Probability

• Probability is defined as the likelihood of an event occurring. The practice of genetics uses the concept of probability to predict the outcome of phenotypic expression based on the alleles present

Punnett Square • A punnett square can be used to

predict the allelic frequencies when true breeding parents are crossed.– draw a square having twice the

number of side boxes as number of alleles.

– segregate the alleles on the side of the boxes (if more than one trait you must segregate as groups)

– recombine the alleles in the boxes – count the ratios

• genotype ratio for the F1 cross is 1:2:2:1 • phenotype ratio is 3:1 with the

dominant trait (purple) expressed in the hybrids.

dihybrid crosses

• For dihybrid crosses the results are still predictable and are as follows: – R - round, r – wrinkled– Y - yellow, y – green

• Ratios:– Phenotypic ratio is 9:3:3:1

Multiplication- addition rule:• Multiplication- addition rule:

– You can use the allelic frequencies to predict the outcome of a cross.• Each trait is calculated by its own punnett square to create the probability

• Given the Genotypes RrYy & rrYy, what is the frequency of offspring exhibiting both recessive traits?– rr = 1/2– yy = 1/4– 1/2 x 1/4 = 1/8

• Chance for both dominant?– Rr = 1/2– YY = 1/4– Yy = 1/2– 1/2 x (1/4 + 2/4) = 3/8