Predicting Genotypes and Phenotypes

Punnett Squares-used to predict genotypes and phenotypes of

offspring based on the parents genotype

Punnett SquaresMonohybrid crosses- 1 heterozygous trait is crossed

(Bb X Bb)

Dihybrid crosses- 2 heterozygous traits are crossed

(BbSs X BbSs)

Punnett Squares (parent generation)Eye Color mom BB

B B

b

dad

bb

b

Offspring 1 Offspring 2

Offspring 3 Offspring 4

Bb

Bb Bb

Bb

Genotypic Ratio Phenotypic Ratio

Punnett Squares (F1)Eye Color mom Bb

B b

B

dad

Bb

b

Offspring 1 Offspring 2

Offspring 3 Offspring 4

BB

bb Bb

Bb

Genotypic Ratio? Phenotypic Ratio?

Punnett SquaresEye Color and Height mom BbSs

BS Bs bS bs

BS

Dad

BbSs Bs

bS

bs

BBSS BBSs BbSS BbSs

BBSs BBss BbSs Bbss

bbSsbbSSBbSs

BbSs

BbSS

Bbss bbSs bbss

Genotypic Ratio? Phenotypic Ratio?

Laws of Probability • Used to predict genotype or phenotype• -Multiplication rule-to determine the probability of

different events happening in combination, multiply the probability of each event.

• -Addition rule-to determine the probability of the same events happening add the probability of even event.

• See examples

Forkline Method

• this is just another way to be able to predict genotype and phenotype ratios in dihybrid problems

• this way you don’t have to write the box

• but it does require you to know the basic ratios that arise from monohybrids

• based on the idea that: in a dihybrid, the two traits sort INDEPENDENTLY of one another

• i.e. what happens with one trait is completely unrelated to what happens with the other trait

PpYy x PpYy

for example, the following dihybrid cross:

normally to solve this we would1. use FOIL for the gametes, then 2. assemble the Punnet square, then 3. count up the genotypic and phenotypic ratios.However, we can make use of two simple concepts:1. the traits (flower color and seed color) sort out

independently of each other2. there are essentially only three different ratios that can

result in a monohybrid cross (it doesn’t matter what the traits are; I’ve used P here, but it could be anything):

heterozyg x heterozyg: Pp x Pp: ¼ PP ½ Pp ¼ pp

heterozyg x homozyg: Pp x PP -----------> ½ Pp ½ PPor Pp x pp ---------------------------------------------> ½ Pp ½ PP

homozyg x homozyg: PP x PP -----------> 100% PPor pp x pp -–-----------------------------------------> 100% pp

1

3

2

PpYy x PpYyso to solve this dihybrid, separate the two traits (since they sort independently):

Pp x Pp will give:

¼ pp

½ Pp

¼ PP

PpYy x PpYy

so to solve this dihybrid, separate the two traits (since they sort independently):

Pp x Pp will give:

¼ pp

½ Pp

¼ PP

similarly, Yy x Yy will give:

¼ yy½ Yy¼ YY

PpYy x PpYy

so to solve this dihybrid, separate the two traits (since they sort independently):

Pp x Pp will give:

¼ pp

½ Pp

¼ PP¼ yy½ Yy¼ YY

multiply fractions1/16 PPYY1/8 PPYY1/16 PPYY

Yy x Yy will give:

¼ yy½ Yy¼ YY

PpYy x PpYy

so to solve this dihybrid, separate the two traits (since they sort independently):

Pp x Pp will give:

¼ pp

½ Pp

¼ PP¼ yy½ Yy¼ YY

multiply fractions1/16 PPYY1/8 PPYy1/16 PPyy

¼ yy½ Yy¼ YY 1/8 PpYY

1/4 PpYy1/8 Ppyy

¼ yy½ Yy¼ YY 1/16 ppYY

1/8 ppYy1/16 ppyy

PpYy x PpYy

so to solve this dihybrid, separate the two traits (since they sort independently):

Pp x Pp will give:

¼ pp

½ Pp

¼ PP¼ yy½ Yy¼ YY

multiply fractions1/16 PPYY1/8 PPYy1/16 PPyy

¼ yy½ Yy¼ YY 1/8 PpYY

1/4 PpYy1/8 Ppyy

¼ yy½ Yy¼ YY 1/16 ppYY

1/8 ppYy1/16 ppyy

121242121

convert all to 16ths for consistency