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!