Section 9-2 Dihybrid Crosses

1. Section 9-2 Dihybrid Crosses

2. What is a Dihybrid Cross? A monohybrid cross involves two parents that each have two alleles for one trait. The Punnett Square has four offspring possibilities. A dihybyrid cross involves two parents that each have two alleles for two traits. The Punnett Square has sixteen offspring possibilities. This is because each parent has four alleles for the combination of traits and these are multiplied together.

3. Why do a Dihybrid Cross? A dihybrid cross is done when it makes sense to look at two traits that both contribute to the phenotype. An example is eyebrows. They are determined by both thickness and connectedness. Having a unibrow is more noticeable when the person also has the allele for thick eyebrows.

4. Setting up a Dihybrid Cross An example involves two traits for rabbit fur production. The first trait is for color and the black (B) fur allele is dominant over the brown (b) fur allele. The second trait is for length and the normal (N) fur allele is dominant over the short (n) fur allele.

5. Setting up a Dihybrid Cross A breeder has a male and a female rabbit that are both heterozygous for each of the two traits. They both have the phenotype of black, normal fur. They both have the genotype of BbNn.Notice that each parent has two alleles for each trait.

6. Setting up a Dihybrid Cross When setting up the parent allele combinations above and beside the Punnett Square, only one of each of the two alleles for each trait is used. If you have the alleles BbNn so you combine the first B with the first N, the first B with the second n, the second b with the first N and finally the second b with the second n. You four combinations are BN, Bn, bN, bn for each parent. In some problems there may be repeat combinations of alleles.

7. Filling in a Dihybrid Cross Each of the 16 boxes for the offspring will have some combination of twob’s and twon’s. Keep the two b’stogether as well as the two n’s. BN BnbNbn BN Bn bN bn

8. Interpreting a Dihybrid Cross When tallying the results for dihybrid crosses, we will only count the different phenotype possibilities. There are always going to be four phenotypes for each problem. They are both dominant, one dominant + one recessive, one recessive + one dominantand both recessive. Stop and do the dice activity to predict the expected offspring ratios for the cross of two heterozygous parents.

9. Interpreting a Dihybrid Cross Each number on the die represent a combination of one of each of the two alleles as follows: 1 2 3 4 BN BnbNbn Make tally marks below for each combination that you get when you roll two dice (number is on the point of each die) for a total of 16 times. B_N_ (Black/Normal) (1 + 1) or (1 + 2) or (1 + 3) or (1 + 4) or (2 + 3) B_nn (Black/short) (2 + 2) or (2 + 4) bbN_ (brown/Normal) (3 + 3) or (3 + 4) bbnn (brown/short) (4 + 4)

10. Interpreting a Dihybrid Cross For the Black/Normal phenotype you are looking to count any square that has at least one B and one N. These will include BBNN, BBNn, BbNNand BbNn as genotypes. For the Black/short phenotype you are looking to count any square that has at least one B but both little n’s. These will include only BBnn or Bbnn.

11. Interpreting a Dihybrid Cross For the brown/Normal phenotype you are looking to count any square that has both little b’sand at least one N. These will include only bbNNand bbNn as genotypes. For the brown/short phenotype you are looking to count any square that has at only little b’s and n’s. These will include only bbnn.

12. Interpreting a Dihybrid Cross The four numbers for your ratio should add up to sixteen. The square below gives the expected ratio of 9 : 3 : 3 : 1 for the combinations listed in the order they are counted in. How close was your actual dice rolling data? Notice that a cross of two heterozygous traits gives a multiplied version of the 3 : 1 ratio seen in a monohybrid cross.