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MANDELIAN GENETICS
Crosses that deviate
from Mandelian
inherintance
TO THE STUDENTS…
Explain codominant alleles.
Calculate the genotypic and phenotypic ratio
(1:2:1).
Explain incomplete dominant alleles.
Calculate the genotypic and phenotypic ratio
(1:2:1).
Explain multiple alleles
Explain polygenes/polygenic inheritance
CODOMINANT ALLELE
INCOMPLETE DOMINANT ALLELES
MULTIPLE ALLELES
POLYGENES/POLYGENIC INHERITANCE
LETHAL ALLELES
LINKED GENES
SEX-LINKED GENES
Monohybrid
No. Character
One
Genes/ character One
Flower colour
Gene for flower
colour
Alleles/gene
Two
Phenotype
Two
Genotype
Three
Dominant: Red (R)
Recessive: White (r)
Red (RR, Rr) and
White (rr)
RR, Rr, rr
Dihybrid
No.
Character
Genes/
character
Alleles/gene
Two
One/
character
Two/gene
Seed colour
Seed shape
Gene of seed colour
Gene for seed shape
Seed colour
Dominant: Green
Recessive: Yellow
Seed Shape
Dominant: Round
Recessive: Wrinkle
CODOMINANT
ALLELE
both alleles of a pair are fully expressed in a
heterozygous form
It occurs when both of the contributions of
both alleles are visible and do not over power
each other in the phenotype.
Eg :MN blood group  MM
 NN
 MN
Mendel Complete dominance
--->
x
Codominance
--->
x
CODOMINANT ALLELE
The MN blood system is a third (in
addition to the ABO and the Rh)
system of blood antigens also
related to proteins of the red blood
cell plasma membrane.
Two allele = LM and LN
•Ability to produce antigen M
•LM
LM
•LM
LN
•LN
LN
•Ability to produce antigenN
have only the M antigen on their red
blood cells, and will be type M.
Produce both antigen M and N,
are type MN
have only the N antigen on their red
blood cells, and will be type N.
CODOMINANT ALLELE
Inheritance of MN blood group
TRUE BREEDING
P
:
LMLM
G
:
LM
F1
:
X
LM
LNLN
LN
LM LN
self cross
LN
CODOMINANT ALLELE
Inheritance of MN blood group
F1xF1:
LM LN
G:
LM
LN
F2
: LM LM
1
LM LN
X
LM
LN
LM LN
LMLN
2
LNLN
1
CODOMINANT ALLELE
x
Chestnut Colored Horse
Cremello colored horse
Palomino colored Horse
golden colored coat with
a white mane and tail
CODOMINANT ALLELE
I remember codominance in the form of
an example like so:
red x white ---> red & white spotted
--->
x
a cross between organisms with two different phenotypes produces offspring
with a third phenotype in which both of the parental traits appear together.
A cattle can be red (RR = all red hairs), white (WW
= all white hairs), or roan (RW = red & white hairs
together).
a) Predict the phenotypic ratios of offspring when a
homozygous white cow is crossed with a roan bull.
b) What should the genotypes & phenotypes for
parent
if a farmer wanted only cattle with red fur?
A cross between a black cat & a tan cat produces a tabby
pattern (black & tan fur together).
a) What pattern of inheritence does this illustrate?
b) What percent of kittens would have tan fur if a tabby
cat is crossed with a black cat?
INCOMPLETE DOMINANT ALLELE
a form of intermediate inheritance in which
one allele for a specific trait is not completely
dominant over the other allele
x
--->
Eg : Colour ofAntirrhinum (snapdragon)
flower
Mendel Complete dominance
--->
x
Incomplete dominance
--->
x
INCOMPLETE DOMINANT ALLELE
The effect of
heterozygous
can be
detected in the
phenotype
1:2:1 ratio of
red:pink:white flowers.
INCOMPLETE DOMINANT ALLELE
I remember Incomplete Dominance in the form
of an example like so:
RED Flower x WHITE Flower ---> PINK Flower
With incomplete dominance, a cross between
organisms with two different phenotypes produces
offspring with a third phenotype that is a blending of
the parental traits.
INCOMPLETE DOMINANT ALLELE
A cross between a blue blahblah bird & a white blahblah
bird produces offspring that are silver. The color of
blahblah birds is determined by just two alleles.
a) What are the genotypes of the parent blahblah
birds in the original cross?
b) What is/are the genotype(s) of the silver offspring?
c) What would be the phenotypic ratios of offspring
produced by two silver blahblah birds?
INCOMPLETE DOMINANT ALLELE
The color of fruit for plant "X" is determined
by two alleles. When two plants with orange
fruits are crossed the following phenotypic
ratios are present in the offspring: 25% red
fruit, 50% orange fruit, 25% yellow
fruit. What are the genotypes of the parent
orange-fruited plants?
Inheritance
Incomplete
dominant allele
Codominant allele
Multiple allele
Polygenic
inheritance
Lethal gene
Linked gene
Sex-linked gene
Definition
Example
Phenotype Genotype
MULTIPLE ALLELES
One gene having more than two alleles.
All alleles control the same character
Eg: ABO blood type ( IA , IB , IO )
Monohybrid
No. Character
One
Genes/ character One
Flower colour
Gene for flower
colour
Alleles/gene
Two
Phenotype
Two
Genotype
Three
Dominant: Red (R)
Recessive: White (r)
Red (RR, Rr) and
White (rr)
RR, Rr, rr
No. Character
One
Genes/ character One
Human ABO blood
group
Gene for human
blood group
Alleles/gene
Three
IA , IB , IO
Phenotype
•Four
•A, B, O, AB
•Six
IA IO, IA IA , IO IB ,
IB IB , IA IB , IO IO
Genotype
MULTIPLE ALLELES
MULTIPLE ALLELES
TRY THIS…
P:
F1:
TYPE A
I A IA
IA IA
A
IA IB
AB
X
IA IB
AB
TYPE AB
IA IB
IA IA
A
MULTIPLE ALLELES
Brandon is blood Type O. He has an older
brother with blood types A and a sister with
blood group B.
a) What are the genotypes of his parents with
respect to this trait?
b) If Brandon’s sister married with a man with
blood group AB, What is the percentage of
their child having blood group B?
POLYGENES/POLYGENIC
INHERITANCE
an additive effect of two or more genes
on a single phenotypic character.
Quantitative
Variation
e.g: height
POLYGENES
• AABBCC - very dark
• AaBbCc - intermediate
• Aabbcc - very light
Differences between
multiple alleles and polygenes
Multiple Alleles
 Not influence by
environmental
factors
Polygene
 Influence by
environmental
factors
 genes that have
multiple alleles
 more than one gene
 discontinuous
variation
 continuous
variation/
quantitative
variation
• the interaction between two or more genes to
control a single phenotype
types of combs on chickens
Phenotypes
Genotypes
Walnut
R_P_
Rose
R_pp
Pea
rrP_
Single
rrpp
• One gene masks another
– coat color in mice =
2 genes
• pigment (C) or
no pigment (c)
• more pigment (black=B)
or less (brown=b)
• cc = albino,
no matter B allele
• 9:3:3:1 becomes 9:3:4
•How would you
know that difference
wasn’t random chance?
•Chi-square test!
Epitasis in Labrador retrievers
• 2 genes: E & B
– pigment (E) or no pigment (e)
– how dark pigment will be: black (B) to brown (b)
2005-2006
QUESTIONS
1. A dominant allele W confers black fur
for mice. A mice that is homozygous
recessive (ww) has white fur. David
would like to know whether his pet
black-furred mice is homozygous
dominant (WW) or heterozygous (Ww).
How might he determine his pet’s
genotype?
1.
Two pairs of genes affect comb type in chickens.
When both genes are recessive, a chicken has a
single comb. A dominant allele of one gene, P , gives
rise to pea comb. Yet a dominant allele of the other
(R) gives rise to rose comb. An epistatic interaction
occurs when a chicken has at least one of both
dominants, P_R_, which gives rise to walnut comb.
Predict the phenotypic ratios resulting from a cross
between two walnut-combed chickens that are
heterozygous for both genes (PpRr).
Given information:
• Single comb = pprr
• Pea comb
= P_ _ _
• Rose comb
=__R_
• Walnut comb = P_R_
P:
PpRr
X
G : •PR •Pr •pR •pr
F1 :
PpRr
•PR •Pr •pR •pr
Punnet square
PR
Pr
pR
pr
PR
Pr
pR
pr
PPRR
walnut
PPRr
walnut
PpRR
walnut
PpRr
walnut
PPRr
walnut
PPrr
pea
PpRr
walnut
Pprr
pea
PpRR
walnut
PpRr
walnut
ppRR
rose
ppRr
rose
PpRr
walnut
Pprr
pea
ppRr
rose
pprr
single
•9 walnut : 3 pea : 3 rose : 1 single