AP Genetics Problems
**So Much More Than Your Average Punnett Square  **
Background- original parents in any given set of crosses are called the parent generation, while the two
subsequent generations are denoted with symbols F1 and F2 (a cross of two F1 individuals). Punnett squares
are one method for visually demonstrating the probability of offspring genotypes and phenotypes.
Results of Mendel’s Experiments
1.
P1 Cross
Tall X short stems
F1 Plants
all tall
F2 Plants
787 tall
277 short
1,064 total
Round X wrinkled seeds
all round
5,474 round
1,850 wrinkled
7,324 total
Yellow X green seeds
all yellow
6,022 yellow
2,001 green
8,023 total
Axial X terminal flowers
all axial
651 axial
207 terminal
858 total
Are these results what you would expect? Explain why in detail.
Example: Monohybrid Crosses
Directions: ALWAYS show ALL work on genetics problems! No work = no credit!
2. Predict the genotypic and phenotypic ratios for the following monohybrid crosses.
T = tall
t = dwarf
a. TT x tt
Genotypic ratio: _____________________
Phenotypic ratio: ____________________
b. Tt x Tt
Genotypic ratio: _____________________
Phenotypic ratio: ____________________
c. Tt x tt
Genotypic ratio: _____________________
Phenotypic ratio: ____________________
d. An experimenter cross – pollinates 2 tall plants, and in the next generation counts 290 tall plants
and 102 short plants. What genotype would you expect of the parents? WHY?
3. Cystic fibrosis is a recessive genetic disorder. Ron is a homozygous dominant (FF) and Nancy is a
carrier (Ff) of cystic fibrosis. Use a Punnett square to predict the probability that one of their children
will have cystic fibrosis.
4. In sheep, White (W) is dominant to black (w).
a. Give the F-2 phenotypic and genotypic ratios resulting from the cross of a pure-breeding white
ram with a pure-breeding black ewe.
F2 Genotypic ratio: _____________________
F2 Phenotypic ratio: ____________________
b. If you found a white sheep and wanted to determine its genotype, what color animal would you
cross it with and why? What is this kind of cross called?
Example: Dihybrid Crosses
Dihybrid crosses use Punnett squares to distribute parental alleles from two genes into gametes (eggs and
sperm or pollen and ovum) as would be predicted by meiosis.
5. In garden peas, the gene for Purple flowers (P) is dominant to the allele for white flowers (p). The
gene for axial flowers (A) is dominant to the allele for terminal flowers (a).
Predict the outcome for the following crosses.
a. PPAA x ppaa
Genotypic ratio: __________X_________
Phenotypic ratio: ____________________
b. PpAa x PpAa
Genotypic ratio: __________X_________
Phenotypic ratio: ____________________
6. Imagine that a couple is planning to have children. The male is heterozygous for Huntington’s
disease and homozygous dominant for Tay-Sachs. The female is homozygous recessive for
Huntington’s disease and heterozygous for Tay-Sachs. The couple is curious about the possibility
and probability of their offspring inheriting Tay-Sachs and/or Huntington’s. For humans,
Huntington’s disease is dominant (H) over the “normal” condition (h), and the “normal”
condition is dominant (T) over Tay-Sachs (t). Complete a Punnett square for this cross and
record the probabilities for phenotypes of the offspring as ratios.
Example: Non-Mendelian Inheritance Patterns
7. In some breeds of cattle, when a red bull is mated with a white cow the offspring shows a “roan”
color (a mixture of red and white hair). What would be the result of a cross between two roan
cattle?
Genotypic ratio: ___________________________
Phenotypic ratio: ___________________________
What type of inheritance is this?
8. A man with blood type A marries a woman with blood type AB. The man’s father had blood
type O. Show the phenotypic and genotypic ratios of their potential children.
Genotypic ratio: ___________________________
Phenotypic ratio: ___________________________
What type of inheritance is this?
9. There are two genes that work together to determine kernel color in corn. Dark red kernel plants
are AABB. White kernel plants are aabb. When you cross a dark red with a white, the offspring
(AaBb) are a light pink color. What type of inheritance pattern does this example display?
Explain!
10. Discuss how the expression of the genes in human blood types is different from the example of
roan cattle and kernel color in corn.
Example: Sex-Linked and Chromosomal Linkage Cross
The normal female condition is a result of the chromosomal pairing XX, while the normal male condition is
XY. Certain genes located on the X chromosome, not associated with female sex characteristics, cause sexlinked recessive traits. As a result, females must receive two recessive alleles to exhibit any particular
characteristic associated with one of these genes, while males need only receive one allele. The reason for
the male anomaly is that the Y chromosome does not carry versions of the same genes as the X
chromosome. Consequently, only females can be true heterozygotes (one dominant allele and one
recessive allele).
11. Red-green color blindness is inherited as a sex-linked recessive disorder. If a color-blind woman
marries a man who has normal vision, what would be the expected phenotypes of their children?
Genotypic ratio: ____________________________
Phenotypic ratio: ___________________________
12. A man and his wife both have normal color vision, but a daughter has red-green color blindness,
a sex-linked recessive trait. The man sues his wife for divorce on grounds of infidelity. Can
genetics provide evidence supporting his case? Show your work/evidence.
13. In D. melanogaster, there is a dominant allele for grey body color and a dominant allele of
another gene for normal wings. The recessive alleles of these two genes result in black body color
and vestigial wings. Flies homozygous for gray body and normal wings were crossed with flies
that had black bodies and vestigial wings. The F-1 offpsring were then test-crossed, with the
following results.
Gray body, normal wings
Black body, vestigial wings
Gray body, vestigial wings
Black body, normal wings
236
253
50
61
Are the two genes linked? If so, how many map units apart are they on the chromosome?
14. The recombination frequency between linked genes A and B is 40 percent; between B and C is
20 percent; between C and D is 10 percent; between C and A is 20 percent, and between D and B
is 10 percent. What is the sequence of genes on the chromosome?