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!
Working out Gametesin Dihybrid Crosses
T T Y Y
TY TY TY TY
Working out Gametesin Dihybrid Crosses
T t Y Y
TY TY tY tY
Working out Gametesin Dihybrid Crosses
T t Y y
TY Ty tY ty
TTYY TTYy TtYY TtYy
TTyy TtYy Ttyy
TtYy ttYY ttYy
TTYy
TtYY
ttyyttYyTtyyTtYy
tytYTyTY
TY
Ty
tY
ty
Dihybrid Cross
TtYy X TtYy
9:3:3:1PhenoRatio
Dihybrid crosses involve 2 traits
P Generation
P Gametes
F1 Generation
Predict the phenotypic outcome if a hetero tall, homozygous yellow pea plant is crossed with TTYY
PhenotypesAll Tall &Yellow
Genotypes 2 TTYY & 2 TtYY
TY tY
TY
TY
TTYY TtYY
TTYY TtYY
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 ofDwarf, red snapdragon
ttRR
Genotype of Homozygous Tall white
snapdragon
TTWW
Gametes from ParentstR X TW
tR
TtRWTW
Phenotype of offspring: all Tall & PinkGenotype of offspring: TtRW
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
• 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
rB
RRBB
RrBbRrBB
RRBb
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.
Phenotypic Ratio1:1:1:1
Genotypic Ratio1:1:1:1
bC bc
Bc
bc
BbCc Bbcc
bbCc bbcc
C = crestedc = plainB = black
b = red
ParentalGenotypes
bbCc&
Bbcc
Gametes would beabc & ABC
Gametes would beABC, AbC, aBC, abC
ABc, Abc, aBc, abc
All offspring: AaBbCc
Trihybrid Cross
YIKES!!
Working out Gametes!!
2n where n = number of different
sets of alleles
n= 1 for mono
n= 2 for dihybrid
n= 3 for trihybrid
Lethal AllelesDominant
allele isLethal!
Mexican hairless dog!!
Hh
Gene Interaction1.Polygenic Inheritance
– Where a trait is controlled by more than one gene
•Skin color
•Eye color
•Height
• Example: Coat color in dogs• B = black coat
• b = brown coat
ALSO W = prevents pigment
w = does not prevent color
W is a separate gene located on a separate chromosomeW allele masks the effect of the B gene
2. Epistatic Interaction Genes that mask the expression of other genes
Dog example continued
• wwBb genotype = ______ phenotype
• WwBb genotype = ______ phenotype
• wwbb genotype = _______ phenotype
black
white
brown
• Occurs when two genotypes combine to create a phenotype that neither can produce alone
3. Complementary Interaction
Genotype A
+ Genotype B
= Phenotype C
Chicken ExampleR = rose combP = pea combR + P = walnutr + p = single comb
Walnut Rose Pea Single
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!!
RRPP RRPp RrPP RrPp
RRpp RrPp Rrpp
RrPp rrPP rrPp
RRPp
RrPP
rrpprrPpRrppRrPp
rprPRpRP
RP
Rp
rP
rp
Dihybrid Cross
RrPp X RrPp
9:3:3:1PhenoRatio
Walnut Rose Pea Single
Pleiotropic Genes• Affect many characteristics throughout body
• Sickle Cell Anemia– mutations can lead to fatigue and weakness,
enlarged spleen, rheumatism, pneumonia
• Sickle cell disease (easy to follow info!)
Blood smear showing sickle cell anemia
Notice sickle shapeof cells…can’t carry
enough oxygen &get stuck in caps
Normal & Abnormal Red Blood Cells
• Eg. Marfan’s Syndrome– inability to produce connective tissue– eye, skeleton and cardiovascular defects
show up
Pleiotropic Genes
Marfan’s Syndrome
Abraham Lincoln
Symptoms of
Marfan’s??
Marfan’s Syndrome
Long thin limbs&
Abnormal joints
Environment and Phenotype
• Primrose flowers
• Red if raised at room temp
• White when raised above 30 0 C
Effect of Environment on Phenotype
Submerged leaves are finely divided,Compared with
the onesGrowing in air
Sun lightens hair&
Darkens freckles
Genetics Problems 1:Dihybrid crosses
Practice Genetics Problems
More Practice!