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Biology
Genetics #4: Mr. Mason is evil…will this ever end??? (or Epistasis, if you prefer)
2.1.11
1. Comb shape in chickens is controlled by two genes and is a classic example of epistasis.
Remember epistasis? Epistasis is when the action of one gene controls the expression of
another. Hair color in humans was the example given in class earlier today: any dominant
allele of black/brown color (B_) would override the gene for red/blonde (R or r).
Remember, blondes and redheads only exist when the individual is homozygous recessive
for the black/brown gene. In addition, epistatatic genes can be either dominant (B_, like
human hair color) or recessive (aa, like albinism). As for the chickens, their combs are
either walnut, pea, rose, or single. When crossing purebreds, the following ratios are
always observed:
single x single = all single
walnut x walnut = all walnut
rose x pea = all walnut
When we take the walnut birds from the above pea x rose cross we get the following ratios:
9/16 walnut
3/16 rose
3/16 pea
1/16 single
In addition, sometimes a non-purebred pea x rose cross will result in the following ratios:
1/4 walnut
1/4 rose
1/4 pea
1/4 single
What are the genotypes responsible for
the four comb shapes in chickens? Show punnetts!
2. Now that you’ve solved that, here’s another epistasis problem involving coat color in mice.
Agouti (gray) is dominant (G_), and black is recessive (gg). A purebred agouti was crossed
with a pure white (albino) mouse of black parents. All of the offspring were agouti. Two of
these agouti offspring were crossed, and the following ratios were obtained:
9/16 agouti
4/16 albino
3/16 black
What are the genotypes responsible for the three coat colors in mice? Show punnetts!
3. In carrots, flower color can be yellow, green, or blue. The carrot itself may be long or
short. As for flower color, the following crosses and ratios were always obtained:
yellow x yellow = yellow
blue x green = 1/2 blue and 1/2 green
yellow x blue = green
As for shape, the following crosses and ratios were some of the potential results:
long x short = 1/2 long 1/2 short
long x short = all long
long x long = all long
short x short = all short
What would you have to cross with a short carrot with green flowers to get the following
results?
103 yellow long, 101 green short, 99 yellow short, and 100 green long
Give phenotype, genotype, and show all punnetts, please.
4. In rabbits, a series of multiple alleles controls coat color in the following way: C is dominant
over all other alleles and causes normal color, while the chinchilla phenotype (light
highlights) is created by cch and is dominant over all alleles except C. The ch allele,
dominant to only ca (albino), creates the Himalayan color pattern (black ears, white body).
Thus, the order of dominance is: C>cch>ch>ca. The following crosses were obtained:
a. Himilayan x Himilayan = albino and full color x albino = chinchilla.
b. albino x chinchilla = albino and full color x albino = full color
c. chinchilla x albino = Himilayan and full color x albino = Himilayan
For each of the above scenarios (a,b,c), determine genotypes of all rabbits involved as well
as the results (genotype and phenotype) of a cross between the offspring from each pair of
crosses (e.g. albino x chinchilla in scenario “a”).