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

2. 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.

4. 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.

5. 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)

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

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

8. Punnett Square for two traitsRX Rx rX rx RX Rx rX rx

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

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

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

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

13. 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?

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

15. RX rx
RrXx

16. 100 % of the offspring will be heterozygous for both traits and 100% will have Red, Long Fur

17. 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?

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

19. RX Rx rX
RrXX
RrXx rx
RrXx
Rrxx

20. 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