1. Dihybrid Crosses Involve 2 traits
Eg. Crossing tall yellow pea plants with short green pea plants
Procedure is same as with monohybrids
Must first find gametes
Then cross in punnett square
Then show gametes coming together, making offspring!

2. Working out Gametes
in Dihybrid Crosses
T T Y Y TY TY TY TY

3. Working out Gametes
in Dihybrid Crosses
T t Y Y TY TY tY tY

4. Working out Gametes
in Dihybrid Crosses
T t Y y TY Ty tY ty

5. ty TY Ty tY TY Ty tY ty Dihybrid Cross TtYy X TtYy 9:3:3:1 Pheno Ratio

6. Dihybrid crosses involve 2 traits

7. P Generation
P Gametes
F1 Generation

8. TY tY TY TY Genotypes Phenotypes
Predict the phenotypic outcome if a hetero tall,
homozygous yellow pea plant is crossed with TTYY TY tY TY
TTYY
TtYY TY
TTYY
TtYY
Phenotypes
All Tall &Yellow
Genotypes
2 TTYY & 2 TtYY

9. ttRR TTWW tR TW Gametes from Parents tR X TW
A dwarf, red snapdragon is crossed with a homozygous tall white
plant. Determine the pheno and geno of the F1’s.
(Tall is dom, red incompletely dom to white)
Genotype of
Dwarf, red snapdragon
ttRR
Genotype of
Homozygous Tall white
snapdragon
TTWW
Gametes from Parents
tR X TW tR
Phenotype of offspring: all Tall & Pink
Genotype of offspring: TtRW TW
TtRW

10. Summary of Dihybrid Cross
Most complex punnett square involves 2 parents heterozygous for both traits
eg. RrBb X RrBb
4 possible gametes from each parent
Punnett square is 4 X 4
Classic 9:3:3:1 phenotypic ratio in offspring

13. RB Rb RB rB RRBB RRBb RrBB RrBb If parents are RRBb and RrBB…..
Only 2 possible gametes from each parent (RB or Rb) and (RB or rB)
Punnett square becomes 2 X 2 instead of 4 X 4! RB Rb RB
RRBB
RRBb rB
RrBB
RrBb

14. bC bc Bc bc Phenotypic Ratio Genotypic Ratio 1:1:1:1 1:1:1:1 Parental
In poultry, a crested head is produced by a dom. allele and a plain head by it’s rec
allele. Black feather is dom to red. A homozygous red feathered, hetero. crest
headed bird is mated to a hetero. black, plain headed bird!
Find the pheno’s and geno’s of the F 1’s. bC bc
Parental
Genotypes
bbCc
&
Bbcc
C = crested
c = plain
B = black
b = red Bc
BbCc
Bbcc bc
bbCc
bbcc
Phenotypic Ratio
1:1:1:1
Genotypic Ratio
1:1:1:1

15. Gametes would be abc & ABC Gametes would be ABC, AbC, aBC, abC
All offspring: AaBbCc
Gametes would be
ABC, AbC, aBC, abC
ABc, Abc, aBc, abc

16. Trihybrid
Cross
YIKES!!

17. 2n where n = number of different
Working out Gametes!!
2n where n = number of different
sets of alleles
n= 1 for mono
n= 2 for dihybrid
n= 3 for trihybrid

18. Lethal Alleles
Dominant
allele is
Lethal!

19. Mexican hairless dog!! Hh

20. Gene Interaction 1.Polygenic Inheritance Skin color Eye color Height
Where a trait is controlled by more than one gene
Skin color
Eye color
Height

21. 2. Epistatic Interaction
Genes that mask the expression of other genes
ALSO
Example: Coat color in dogs
B = black coat
b = brown coat
W = prevents pigment
w = does not prevent color
W is a separate gene located on a separate
chromosome
W allele masks the effect of the B gene

22. Dog example continued yellow/white wwBb genotype = ______ phenotype
black
Yellow/white
brown
yellow/white
brown
black

23. 4 Yellow/white:3 Black: 1 Brown
What would be the genotypes and phenotypes produced by a cross between two dogs: WwBb x wwBb? wB wb WB
WwBB
WwBb
Wwbb
wwBB
wwBb
wwbb Wb wB wb
4 Yellow/white:3 Black: 1 Brown

24. 3. Complementary Interaction
Occurs when two genotypes combine to create a phenotype that neither can produce alone
Genotype A +
Genotype B =
Phenotype C
Chicken Example
R = rose comb
P = pea comb
R + P = walnut
r + p = single comb

25. Walnut
Rose
Pea
Single

26. Question on Complimentary Interaction
Find the F1’s and the F2’s for the following parents:
RRpp x rrPP
Gametes would be Rp x rP
All offspring would be RrPp
Phenotype: all walnut
Must use punnett square for second cross!!

27. RP Rp rP rp RP Rp rP rp Dihybrid Cross RrPp X RrPp 9:3:3:1 Pheno Ratio

28. Walnut
Rose
Pea
Single

29. Pleiotropic Genes Example 1: Sickle Cell disease
A gene that affects many characteristics throughout body
Example 1: Sickle Cell disease
HbAHbA= normal hemoglobin
HbSHbS = sickle cell anemia
mutations can lead to fatigue and weakness,
enlarged spleen, rheumatism, pneumonia
HbAHbS = Resistance to malaria (sickle cell trait)

30. Blood smear showing sickle cell anemia
Notice sickle shape
of cells…can’t carry
enough oxygen &
get stuck in caps
Blood smear showing sickle cell anemia

31. Normal & Abnormal Red Blood Cells

32. Example 2: Marfan’s Syndrome
Long thin limbs
& Abnormal joints
Example 2: Marfan’s Syndrome
inability to produce connective tissue
eye, skeleton and cardiovascular defects show up

33. Abraham Lincoln
Symptoms of
Marfan’s??

34. Environment and Phenotype
Examples:
Primrose flowers
Red if raised at room temp
White when raised above 30 0 C

35. Effect of Environment on Phenotype
2. Water Plants
Submerged leaves
are finely divided,
Compared with
the ones
Growing in air

36. 3. Sun
lightens
hair
&
darkens
freckles

37. Genetics Problems 1:Dihybrid crosses
Practice Genetics Problems
More Practice!