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Genetics Practice Problems
1. For each genotype, indicate whether it is heterozygous (A) or homozygous (B).
AA ____ Bb ____ Cc ____ Dd ____
Ee ____ ff ____ GG ____ HH ____
Jj ____ kk ____ Ll ____ Mm ____
nn ____ OO ____ Pp ____
2. For each of the genotypes below, determine the phenotype.
A) Purple flowers are dominant to white flowers
QQ _________________________
Qq _________________________
qq _________________________
B) Brown eyes are dominant to blue eyes
BB _________________________ Bb _________________________ bb _________________________
C) Round seeds are dominant to wrinkled
RR _________________________
Rr _________________________
rr __________________________
D) Bobtails are recessive (long tails dominant)
TT _________________________ Tt _________________________ tt __________________________
3. For each phenotype, list the genotypes. (Use the letter of the dominant trait)
Straight hair is dominant to curly. ____________ straight ____________ straight ____________ curly
Pointed heads are dominant to round heads. ____________ pointed
____________ pointed
____________ round
4. Set up the Punnett square for each of the crosses listed below. The trait being studied is round seeds (dominant) and wrinkled seeds (recessive).
A) Rr x rr
B) Rr x Rr
C) RR x Rr
What percentage of the offspring will be round?
________%
What percentage of the offspring will be round?
________%
What percentage of the offspring will be round?
________%

5. Practice with crosses and Punnett squares. Show all work!
A) A TT (tall) plant is crossed with a tt (short plant). What percentage of the
offspring will be tall?
B) A Tt plant is
crossed with a Tt plant. What percentage of the offspring will be short?
C) A heterozygous round seeded plant (Rr) is crossed with a homozygous round seeded plant (RR). What percentage of the offspring will be homozygous (RR)?
D) Ahomozygousroundseededplant is crossed with a homozygous wrinkled seeded plant. What are the genotypes of the parents? What percentage of the offspring
will also be homozygous?
E) In pea plants yellow flowers
are dominant to white flowers.
If two white flowered plants are crossed, what percentage of their offspring will be white flowered?
F) A white flowered plant is crossed with a plant that is heterozygous for the trait. What percentage of the offspring will have yellow flowers?
G) Two plants, both heterozygous for
the gene that controls flower color
are crossed. What percentage of
their offspring will have yellow flowers? What percentage will have white flowers?
H) In guinea pigs, the allele for long hair is dominant.
What genotype would a heterozygous longhaired guinea pig have?
What genotype would a pure breeding longhaired guinea pig have? What genotype would a shorthaired guinea pig have?
I) Show the cross for a pure breeding longhaired guinea pig and a shorthaired guinea pig. What percentage of the offspring will have long hair?
J) Show the cross for two heterozygous guinea pigs. What percentage of the offspring will have long hair?
What percentage of the offspring will have short hair?

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josinclair73

Answers and explanations:

1. For each genotype, indicate whether it is heterozygous (A) or homozygous (B).

Homozygous (A) and heterozygous (B) refers to the presence of two equal or different alleles for the same trait, respectively.

  • AA (A).
  • Bb (B).
  • Cc (B).
  • Dd (B).
  • Ee (B).
  • ff (B).
  • GG (A).
  • HH (A).
  • Jj (B).
  • kk (A).
  • Ll (B).
  • Mm (B).
  • nn (A).
  • OO (A).
  • Pp (B).

2. For each of the genotypes below, determine the phenotype.

A) Purple flowers are dominant to white flowers

  • QQ Purple flowers.
  • Qq Purple flowers.
  • qq White flowers.

B) Brown eyes are dominant to blue eyes

  • BB Brown eyes.
  • Bb Brown eyes.
  • bb Blue eyes.

C) Round seeds are dominant to wrinkled

  • RR Round seeds.
  • Rr Round seeds.
  • rr Wrinled seeds.

D) Bobtails are recessive (long tails dominant)

TT Longtails.

Tt Longtails.

tt Bobtails.

3. For each phenotype, list the genotypes. (Use the letter of the dominant trait)

Straight hair is dominant to curly.

SS Straight.

Ss Straight.

ss Curly.

Pointed heads are dominant to round heads.

PP Pointed .

Pp Pointed .

pp Round.

4. Set up the Punnett square for each of the crosses listed below. The trait being studied is round seeds (dominant) and wrinkled seeds (recessive).

A) Rr x rr

Alelles     R       r

r             Rr      rr

r             Rr      rr

Results of the crossing

  • RR (Round seed homozygous) None.
  • Rr (Round seed heterozygous) 50%.
  • rr (Wrinkled seeds homozygous) 50%.

What percentage of the offspring will be round?

- 50%

B) Rr x Rr

Alelles     R       r

R            RR      Rr

r             Rr       rr

Results of the crossing

  • RR (Round seed homozygous) 25%.
  • Rr (Round seed heterozygous) 50%.
  • rr (Wrinkled seeds homozygous) 25%.

What percentage of the offspring will be round?

- 75 %

C) RR x Rr

Alelles     R       R

R            RR      RR

r             Rr       Rr

Results of the crossing

  • RR (Round seed homozygous) 50%.
  • Rr (Round seed heterozygous) 50%.
  • rr (Wrinkled seeds homozygous) None.

What percentage of the offspring will be round?

- 100%

5. Punnett squares.

A) A TT (tall) plant is crossed with a tt (short plant). What percentage of the  offspring will be tall?

The result of the cross between two pure lines (TT and tt) is always a 100% heterozygous offspring (Tt), with the phenotype of the dominant trait.

Alelles     T       T

t              Tt      Tt

t              Tt      Tt

100% of the offspring will be tall.

B) Tt X Tt

Alelles     T       t

T             TT      Tt

t              Tt      tt

Results of the crossing

  • TT (Tall plants homozygous) 25%.
  • Tt (Tall plants heterozygous) 50%.
  • tt (Short plants homozygous) 25%.

25% of the offspring will be short.

C) RR X Rr

Alelles     R       R

R            RR      RR

r             Rr       Rr

Results of the crossing

  • RR (Round seed homozygous) 50%.
  • Rr (Round seed heterozygous) 50%.
  • rr (Wrinkled seeds homozygous) None.

50% of the offspring will be homozygous (RR).

D) RR X rr

Parent Genotypes: Homozygous round seeded RR and homozygous wrinkled seed rr.

Alelles     R       R

r              Rr      Rr

r              Rr       Rr

There are no homozygous offspring at this crossing.

E) White/Yellow flowered (pea) plants.

Q for yellow flowers.

q for white flowers.

Parent Genotypes:  Pea plants white flowers  qq.

Alelles     q       q

q             qq      qq

q             qq      qq

100% of the offspring will be white flowered (qq).

F) White/Yellow flowered plants.

- Q for yellow flowers.

- q for white flowers.

Parent Genotypes:  White flowered plant  qq and yellow flowered plant (heterozygous) Qq.

Alelles     Q       q

q             Qq      qq

q             Qq      qq

Results of the crossing

  • QQ (Yellow flowered homozygous) none
  • Qq (Yellow flowered heterozygous) 50%.
  • qq (White flowered homozygous) 50%.

50% of the offspring will be yellow flowered (Qq).

G) Qq X Qq

Parent Genotypes:  Yellow flowered plant  Qq (both heterozygous).

Alelles     Q       q

Q             QQ      Qq

q              Qq      qq

Results of the crossing

  • QQ (Yellow flowered homozygous) 25%.
  • Qq (Yellow flowered heterozygous) 50%.
  • qq (White flowered homozygous) 25%.

75% of the offspring will have yellow flowers (QQ and Qq).

25% of the offspring will have white flowers (qq).

H) Guinea pigs, the allele for long hair is dominant.

C for longhaired guinea pig.

c for shorthaired guinea pig.

  • Heterozygous longhaired guinea pig have Cc genotype.
  • Pure breeding longhaired guinea pig have CC genotype.
  • Shorthaired guinea pig have cc genotype.

I) Guinea pigs CC X cc

Parent Genotypes:

  • Pure breeding longhaired guinea pig have CC genotype.
  • Shorthaired guinea pig have cc genotype.

CC X cc

Alelles     C         C

c             Cc       Cc

c             Cc       Cc

100% of the offspring will have long hair (Cc)

J) Guinea pigs Cc X Cc

Alelles     C         c

C            CC       Cc

c             Cc       cc

Results of the crossing

  • CC (Long hair homozygous) 25%.
  • Qq (Long hair heterozygous) 50%.
  • qq (Short hair homozygous) 25%.

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