Dihybrid Crosses What are dihybrid crosses Dihybrid crosses

Dihybrid Crosses

What are dihybrid crosses? Dihybrid crosses are those where we consider the inheritance of two characteristics at the same time. How is this different from a monohybrid cross?

Famous Example of Dihybrid Inheritance: Mendel’s Peas Mendel had observed that the seeds of his pea plants varied in several ways- amongst them, whether the peas were round or wrinkled AND whether they were green or yellow.

There are four possible combinations of the two characteristics. Peas can be: 1. round and yellow 2. round and green 3. wrinkled and yellow 4. wrinkled and green

Mendel first did monohybrid crosses on both characteristics, then he decided to look at how these traits were inherited together….

• From his monohybrid trials, Mendel had discovered that the allele for yellow colour (Y) was dominant to that for green (y). • He had also found that the allele for round peas (R) was dominant to the allele for wrinkled peas (r). • Starting with individuals that were homozygous, either for both dominant characteristics or both recessive characteristics, he set off breeding again…

Phenotype Round, yellow Genotype RRYY Gametes all F 1 X rryy RY all Wrinkled, green all ry Rr. Yy All of the F 1 generation would be heterozygous for both characteristics, meaning that the phenotype of the seedlings would all be round and yellow.

He then crossed two of the F 1 (Rr. Yy) generation together…

Crossing members of the F 1 Generation Phenotype Round, yellow Genotype Rr. Yy Gametes RY, Ry, r. Y, ry F 2 X Round, yellow Rr. Yy RY, Ry, r. Y, ry We really need a punnet square for this…

RY RY Ry r. Y ry

RY Ry r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy RRyy Rr. Yy Rryy r. Y Rr. Yy rr. YY rr. Yy ry Rr. Yy Rryy rr. Yy rryy

F 2 Generation Phenotype ratios 9 : 3 : 1 Round Wrinkled Yellow Green This is the typical ratio expected in a dihybrid cross.

Step-by-step 1. Find out what your parent genotypes are and write them down e. g. Aa. BB and aabb 2. Determine what different combinations of alleles could be in the gametes of each parent. You can use the technique "FOIL" to work out all possible combinations. FOIL stands for "Firsts“, "Outsides“, "Insides", Lasts. 3. Write the different gamete allele combinations for the female parent in the horizontal row and those of the male parent in the vertical rows of the Punnett square. 4. Determine the final genotypes of the offspring, just like we did in monohybrid crosses, however this time, the offspring genotypes will consist of 4 alleles.

Step-by-step REMEMBER: When writing the offspring genotypes, keep alleles from the same trait together and always put a dominant allele first if present e. g. Aa. BB (correct) NOT a. BAB (alleles from the same trait not together, recessive allele in front of dominant allele). We can then work out the genotype ratios and phenotype ratios of the offspring as usual.

Scipad pages 137 -143 relate to Dihybrid crosses with no linkage

This was an example of a dihybrid cross with no linkage… However, what happens if chromosomes cross over during meiosis? This leads to a slightly different scenario if the genes are linked (close together on the chromosome).