long version
Monohybrid Cross
-two hybrids, single character (Aa X Aa)
PREDICTIONS (Punnet square/tree diagram/FOIL)
AA = (0.5)(0.5) = 25% homozygous dominant
Aa = 2(0.5)(0.5) = 50% heterozygous dominant 75% dominant
aa = (0.5)(0.5) =25% homozygous recessive 25% recessive
REMEMBER it is sufficient to possess one dominant allele to show the dominant trait, it is necessary to possess two recessive alleles to show the recessive trait. A recessive phenotype is a known genotype.
Try this: In a cross between two purple flowers, one of the ten offspring has white flowers. What are the genotypes of the parents?
Dihybrid cross
-two hybrids, two characters (AaBb X AaBb)
USING PROBABILITY RULES
A_B_ = (3/4)(3/4) = 9/16 dom/dom phenotype
A_bb = (3/4)(1/4) = 3/16 dom/rec phenotype
aaB_ = (1/4)(3/4) = 3/16 rec/dom phenotype
aabb = (1/4)(1/4) = 1/16 rec/rec phenotype
A_bb and aaB_ represent recombinants
REMEMBER You must give answers to probability questions in the form of a fraction, decimal, or ratio.
PRACTICE (in class)
genetics problems with answers