1. Dihybrid Genetics

2. Dihybrid Cross: Study of the inheritance of two gene pairs
Complications:
Number of possible gametes increases
Number of Possible Gametes = 2n where “n” = number of gene pairs
Monohybrid = 21 = 2 possible gametes
Dihybrid = 22 = 4 possible gametes
Trihybrid = 23 = 8 possible gametes
Tetrahybrid = 24 = 16 possible gametes

3. Homozygous Tall plant with Purple Flowers X Homozygous short plant with white flowers
Key: T = Tall P = purple
t = short p = white
P Generation TTPP x ttpp
Each gamete must get one of each gene
Gametes: TP, TP, TP, TP or tp, tp, tp, tp
-gametes for each individual are the same because they are homozygous
F1 Genotypes: TtPp

4. F1 Gametes: formed from TtPp Genes separate in meiosis (Law of Segregation) Independent assortment allows for several combinations
Tt Pp OR
Tt Pp

5. F1 Genotypes: TtPp Genes separate in meiosis – each gamete gets one of each gene Independent assortment allows for several combinations
Tt Pp OR
Tt Pp
T P
t p
T p
t P
Dihybrid Genotype results in 4 possible gametes.
Expands the Punnett to a 4x4 grid

6. TtPp First = Outer = Inner = Last =
FOIL the Gametes
TtPp First = Outer = Inner = Last =

7. TtPp First = TP Outer = Tp Inner = tP Last = tp
FOIL the Gametes
TtPp First = TP Outer = Tp Inner = tP Last = tp

8. Possible gametes for:
TTPp = TtPP =
ttPp =

9. Possible gametes for: TTPp = TP,Tp,TP,Tp TtPP = ttPp =

10. Possible gametes for: TTPp = TP,Tp,TP,Tp TtPP = TP,tP,TP, tP ttPp =

11. Possible gametes for: TTPp = TP,Tp,TP,Tp TtPP = TP,tP,TP, tP ttPp = tP,tp, tP, tp

12. F1 Cross: TtPp x TtPp

13. TP Tp tP tp TP Tp tP tp

14. TP Tp tP tp
TTPP
TTPp
TtPP
TtPp TP
TtPp
Ttpp
TTPp
TTpp Tp
TtPP
TtPp
ttPP
ttPp tP
TtPp
ttpp tp
Ttpp
ttPp

15. Dihybrid Cross Ratios
Genotypic Ratio: Phenotypic Ratio:

16. Dihybrid Cross Ratios
Genotypic Ratio: TTPP:TTPp:TtPP:TtPp:TTpp:Ttpp:ttPP:ttPp:ttpp Phenotypic Ratio:

17. Dihybrid Cross Ratios Genotypic Ratio: 1:2:2:4:1:2:1:2:1
TTPP:TTPp:TtPP:TtPp:TTpp:Ttpp:ttPP:ttPp:ttpp
1:2:2:4:1:2:1:2:1
Phenotypic Ratio:

18. Dihybrid Cross Ratios Genotypic Ratio: 1:2:2:4:1:2:1:2:1
TTPP:TTPp:TtPP:TtPp:TTpp:Ttpp:ttPP:ttPp:ttpp
1:2:2:4:1:2:1:2:1
Phenotypic Ratio:
Tall Purple: Tall White: Short Purple: Short White

19. Dihybrid Cross Ratios Genotypic Ratio: 1:2:2:4:1:2:1:2:1
TTPP:TTPp:TtPP:TtPp:TTpp:Ttpp:ttPP:ttPp:ttpp
1:2:2:4:1:2:1:2:1
Phenotypic Ratio:
Tall Purple: Tall White: Short Purple: Short White
9:3:3:1
Phenotypic Ratio for ANY Dihybrid Cross where both parents are heterozygous for both traits.

20. A new species of spider was discovered in the boiler room of NDHS
A new species of spider was discovered in the boiler room of NDHS. These spiders either have large hairy bodies or small smooth bodies. They can also have red legs or brown legs. Extensive genetic research has shown that hairy bodies are dominant over smooth and brown legs are dominant over red.
If a smooth bodied male with brown legs (heterozygous) mates with a hairy bodied female (heterozygous) with red legs, what would the expected offspring look like?

21. If a purebred hairy bodied, red legged male spider is mated with a female that is heterozygous for both traits, how many offspring would be expected to have hairy bodies and red legs?