TWO-FACTOR CROSSES
INTRODUCTION
Crosses that involve two traits, such as pea color and pea shape, are called two-factor
crosses. Predicting the outcome of two-factor crosses requires basically the same procedure
as one-factor crosses. Keep in mind though that during meiosis, chromosomes assort
independently. This means that each of the chromosomes of any pair of homologous
pair of chromosomes has an equal opportunity of ending up in a gamete with either
chromosome from any other pair of homologous chromosomes. Since genes are located on
chromosomes, genes also assort independently as shown below.
EXAMPLE
What are the genotype and phenotype ratios of the offspring resulting from a
cross between two pea plants that are heterozygous for pod color and shape?
step 1
Choose colors to represent the alleles.
i.e.
G for green and g for yellow, N for smooth and n for constricted
step 2
Write the genotypes for the parents.
i.e.
GgNn x GgNn
step 3
Determine the gametes that the parents can produce.
ie.
GN or Gn or gN or gn
step 4
Enter the gamete possibilities on the sides of the Punnett square.
step 5
Complete the Punnett square.
GN
GN GGNN
Gn GGNn
gN GgNN
gn GgNn
step 6
Gn
GGNn
GGnn
GgNn
Ggnn
gN
GgNN
GgNn
ggNN
ggNn
Determine the phenotypes
i.e.
9 green smooth
3 green constricted
step 7
Determine the genotypes
i.e.
1 GGNN :
2 GGNn :
1 GGnn :
2 GgNN :
4 GgNn :
2 Ggnn :
1 ggNN :
2 ggNn :
1 ggnn
gn
GgNn
Ggnn
ggNn
ggnn
3 yellow smooth
1 yellow constricted
PROBLEMS
1. What would be the results of a cross between a pea plant that is both heterozygous green
and heterozygous smooth and a pea plant that is yellow and homozygous smooth?
green and smooth pea plant genotype: _____________________
yellow and smooth pea plant genotype: _____________________
offspring genotype ratio:
______________________________________________
______________________________________________
offspring phenotype ratio:
______________________________________________
______________________________________________
2. In tomatoes, red fruit color (R) is dominant over yellow fruit color (r) and tall vines (T)
are dominant over short vines (t). A variety called Yellow Giant has yellow fruit and tall
vines. Another variety, Tiny Boy, bears red fruit on short vines. Could a breeder cross
Yellow Giant and Tiny Boy tomatoes and obtain a variety that is short with yellow fruit?
Yellow Giant genotype: _____________________________________
Tiny Boy genotype:
_____________________________________
offspring genotype ratio:
______________________________________________
______________________________________________
offspring phenotype ratio:
______________________________________________
______________________________________________
3. Suppose that on the newly discovered planet Metatheres there are creatures called prafs.
Yellow praf eyes (Y) are dominant over pink (y). Webbed feet (D) are dominant over
hoofed feet (d). Suppose a homozygous yellow-eyed, heterozygous web-footed praf
mates with a pink-eyed, hoof-footed praf. Could they have a baby praf with yellow
eyes and hoofed feet? ________
yellow-eyed, web-footed genotype:
pink-eyed, hoof-footed genotype:
offspring genotype ratio:
_____________________
_____________________
______________________________________________
______________________________________________
offspring phenotype ratio:
______________________________________________
______________________________________________
4. In Japanese four o’clock flowers the alleles for red flower color (R) and white flower
color (W) are codominant. Thus a heterozygous combination (RW) is white with red
streaks. What are the results of a cross between a red, heterozygous tall (Tt) and
streaked, short plant?
red, tall plant genotype:
streaked, short plant genotype:
offspring genotype ratio:
_____________________
_____________________
______________________________________________
______________________________________________
offspring phenotype ratio:
______________________________________________
______________________________________________