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XBIO: Genetics Problem Packet
Monohybrid Crosses – One Factor Crosses
Trait
Dominant Allele
Pod Shape
Smooth (N)
Pod Color
Green (G)
Flower Position
Axial (A)
Plant Height
Tall (T)
Steps
1.
2.
3.
4.
5.
6.
Recessive Allele
Constricted(n)
Yellow (g)
Terminal (a)
Short/dwarf (t)
Choose a letter to represent the alleles in the cross
Write the genotypes of the parents
Determine the possible gametes (reproductive cells) the parents can produce
Enter the possible gametes at the top and side of the Punnett square
Complete the Punnett square by writing the alleles from the gametes in the appropriate boxes
Determine the phenotypes of the offspring
Practice Problems
Nn x NN
Genotype ratio:
Phenotype ratio:
Aa x aa
Genotype ratio:
Phenotype ratio:
Tt x Tt
Genotype ratio:
Phenotype ratio:
Two heterozygous green
plants
Genotype ratio:
Phenotype ratio:
Heterozygous for axial
flowers with a plant with
terminal flowers
Genotype ratio:
Phenotype ratio:
Homozygous tall plant
with a short plant
Genotype ratio:
Phenotype ratio:
Heterozygous smooth
with a homozygous smooth
plant
Genotype ratio:
Phenotype ratio:
Dihybrid Crosses –Two Factor Crosses
Sample Problem – What are the genotypic and phenotypic rations in the offspring resulting from a cross
between two pea plants that are heterozygous for pod color and pod shape?
1. Choose letters to represent the genes in the cross
2. Write the genotypes of the parents
o Heterozygous for both traits – GgNnxGgNn
3. Determine the possible gametes that the parents can produce
o Each parent produces four types of gametes - FOIL – GN, Gn, gN, gn
4. Enter the possible gametes at the top and side of the Punnett square
5. Complete the Punnett square by writing the alleles from the gametes in the appropriate boxes
GN
Gn
gN
gn
GN
GGNN
GGNn
GgNN
GgNn
Gn
GGNn
GGnn
GgNn
Ggnn
gN
GgNN
GgNn
ggNN
ggNn
Gn
GgNn
Ggnn
ggNn
ggnn
6. Determine the phenotypes of the offspring: 9 green smooth, 3 green wrinkled, 3 yellow smooth, 1
yellow wrinkled 9:3:3:1
Practice Problems
In mice the ability to run normally is a dominant trait. Mice with this trait are called running mice (R).
The recessive trait causes mice to run in circles only. Mice with this trait are called waltzing mice (r).
Hair color is also inherited in mice. Black hair (B) is dominant over brown (b) hair.
1.
Cross a heterozygous running, heterozygous black mouse (________) with a homozygous running,
homozygous black mouse (________).
Parental genotypes: ____________________________
Phenotypic ratio: ______________________________
2. Cross a homozygous running, homozygous black mouse (________) with a heterozygous running, brown
mouse (________).
Parental genotypes: ____________________________
Phenotypic ratio: ______________________________
3. Cross a waltzing brown mouse (________) with a waltzing brown mouse (________).
Parental genotypes: ____________________________
Phenotypic ratio: ______________________________
4. Cross a heterozygous running, brown mouse (________) with a heterozygous running homozygous
black mouse (________).
Parental genotypes: ____________________________
Phenotypic ratio: ______________________________