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.