BIOLOGY 2/2A
Lab: Selective Breeding and Dihybridization
Introduction: For the last 20,000 years, human civilization has benefited greatly from
the application of selective breeding/artificial selection principles to agriculture,
silviculture and animal husbandry. In artificial selection, various desired phenotypes are
obtained by choosing which animals/plants should be mated such that the combination of
their genetic information will produce progeny with said phenotype. However, the
differences that exist between phenotype and genotype can make this a challenging
pursuit.
In this simulation, you will represent a farmer/animal breeder with the goal of
producing a line of plants/animals that will be pure-breeding for a specific
combination of traits. To obtain this pure-breeding line, you will need to make selective
crosses of parental stock and progeny/offspring to obtain a population that only produces
the phenotypes in question. Use dihybrid Punnett squares to assist you in this process.
The Scenario:
1. You will be asked to produce one of the three following combinations of phenotypes.
The alleles for each combination of traits are listed below.
Species
Traits
Appaloosa
Horses
Balinese Pigs
Vermeer
Tulips
Mane Color
Dominant
Allele
Black (B)
Recessive
Allele
White (b)
Desired
Phenotypes
Black
Coat Pattern
Bristle Length
Coat Color
Coronal Pattern
Spotted (S)
Long (L)
Copper (C)
One color (O)
No spots (s)
Short (l)
Dark Brown (c)
Multicolor (o)
Spotted
Long
Dark Brown
Multicolor
Petal width
Narrow (N)
Wide (n)
Wide
In other words, if you are a horse breeder, you want to produce a herd of horses that has a
black mane and spotted coats and will always produce spotted foals with black manes.
If you are a pig breeder, you want dark brown pigs with long bristles.
If you are breeding tulips, you want tulips with wide, multicolored petals.
2.
On your desk you should have a card with the genotype of your original parent
and its sex. Write the phenotype of this parent on the other side of the card.
YOU MAY SHOW ONLY THE PHENOTYPE OF YOUR PARENT (NOT
ITS GENOTYPE) TO PROSPECTIVE MATES.
3.
Circulate around the room looking for prospective mates. Obviously, they need
to be the same species and the opposite sex. You may only see their phenotype so
choose your mates carefully. You may not be able to produce offspring with both
desired traits in the first cross, but you may be able to obtain the proper alleles, so
consider carefully which phenotypes may carry which alleles.
4.
When you have found a suitable mate for the first cross, sit down and reveal
your genotype to your mate. Perform a dihybrid cross for these traits using the
Punnett Square below. When you have combined all of the possible gamete
combinations, analyze the resulting progeny by listing a phenotypic ratio for this
F1 generation.
Your Phenotype: _______ & ________
Mate‟s Phenotype: _________ & _______
Your Genotype: ________
Mate‟s Genotype: __________
Your Gametes: ____, ____, ____, _____ (place on top of columns in Punnett square)
Mate‟s Gametes: ____, ____, ____, _____ (place on sides of rows in Punnett square)
Phenotypic Ratio (dom/dom, dom/rec, rec/dom, rec/rec):
______:_____:_____:______
5.
When you have finished with your first cross, select one of your offspring from
the F1 generation and write its genotype on a blank slip of paper. Write the
phenotype on the opposite side. Assume the sex of this chosen offspring is the
same as the original parent. As before, you may only show this individual’s
phenotype to prospective mates.
6. Recirculate and find a new mate. Remember that ultimately you want to
produce sixteen progeny that ALL have the same phenotype and that will always
have the same phenotype in successive generations. Once you have found a suitable
mate, perform another cross and analyze the offspring as before for this F2
Generation
Your Phenotype: _______ & ________
Mate‟s Phenotype: _________ & _______
Your Genotype: ________
Mate‟s Genotype: __________
Your Gametes: ____, ____, ____, ___ (place on top of columns in Punnett square)
Mate‟s Gametes: ___, ____, ____, _____ (place on sides of rows in Punnett square)
Phenotypic Ratio (dom/dom, dom/rec, rec/dom, rec/rec):
______:_____:_____:______
6.
Were ALL sixteen of your offspring phenotypically and genotypically identical?
Did every individual display BOTH of the desired traits? If your answer to both
questions is true, proceed to the analysis questions section of this lab and have
your Punnett squares checked by your instructor. If you cannot answer “yes” to
BOTH of these questions, you need to continue with step 7.
7.
Repeat this process by selecting another offspring from your F2 generation
(above), making a phenotype/genotype label with your second blank and
assigning the proper sex. Find a suitable mate and perform the F3 cross.
Your Phenotype: _______ & ________
Mate‟s Phenotype: _________ & _______
Your Genotype: ________
Mate‟s Genotype: __________
Your Gametes: ____, ____, ____, ___ (place on top of columns in Punnett square)
Mate‟s Gametes: ____, ____, ____, _____ (place on sides of rows in Punnett square)
Phenotypic Ratio (dom/dom, dom/rec, rec/dom, rec/rec):
_____:_____:_____:______
8. Were all of your offspring identical (phenotypically and genotypically)? Did
every individual display BOTH of the desired traits? If both are true, proceed to the
analysis questions section of this lab and have your Punnett square checked by your
instructor. If you cannot answer “yes” to BOTH of these questions at this point, you
need to continue with your artificial selection.
If you cannot finish during the course of this period, select offspring from your
F3 and continue the dihybridization process until you reach the point where all
sixteen have the correct phenotypic combination and are all homozygous for
these traits. Attach your additional work to this lab.
ANALYSIS QUESTIONS:
1.
Why did it take most breeders more than one cross to produce a “pure
breeding” group with both of the desired traits? Use the proper genotypic
terminology in your answer.
2.
Read the section in Chapter 11 about incomplete dominance. Do you think
this process of producing a pure-breeding group would have been easier or
more difficult if both of the traits had been incompletely dominant? Why? In
your answer, explain what incomplete dominance is and how it would have
affected this process.
3.
What do the results of this lab suggest about the concept of “ethnic cleansing”
(i.e. killing or sterilizing individuals showing a particular trait) to remove
various human phenotypic traits associated with certain ethnic groups? Why is
such a practice doomed to fail?
4.
Look at the definition of artificial selection in the introduction and then look up
the term “natural selection” in your text. How do you suppose the process of
natural selection differs from the artificial selection modeled in this lab?
Which process most likely takes a longer time? Why?
5.
Suppose one population of Appaloosa were isolated on a barrier island of the
coast of North Carolina such that they could not naturally breed with other
horse populations. How might you expect their populations to change
differently than populations that are not isolated?
6.
Why does the establishment of “pure breeding lines” not guarantee that the
offspring will show the same traits as the parental generation?
7.
All physiological differences aside, why can‟t Appaloosa horses and Balinese
Pigs produce viable offspring (i.e. offspring that can survive and reproduce)?
Explain in terms of the chromosomal content of each species.
8.
Crowned Lemurs, Black Rats and Grevy‟s Zebras all have 46 chromosomes?
Once again, all physiological differences and ethical concerns aside, why can‟t
humans be „hybridized‟ with these species? Explain in terms of homologous
chromosomes.