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Dihybrid Crosses

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Dihybrid Crosses

Crosses involving two traits

- 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

- In Mendel’s peas, let’s 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

- Let’s use a YyRr x YyRr cross since it is the most complex

- Mendel’s 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

- 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

- 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 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

YYRR

YYRr

YyRR

YyRr

YYRr

YYrr

YyRr

Yyrr

Yr

YyRr

yyRR

yyRr

yR

YyRR

YyRr

Yyrr

yyRr

yyrr

yr

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%

Differentiate between monohybrid and dihybrid crosses.

Explain the four basic steps to solving a dihybrid cross.

Explain how to determine the alleles for each parent that are part of the Punnett square.

Explain how to combine gametes and alleles in a dihybrid Punnett square.

Use a Punnett square to solve dihybrid genetics crosses.