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Fruit Fly Dihybrid Cross Worksheet
Name: ___________________
Drosophila melanogaster
Three organisms are legends in genetics, and much of our modern knowledge can
be traced back to research upon them. Mendel’s garden peas, the genus Pisum, first
illuminated the mathematical predictability of inheritance. The bacterium Escherichia
coli, the very same E. coli that can cause food poisoning, was instrumental in the
development of biotechnology. And the humble fruit fly, Drosophila melanogaster, not
only revealed the complicated truth of Mendel’s inheritance patterns, but it also set the
stage for Watson, Crick, Franklin, and Wilkins’ discovery of the structure of DNA.
The scientist who is most famous for the study of fruit flies is Thomas Hunt
Morgan. An American, his famous Fly Room at Columbia University was the first
biological study on such a massive scale, eventually involving dozens of labs and more
than a hundred scientists. Drosophila became what is called a “model organism,” an
organism that is extensively studied with the understanding that knowledge about how it
works is also knowledge about how nearly everything else works, too. What Morgan
learned about how Drosophila passes on its traits is also true of all other animals.
Scientific breakthroughs
Before Morgan and his fruit flies, very little was really known about inheritance.
In his 1859 publication Origin of Species, Darwin (and everyone else at the time) had
known nothing about the existence of DNA or genes, but if the theory of evolution was to
be correct, it predicted that one day an actual physical factor of inheritance would be
discovered. Furthermore, evolution predicted that physical factor would be something
that underwent infrequent random changes. Mendel’s work, surfacing in 1900, showed
that inheritance could be quantified as Darwin had predicted, but it was still unknown
exactly how inheritance worked.
Morgan began working with fruit flies in 1908. He chose fruit flies because they
were inexpensive, had many offspring quickly, and had many obvious traits to study. He
made discoveries that would become essential to biology, and that validated both Mendel
and Darwin’s work from many decades before:
1. He demonstrated the existence of mutations.
2. He showed that genes are linked together in a linear fashion.
3. He discovered crossing-over.
4. He confirmed that chromosomes are related to inheritance.
5. He demonstrated that it’s possible to create a map of an organism’s genes
through controlled breeding experiments.
Although Morgan didn’t know that a molecule called DNA was involved, or that
chromosomes involved nitrogenous base pairs, he was able to make remarkably accurate
discoveries just by breeding these “simple” flies. His findings, along with the WatsonCrick discovery 50 years later, fit Mendel and Darwin’s models perfectly.
After the discovery of DNA, scientists were able to match the Morgan lab gene
maps with a complete genome map of Drosophila’s DNA. Morgan’s discoveries from
Fruit Fly Dihybrid Cross Worksheet
Name: ___________________
1908 held up to the scrutiny of modern technology, and Drosophila continues to be an
essential organism in the study of genetics.
How It Worked
How did breeding reveal all of this? Consider what happened to Morgan in 1910.
The mutation that causes white eyes is recessive, while the red “wild-type,” or natural,
eye color is dominant. Morgan bred some white-eyed males (ww) with red-eyed
homozygous (w+w+) females. All the offspring had red eyes, no surprise there. Half the
offspring were males, and half were females, also no surprise. Make two Punnett
Squares, one for sex and one for eye color, showing why that makes sense (Morgan used
w for the white-eyed allele and w+ for the dominant red-eyed allele. Females are XX,
males are XY.):
He then bred those offspring together. One would expect two heterozygous flies
to have offspring that are 75% red-eyed, and 25% white-eyed. Make a Punnett Square
showing why he expected that 3:1 ratio…
And he did get a 3:1 ratio, and the offspring were still 50%/50% male and female,
but he noticed something interesting. All of the white-eyed offspring were males! Out of
more than 2,000 flies, not a single female had white eyes. Of the males, half had red
eyes, and half had white. Morgan tried several more experiments, crossing different
genotypes and combinations of flies. He could never get any white-eyed females, no
matter what he did, and the offspring continued to always be split 50/50 between the two
sexes.
From these results, Morgan determined that the allele for biological sex and the
allele for eye color were somehow linked. He found other alleles as well that seemed to
be linked: the parents genotypes in one gene would affect their offsprings’ genotypes for
another gene entirely. This led Morgan to the understanding that a chromosome is a line
of genes.
However, for genes that are on different chromosomes, they still follow Mendel’s
laws of segregation and independent assortment. For these unlinked genes, it’s possible
to make Punnett Squares showing offspring genotypes for two or more genes at the same
time. Just as the Punnett Square showing offspring for one kind of gene was called a
monohybrid cross, a Punnett Square showing offspring for two kinds of genes at the same
time (like eye color and hitchhiker’s thumb, for instance) is called a dihybrid cross.
Fruit Fly Dihybrid Cross Worksheet
Name: ___________________
Drosophila alleles
Assume that the following Drosophila alleles are unlinked and can be used to
make dihybrid crosses. (“Wild-type” means normal.)
Wild-type smooth (H) vs hairy body (h)
Lobed eyes (L) vs wild-type unlobed (l)
Notched wings (N) vs wild-type unnotched (n)
Wild-type brown (S) vs sable body (s)
Wild-type red (W+) vs white eyes (w)
Curved (C) vs wild-type straight wings (c)
Minute bristles (M) vs wild-type thick bristles (m)
Example: A fly that is heterozygous wild-type with respect to both hairiness and body color (HhSs) is
crossed with a fly that hair and sable-bodied (hhss).
HS
Hs
hS
hs
hs
HhSs
Hhss
hhSs
hhss
hs
HhSs
Hhss
hhSs
hhss
hs
HhSs
Hhss
hhSs
hhss
hs
HhSs
Hhss
hhSs
hhss
Genotype Probabilities:
25% HhSs, 25% Hhss, 25% hhSs, 25% hhss
Phenotype Probabilities: 25% smooth+brown, 25% smooth+sable, 25% hairy+brown, 25% hairy+sable
1. Cross a fly that is LLCC with a fly that is llcc.
Genotype Probabilities:
Phenotype Probabilities:
Fruit Fly Dihybrid Cross Worksheet
Name: ___________________
2. Cross a fly that is SsMM with a fly that is ssmm.
Genotype Probabilities:
Phenotype Probabilities:
3. Cross two flies that are both heterozygous for notched wings and hairiness.
Genotype Probabilities:
Phenotype Probabilities:
4. Cross a white-eyed, sable-bodied fly with a fully heterozygous fly (W+wSs).
Genotype Probabilities:
Phenotype Probabilities:
Fruit Fly Dihybrid Cross Worksheet
Name: ___________________
5. What genotypes must they have for a white-eyed, sable-bodied fly to have any whiteeyed offspring if it mates with a red-eyed, brown-bodied fly? Show a dihybrid cross that
would permit that outcome.
6. Two flies with curved wings and minute bristles mate. What genotypes must they
have in order to produce offspring with normal wings and normal bristles? Show a
dihybrid cross that would permit that outcome.
7. Can a brown-bodied, minute-bristled fly have a sable-bodied, thick-bristled offspring?
Create a dihybrid cross that illustrates what genotype its mate could have.