Dihybrid CrossesDihybrid Crosses
Crosses involving two traitsCrosses involving two traits
Dihybrid CrossesDihybrid 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
cellFinally, calculate genotypic or
phenotypic ratios
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
cellFinally, calculate genotypic or
phenotypic ratios
Determining allelesDetermining allelesIn Mendel’s peas, let’s consider pea color and pea
shapeY = yellow, y = greenR = 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
Let’s use a YyRr x YyRr cross since it is the most complex
In Mendel’s peas, let’s consider pea color and pea shapeY = yellow, y = greenR = 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
Let’s use a YyRr x YyRr cross since it is the most complex
Constructing a Punnett square
Constructing a Punnett square
Mendel’s laws support the creation of four combinations of gametes for each individualRemember 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
Mendel’s laws support the creation of four combinations of gametes for each individualRemember 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
Combining gametesCombining 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
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
Yr
yR
yr
YYRR YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
Calculating RatiosCalculating Ratios
Yellow & Round (Y_R_): 9green & Round (yyR_): 3Yellow & wrinkled (Y_rr): 3green & wrinkled (yyrr): 1
The phenotypic result of 2 heterozygotes in a dihybrid cross will always be 9:3:3:1 (2 dom; 1dom/1rec; 1rec/1dom; 2 rec)
The genotypic result is a bit more complex
Yellow & Round (Y_R_): 9green & Round (yyR_): 3Yellow & wrinkled (Y_rr): 3green & wrinkled (yyrr): 1
The phenotypic result of 2 heterozygotes in a dihybrid cross will always be 9:3:3:1 (2 dom; 1dom/1rec; 1rec/1dom; 2 rec)
The genotypic result is a bit more complex
YR
Yr yR yr
YR
Yr
yR
yr
YYRR YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
Practice!!!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:
1. Tall, Purple
2. Tall, White
3. Short, Purple
4. Short, White
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:
1. Tall, Purple
2. Tall, White
3. Short, Purple
4. Short, White
8/16 = 50%0/16 = 0%8/16 = 50%
0/16 = 0%
Review QuestionsReview 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.