Crosses involving two traits Dihybrid Crosses Crosses involving two traits

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

Determining alleles 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

Constructing a Punnett square 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 Gamete 1 Gamete 2 Gamete 3 Gamete 4

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 Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr YyRr Yyrr yyRr yyrr

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 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 Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr YyRr Yyrr yyRr yyrr

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%

Review Questions 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.