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
LETHAL ALLELES
POLYGENES/POLYGENIC INHERITANCE
LINKED GENES
SEX-LINKED GENES
Monohybrid
No. Character One Flower colour
Genes/ character One Gene for flower
colour
Alleles/gene Two Dominant: Red (R)
Recessive: White (r)
Phenotype Two Red (RR, Rr) and
White (rr)
Genotype Three RR, Rr, rr
Dihybrid
No.
Character
Two Seed colour
Seed shape
Genes/
character
One/
character
Gene of seed colour
Gene for seed shape
Alleles/gene Two/gene Seed colourDominant: Green
Recessive: Yellow
Seed Shape
Dominant: Round
Recessive: Wrinkle
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
CODOMINANT ALLELE
MM
NN
MN
--->x--->x
Mendel Complete dominance
Codominance
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.
CODOMINANT ALLELE
Two allele = LM and LN
•Ability to produce antigen M •Ability to produce antigenN
•LM LM
•LM LN
•LN LN
have only the M antigen on their red
blood cells, and will be type M.
have only the N antigen on their red
blood cells, and will be type N.
Produce both antigen M and N,
are type MN
Inheritance of MN blood group
P : LMLM X LNLN
TRUE BREEDING
CODOMINANT ALLELE
self cross
G : LM LM LN LN
F1 : LM LN
Inheritance of MN blood group
F1xF1: LM LN X LM LN
G: LM LN LM LN
F2 : LM LM LM LN LMLN LNLN
1 12
CODOMINANT ALLELE
CODOMINANT ALLELE
x
Chestnut Colored Horse Cremello colored horse
Palomino colored Horse
golden colored coat with
a white mane and tail
I remember codominance in the form of an example like so:
CODOMINANT ALLELE
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
Eg : Colour ofAntirrhinum (snapdragon)
flower
a form of intermediate inheritance in which one allele for a specific trait is not completely
dominant over the other allele
x --->
--->x--->x
Mendel Complete dominance
Incomplete dominance
1:2:1 ratio of
red:pink:white flowers.
INCOMPLETE DOMINANT ALLELE
The effect of heterozygous
can be detected in the
phenotype
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 blahblahbird produces offspring that are silver. The color ofblahblah birds is determined by just two alleles.
a) What are the genotypes of the parent blahblahbirds 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?
INCOMPLETE DOMINANT ALLELE
Inheritance Definition Example Phenotype -
Genotype
Incomplete
dominant allele
Codominant allele
Multiple allele
Polygenic
inheritance
Lethal gene
Linked gene
Sex-linked gene
One gene having more than two alleles.
All alleles control the same character
Eg: ABO blood type ( IA , IB , IO )
MULTIPLE ALLELES
Monohybrid
No. Character One Flower colour
Genes/ character One Gene for flower
colour
Alleles/gene Two Dominant: Red (R)
Recessive: White (r)
Phenotype Two Red (RR, Rr) and
White (rr)
Genotype Three RR, Rr, rr
No. Character One Human ABO blood
group
Genes/ character One Gene for human
blood group
Alleles/gene
Phenotype
Genotype
Three IA , IB , IO
•Four •A, B, O, AB
IA IO, IA IA , IO IB ,
IB IB , IA IB , IO IO•Six
MULTIPLE ALLELES
TRY THIS…
P: TYPE A X TYPE AB
IA IA IA IB
F1: IA IA IA IB IA IB IA IA
A AB AB A
MULTIPLE ALLELES
Brandon is blood Type O. He has an olderbrother with blood types A and a sister withblood 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?
MULTIPLE ALLELES
an additive effect of two or more geneson a single phenotypic character.
e.g: height
POLYGENES/POLYGENIC INHERITANCE
Quantitative Variation
• AABBCC - very dark
• AaBbCc - intermediate
• Aabbcc - very light
POLYGENES
Differences between multiple alleles and polygenes
Multiple Alleles Polygene Not influence by
environmental factors
genes that have multiple alleles
discontinuous variation
Influence by environmental factors
more than one gene
continuous variation/ quantitative variation
• the interaction between two or more genes to control a single phenotype
Phenotypes Genotypes
Walnut R_P_
Rose R_pp
Pea rrP_
Single rrpp
types of combs on chickens
• 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 differencewasn’t random chance?
•Chi-square test!
2005-2006
Epitasis in Labrador retrievers
• 2 genes: E & B
– pigment (E) or no pigment (e)
– how dark pigment will be: black (B) to brown (b)
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 PpRr
G :
F1 : Punnet square
•PR •Pr •pR •pr •PR •Pr •pR •pr
PR Pr pR pr
PR PPRR
walnut
PPRr
walnut
PpRR
walnut
PpRr
walnut
Pr PPRr
walnut
PPrr
pea
PpRr
walnut
Pprr
pea
pR PpRR
walnut
PpRr
walnut
ppRR
rose
ppRr
rose
pr PpRr
walnut
Pprr
pea
ppRr
rose
pprr
single
•9 walnut : 3 pea : 3 rose : 1 single