When two traits are involved in a cross, it is called a di-hybrid cross.

Di-Hybrid Crosses If you want to make a cross of two traits on the same chromosome; you would now need a 4 X 4 square with 16 different possible offspring. Here is an example of this larger Punnett square.

For the following crosses: use these symbols. R = Red fur r = white fur X = Long fur x = short fur Each parent will always pass a trait for fur color, and a trait for fur length on to the offspring.

Separating the Gametes One Parent’s Genotype = RrXx Use FOIL to separate the gametes. 1st gamete = RX (first two) 2nd gamete = Rx (outer two) 3rd gamete = rX (inner two) 4th gamete = rx (last two)

Example Problem Cross a male, heterozygous for both traits. Genotype = RrXx with a female that is also heterozygous for both traits.

Gametes for a Di-Hybrid Cross Father = RrXx Possible Gametes are: RX Rx rX rx Mother = RrXx RX Rx rX rx

Punnett Square for two traits RX Rx rX rx RX Rx rX rx

RX Rx rX rx RX Rx rX rx Punnett Square for two traits RRXX RRXx RrXX

Genotypic Ratios RRXX= 1/16 RRXx= 2/16 RRxx=1/16

RX Rx rX rx RX Rx rX rx Punnett Square for two traits RRXX RRXx RrXX

Phenotypic Ratios Red / Long = 9/16 Red / short = 3/16 white / Long = 3/16 white / short = 1/16

Another Di-Hybrid Cross Cross a male homozygous dominant for both traits. Genotype = RRXX with a female who is homozygous recessive for both traits. Genotype = rrxx How many different gametes are possible for each parent?

Separating the Gametes Father = RRXX Possible gametes = RX Mother = rrxx Possible gametes = rx

RX rx RrXx

100 % of the offspring will be heterozygous for both traits and 100% will have Red, Long Fur

Another Di-Hybrid Cross Cross a male homozygous dominant for fur color and heterozygous for fur length. Genotype = RRXx With a female who is homozygous recessive for fur color and heterozygous for fur length. Genotype = rrXx How many different gametes are possible for each parent?

Separating the Gametes Father = RRXx Possible gametes = RX Rx Mother = rrXx Possible gametes = rX rx

RX Rx rX RrXX RrXx rx RrXx Rrxx

Genotypic and Phenotypic Ratios Genotypic Ratio 1/4 RrXX 1/2 RrXx 1/4 Rrxx Phenotypic Ratio 3/4 Red and Long Fur 1/4 Red, short fur