Dihybrid Crosses

1
Dihybrid Crosses

• Crosses involving two traits

2
Dihybrid Crosses

• Constructing a Punnett square for a dihybrid
  cross is similar to the method used for a
  monohybrid cross
• First, determine the alleles produced in the
  gametes of each parent
• Next, construct a Punnett square
• Then, combine the gametes for each cell
• Finally, calculate genotypic or phenotypic ratios

3
Determining alleles

• In Mendels peas, lets consider pea color and
  pea shape
• Y yellow, y green
• R round, r wrinkled
• First determine the genotype for the 2 traits of
  the individual (YYRR, yyrr, YyRr, etc)
• The FOIL method (from Algebra) may be helpful
  in determining allele possibilities for an
  individual
• (YY)(RR) YR, YR, YR, YR
• (yy)(rr) yr, yr, yr, yr
• (Yy)(Rr) YR, Yr, yR, yR
• Lets use a YyRr x YyRr cross since it is the
  most complex

4
Constructing a Punnett square

• Mendels laws support the creation of four
  combinations of gametes for each individual
• Remember that meiosis produces 4 unique daughter
  cells
• Since each parent produces 4 different
  combinations of alleles in the gametes, draw a 4
  square by 4 square Punnett square

Gamete 1 Gamete 2 Gamete 3 Gamete 4
Gamete 1
Gamete 2
Gamete 3
Gamete 4
5
Combining gametes

• List the gametes for Parent 1 along one edge of
  the Punnett square
• List the gametes for Parent 2 along the other
  edge of the Punnett square
• Fill out the squares with alleles from both
  parents (keeping similar letters together)
• The result is the prediction of all possible
  combinations of genotypes for the offspring of
  the dihybrid cross

YR
Yr
yR
yr




YR
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
Yr
YyRr
yyRR
yyRr
yR
YyRR
YyRr
Yyrr
yyRr
yyrr
yr
6
Calculating Ratios

• Yellow Round (Y_R_) 9
• green Round (yyR_) 3
• Yellow wrinkled (Y_rr) 3
• green wrinkled (yyrr) 1
• The phenotypic result of 2 heterozygotes in a
  dihybrid cross will always be 9331 (2 dom
  1dom/1rec 1rec/1dom 2 rec)
• The genotypic result is a bit more complex

YR
Yr
yR
yr




YR
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
Yr
YyRr
yyRR
yyRr
yR
YyRR
YyRr
Yyrr
yyRr
yyrr
yr
7
Practice!!!

• In pea plants, purple flowers (F) are dominant to
  white flowers (f) and tall stems (T) are dominant
  to short stems (t). If a tall (heterozygous),
  purple (homozygous) flowered pea plant is crossed
  with a short, white flowered pea plant. What are
  the possibilities of having offspring of
• Tall, Purple
• Tall, White
• Short, Purple
• Short, White

8/16 50
0/16 0
8/16 50
0/16 0
8
Review Questions

1. Differentiate between monohybrid and dihybrid
   crosses.
2. Explain the four basic steps to solving a
   dihybrid cross.
3. Explain how to determine the alleles for each
   parent that are part of the Punnett square.
4. Explain how to combine gametes and alleles in a
   dihybrid Punnett square.
5. Use a Punnett square to solve dihybrid genetics
   crosses.