Genetics Problems
Show all of your work when answering the following questions. Use Punnett squares whenever
possible.
1. In humans, the allele A for pigment formation is dominant to the allele a for inability to form pigment;
aa individuals are albino. Determine the expected genotype and phenotype ratios expected from a
cross between two individuals heterozygous for this trait.
2. In humans, tasting, T, is dominant to non-tasting, t. Determine the expected genotypic and
phenotypic ratio resulting from a cross between:
a)
a homozygous taster and a non-taster
b)
two heterozygous tasters
c)
a heterozygous taster and a homozygous taster
3. In mice, G for grey coat is dominant to g. The gg individuals are black. Two grey mice are mated to
produce nine grey mice and two black mice. Explain these results.
4. In pea plants, tall (T) is dominant to dwarf (t) and smooth seeds (S) is dominant to wrinkled
seeds(s). Use a Punnet square to determine what fraction of the offspring produced in each of the
following crosses you would expect to be tall with wrinkled seeds.
a)
TtSs x TtSs (this is a dihybrid cross, use your ratios!)
b)
TtSs x ttss
c)
TtSs x TTSs
d)
TtSs x Ttss
e)
TtSs x ttSs
f)
TTss x ttSS
5. In mice, grey coat colour, G, is dominant to white coat, g, and long tail, T, is dominant to short, t.
What fraction of F1 mice resulting from each of the following crosses would you expect to have grey
coats and short tails?
a)
Ggtt x Ggtt
b)
ggtt x GGtt
c)
GgTt x GgTt
d)
GgTt x ggTt
6. In a certain organism, two traits are determined by two pairs of all eles, each of which shows
dominance. One trait is determined by some combination of the alleles A and a. The other trait is
determined by some combination of the alleles B and b, which are located on a different chromosome
from A and a. Construct a complete Punnett square to show the results of a cross between two
parents with genotypes Aabb (parent #1) and AaBb (parent #2). Then determine each of the
following:
a)
What fraction of the offspring is expected to have the same genotype as parent #1?
b)
What fraction of the offspring is expected to have the same phenotype as parent #2?
c)
What fraction of the offspring is expected to be dihybrid?