NAME___________________________
DATE___________________
HOUR___________________
BIOLOGY I – THE GENETICS OF THE LABRADOR RETRIEVER COAT COLOR
HEREDITY – 5.1
INTRODUCTION – Labrador Retrievers are one of the most popular dog breeds in the world. They are loyal,
smart, and eager to please. Most of all it is their temperament in a family setting that makes them such great pets.
Labs come in three colors. They are Black labs, chocolate labs, and yellow labs. The coat colors are controlled by
two genes. We will track the inheritance of Labrador coat color with a dihybrid cross.
GENETIC DETAILS – The first gene at work controls the dark color of the coat. It works through simple
dominance. Black coats (BB) are dominant to chocolate coats (bb). A cross of this nature [BB x bb.] would produce
the following offspring with all black coats.
b
b
B
Bb
Bb
B
Bb
Bb
All of the expected results from a monohybrid with simple dominance would be seen.
However this is not the only gene at work. There is a gene that turns the expression of coat color on and off. This is
called an EPISTATIC gene. This gene is symbolized with the letter E and it is dominant. There are three
possibilities, EE, Ee, and ee. The first two (EE) and (Ee) turn on the coat color genes that control if the dog is black
or chocolate. If the dog has the (E_ ) genotype, then its coat color will show up. The third genotype (ee) then the coat
color will be turned off! The dog that has the genotype (ee) will be a yellow lab no matter what genes it has for black
or chocolate…
BbEe X BbEe
BE
Be
bE
be
`
BE
BBEE
BBEe
BbEE
BbEe
THE RESULTS:
Be
BBEe
BBee
BbEe
Bbee
bE
BbEE
BbEe
bbEE
bbEe
be
BbEe
Bbee
bbEe
bbee
9 BLACK LABS
4 YELLOW LABS
3 CHOCOLATE LABS
1. How does this outcome differ from the ratio of offspring we would expect in a regular dihybrid with four different
phenotypes?
2. Practice writing the genotypes for these dogs:
A Homozygous Black lab Heterozygous for the epistatic gene: ____________________
A Chocolate lab Homozygous for the epistatic gene: ____________________
A Yellow lab: ____________________
3. Cross these two dogs: BBEE X bbee
What is the genotypic ratio _________________
What is the phenotypic ratio__________________
4. Cross a Heterozygous black lab homozygous for the epistatic gene with a Chocolate lab that is heterozygous for
the epistatic gene.
Could this litter of pups produce any Chocolate labs? _______If so, what would their genotype be?____________
What is the probability of any pup from this litter being Chocolate? ______________ (expressed as a fraction)
Could this litter of pups produce any Yellow labs? _______If so, what would their genotype be?____________
What is the probability of any pup from this litter being Yellow? ______________ (expressed as a fraction)
In a litter of 8 pups, how many chocolate pups can be expected, if any?_________________________________