Dihybrid Crosses: Setting Up Punnett Square

• A cross involving two pairs of contrasting traits

• Round & Tall x wrinkled & Short (RrTt x rrtt)

• Use ‘FOIL’ method to find all combinations of alleles in the gametes

(First, Outer, Inner & Last)

• Ex. Heterozygous round & tall = RrTtGametes: RT, Rt, rT, rt

Rr Tt

First

Outer

Inner

Last

RTFirst

RtOuter

rTInner

rtLast

Legend

Round peas = RR, Rr

Wrinkled peas = rr

Yellow peas = YY, Yy

Green peas = yy

RrYy x RrYy

rryyrrYyRryyRrYy ry

Last

rrYyrrYYRrYyRrYY rY

Inner

RryyRrYyRRyyRRYy Ry

Outer

RrYyRrYYRRYyRRYY RY

First

ry

Last

rY

Inner

Ry

Outer

RY

First

Phenotype for a cross involving two parents that are heterozygous for both traits?

–9/16 Round,Yellow peas

–3/16 Round, Green peas

–3/16 Wrinkled, Yellow peas

–1/16 Wrinkled, Green peas

(56%)

(19%)

(19%)

(6%)

Ratio - 9:3:3:1

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