DIHYBRID CROSSES

=Dihybrid Crosses= We can also do a punnett square for two different characteristics at the same time. This is known as a dihybrid cross. An Austrian monk, Gregor Mendel, first noted the inheritance of two different characteristics when looking at pea plants. He looked at two characteristics: Pea shape & colour. When he mated pure breeding yellow, round peas with pure breeding green wrinkled peas he found all the offspring were yellow, round peas [image:http://i.imgur.com/XfjNY4y.png] From this cross, Mendel figured out that the pea shape round (R) is dominant over wrinkled (r) and that the pea colour yellow (Y) is dominant over green (y). [image:http://i.imgur.com/q3UuYqD.png] When the resulting heterozygotes are crossed, they can produce several different types of gametes. Due to Independent assortment, each gamete will carry one of the alleles for each characteristic. This results in the possible combinations shown to the left. If we place the possible gametes of each parent in a punnett square we get the following: [image:http://i.imgur.com/Xl89fIv.png?1] The resulting phenotype ratio (below) is actually a combination of two 3: 1 ratios (you will notice that 3 of every 4 plants are Yellow and that three of every four plants are Round) [image:http://i.imgur.com/tNkHVUr.png] Consider the underlying genotypes of each of the resulting phenotypes... • The 9 Round Yellow peas represent individuals with a genotype of '''R_Y_''' • The 3 Round green peas represent individuals with a genotype of '''R_yy''' • The 3 wrinkled Yellow peas represent individuals with a genotype of '''rrY_''' • The 1 wrinkled green pea represents an individual with a genotype of '''rryy''' [image:http://i.imgur.com/vwe5zuZ.png]