Transcript Dihybrid Cross

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 Gametes
in Dihybrid Crosses
TTYY
TY TY TY TY
Working out Gametes
in Dihybrid Crosses
TtYY
TY TY tY tY
Working out Gametes
in Dihybrid Crosses
TtYy
TY Ty tY
ty
TtYy X TtYy
TY Ty tY ty
TY
TTYY
TTYy
TtYY
TtYy
Ty
TTYy
TTyy
TtYy
Ttyy
tY
TtYY
TtYy
ttYY
ttYy
ty
TtYy
Ttyy
ttYy
ttyy
Dihybrid Cross
9:3:3:1
Pheno
Ratio
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
TY
tY
TY
TTYY
TtYY
TY
TTYY
TtYY
Phenotypes
Genotypes
All Tall &Yellow
2 TTYY & 2 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 of
Dwarf, red snapdragon
ttRR
Genotype of
Homozygous Tall white
snapdragon
TTWW
Gametes from Parents
tR X TW
tR
TW
TtRW
Phenotype of offspring: all Tall & Pink
Genotype 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 RRBB RRBb
rB RrBB 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.
C = crested
c = plain
B = black
b = red
bC
bc
Bc
BbCc
Bbcc
bc
bbCc
bbcc
Phenotypic Ratio
1:1:1:1
Parental
Genotypes
bbCc
&
Bbcc
Genotypic Ratio
1:1:1:1
Gametes would be
abc & ABC
All offspring: AaBbCc
Gametes would be
ABC, AbC, aBC, abC
ABc, Abc, aBc, abc
Trihybrid
Cross
YIKES!!
Working out Gametes!!
n
2
where n = number of different
sets of alleles
n= 1 for mono
n= 2 for dihybrid
n= 3 for trihybrid
Lethal Alleles
Dominant
allele is
Lethal!
Mexican hairless dog!!
Hh
Gene Interaction
1.Polygenic Inheritance
– Where a trait is controlled by more
than one gene
• Skin color
• Eye color
• Height
2. Epistatic Interaction
Genes that mask the
expression of other genes
ALSO
• Example: Coat
• B = black coat
• b = brown coat
color in dogs
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
Dog example continued
black phenotype
• wwBb genotype = ______
• WwBb genotype = Yellow/white
______ phenotype
• wwbb genotype = _______
brown phenotype
What would be the genotypes and phenotypes
produced by a cross between two dogs:
WwBb x wwBb?
wB
WB
Wb
wB
wb
wb
3. Complementary Interaction
• Occurs when two genotypes combine to create
a phenotype that neither can produce alone
Genotype
A
+ Genotype
B
Chicken Example
R = rose comb
P = pea comb
R + P = walnut
r + p = single comb
=
Phenotype
C
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 X RrPp
RP Rp rP
rp
RP
RRPP
RRPp
RrPP
RrPp
Rp
RRPp
RRpp
RrPp
Rrpp
rP
RrPP
RrPp
rrPP
rrPp
rp
RrPp
Rrpp
rrPp
rrpp
Dihybrid Cross
9:3:3:1
Pheno
Ratio
Walnut
Rose
Pea
Single
Pleiotropic Genes
• 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)
Notice sickle shape
of cells…can’t carry
enough oxygen &
get stuck in caps
Blood smear showing sickle cell anemia
Normal & Abnormal Red Blood Cells
• Example 2: Marfan’s Syndrome
– inability to produce connective tissue
– eye, skeleton and cardiovascular defects show up
Abraham Lincoln
Symptoms
of
Marfan’s??
Environment and Phenotype
Examples:
1. Primrose flowers
• Red if raised at room temp
• White when raised above 30 0 C
Effect of Environment on Phenotype
2. Water Plants
Submerged leaves
are finely divided,
Compared with
the ones
Growing in air
3. Sun
lightens
hair
&
darkens
freckles
Genetics Problems 1:Dihybrid crosses
Practice Genetics Problems
More Practice!