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) 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?

Answers and explanations:

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

A) Purple flowers are dominant to white flowers

B) Brown eyes are dominant to blue eyes

C) Round seeds are dominant to wrinkled

D) Bobtails are recessive (long tails dominant)

TT Longtails.

Tt Longtails.

tt Bobtails.

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.

A) Rr x rr

Alelles     R       r

r             Rr      rr

r             Rr      rr

Results of the crossing

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

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

What percentage of the offspring will be round?

- 100%

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

25% of the offspring will be short.

C) RR X Rr

Alelles     R       R

R            RR      RR

r             Rr       Rr

Results of the crossing

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

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

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.

I) Guinea pigs CC X cc

Parent Genotypes:

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

thats not even an aswer choice