
2. Make a key on your answer sheet of dominant and recessive traits. Use a different letter for each
combination. For example, do not use B for both brown eyes and brown hair.
3. Use a different Punnett square diagram for each situation.
4. Determine the probability of each possible genotype and phenotype of the offspring.
5. Remember: pure is an alternate term for homozygous and hybrid is an alternate term for heterozygous.
DOMINANT GENES
brown eyes
brown hair
can roll tongue
regular chin
free earlobes
RECESSIVE GENES
blue eyes
blond hair
can’t roll tongue
cleft chin
attached earlobes
FAMILY 1:
A mother with pure brown hair and a father with hybrid brown hair.
1.
Key:
Genotype:
Phenotype:
FAMILY 2:
A mother with pure blue eyes and a father with pure brown eyes.
2.
Key:
Genotype:
Phenotype:
FAMILY 3:
A hybrid mother who can roll her tongue and a father who can’t roll his tongue.
3.
Key:
Genotype:
Phenotype:
FAMILY 4:
A mother with blond hair and a father with hybrid brown hair.
4.
Key:
Genotype:
Phenotype:
FAMILY 5:
A mother with hybrid free earlobes and a father with hybrid free earlobes.
5.
Key:
Genotype:
Phenotype:
FAMILY 6:
A mother with a cleft chin and a father with a cleft chin.
6.
Key:
Genotype:
Phenotype:
FAMILY 7:
A mother with pure brown hair and a father with pure brown hair.
7.
Key:
Genotype:
Phenotype:
FAMILY 8:
A mother with pure brown eyes and a father with pure brown eyes.
8.
Key:
Genotype:
Phenotype:
In a certain species of animal, black fur is dominant over brown fur.
9. Both parents are heterozygous for black fur.
9.
Key:
Genotype:
Phenotype:
10. The mother is homozygous black and the father is homozygous brown.
10.
Key:
Genotype:
Phenotype:
11. The mother is heterozygous black and the father is homozygous brown.
11.
Key:
Genotype:
Phenotype:
In guinea pigs, black fur is dominant over white fur.
12. One parent is pure black and the other parent is pure white.
12.
Key:
Genotype:
Phenotype:
13. One parent is pure white and the other parent is hybrid black.
13.
Key:
Genotype:
Phenotype:
14. The female is a homozygous black guinea pig and the male is a heterozygous black guinea pig.
14.
Key:
Genotype:
Phenotype:
15. In the cross Bb X Bb, approximately what percentage of the offspring would have the same phenotype as the
parents? _____________________
16. In the cross Bb X Bb approximately what percentage of the offspring would have the same genotype as the
parents?______________________
Part III Two-Trait Crosses
1. Purple (P) is dominant over yellow (p) and that smooth (S) is dominant over rough (s).
PPSS
PpSS
PPSs
PpSs
PpSS
ppSS
PpSs
ppSs
PPSs
PpSs
PPss
Ppss
PpSs
ppSs
Ppss
ppss
Determine the probability of getting a
purple, smooth seed. ____________
2. In Drosophila, long wings (L) are dominant over short wings (l), and gray body (G) is dominant over
black body (g). Complete the results of the following cross and answer the question that follows:
LLGg
LLgg
LlGg
Llgg
How many of the offspring will
have long wings and a black body?
LLGg
LLgg
LlGg
Llgg
_________________________
LlGg
Llgg
llGg
llgg
LlGg
Llgg
llGg
llgg