Download Chapter 5 I. Multiple Alleles

Chapter 5
Extensions and Modifications
of Basic Principles
I. Multiple Alleles
•
•
•
The ABO blood group has
multiple alleles
codominance
and complete dominance.
In codominance, both alleles are expressed
simultaneously.
In the ABO blood system, there are 3 alleles:
IA
IB
i
IA is dominant over i
IB is dominant over i
IA and IB are co-dominant
Universal donor (type O)
and universal recipient (type AB)
Summary points
1. Dominance is a function of interaction
between alleles.
2. Epistatsis is a function of interaction
between genes.
What pattern of inheritance is
demonstrated in the following cross?
1
II. Sex limited and sex influenced inheritance
What pattern of inheritance is demonstrated in the
following cross?
What pattern of inheritance is
demonstrated in the cross above?
What pattern of inheritance is
demonstrated in the following cross?
2
What pattern of inheritance is
demonstrated in the cross above?
III. Dominance / Incomplete dominance
IV. Gene interaction
A. New genotypes can show up in the F1 and F2
generation.
Eg. Comb type in chickens
Walnut
R-P
What are the phenotypes of these parents,
and what F1 and F2 offspring would you
expect from the following crosses?
RRPP x rrpp
walnut x single
F1: all walnut
Note that your first hypothesis here would
probably be a single gene where walnut is
dominant.
F2: __ walnut: __ rose: __ pea: __ single
Rose
R-pp
Pea
rrP-
Single
rrpp
What are the phenotypes of these parents,
and what F1 and F2 offspring would you
expect from the following crosses?
RRpp x rrPP
rose x pea
F1: all walnut
Note that a novel phenotype appears in this
generation.
F2: __ walnut: __ rose: __ pea: __ single
3
B. Epistasis occurs when one gene masks
the effect of a second gene.
Gene interaction and/or epistasis may
produce offspring ratios that are variations
of a 9:3:3:1.
Dominant epistasis
__ white: __ yellow: __ green
W--- white: wwY- yellow: wwyy green
9
3
3
1
A-B-
A-bb
aaB-
aabb
9:3:3:1
12:3:1
10:3:3
9:6:1
9:3:4
15:1
13:3
12:4
10:6
9:7
Duplicate recessive epistasis
__ pigmented: __ white
A-B- pigmented: A-bb or aaB- or aabb white
4
Two pathways that would yield a 9:7 (color : white) ratio.
… with two pathways you can also
get a 9:3:3:1 ratio, but…
A-
gene A
Colorless
ŽŽŽf
gene B
Colorless
ŽŽŽf
Colorless
Colorless
(C?)
gene A
Colorless
ŽŽŽf
Colorless
ŽŽf
gene B
Colorless
ŽŽŽf
Color
ŽŽŽf
ŽŽŽf
RED
ŽŽf
PURPLE
BLUE
aaB3
blue
aabb
1
white
Colorless
aa
RED
Colorless
ŽŽf
BColorless
ŽŽŽf
A-BA- bb
9
3
purple red
aa
Colorless
ŽŽŽf
B-
Color
PURPLE
BLUE
RED
ŽŽf
bb
Colorless
If the functional a gene is recessive?
A-BA- bb aaB- aabb
9
3
3
1
blue
white
purple red
ŽŽŽf
ŽŽŽf
PURPLE
BLUE
If both functional genes are recessive?
A-BA- bb aaB- aabb
9
3
3
1
white blue
red
purple
How do we get so many different ratios?
AColorless
ŽŽŽf
BPINK
ŽŽŽf
RED
Recessive epistasis
A-BA- bb aaB- aabb
9
3
3
1
red
pink
white white
9
3
4
We have already seen that if the
intermediate product is white (colorless) you
get a ___:___ ratio.
5
… and with a slight variation?
aa
Colorless
A-B9
white
B-
ŽŽŽf
A- bb
3
white
12
… but if these are duplicate
pathways…
PINK
aaB3
red
3
ŽŽŽf
ARED
aabb
1
pink
1
What ratio do you get if the intermediate
product is white?
Colorless
ŽŽŽf
Colorless
ŽŽŽf
Summer squash
•9 disk: 6 sphere: 1 elongate
Fowl color
•13 white: 3 colored
Corn kernel color
•9 purple: 7 white
Complementation test
Determines whether two independently
isolated mutations are at the same loci or
different loci.
ŽŽf
B-
A-B9
red
A- bb
3
red
RED
aaB3
red
aabb
1
white
____red :____ white
Summary points
Examples of Epistatic Ratios
Sheperd’s purse
•15 triangular : 1 oval
RED
1. Genes are discrete units that control the
phenotype of organisms.
2. Inheritance follows the rules of segregation
and independent assortment
3. Dominance is a function of interaction between
alleles.
4. Genes control the production of enzymes and
thus the function of biochemical pathways.
5. Epistatsis is a function of interaction between
genes.
Flower color
Wile type is red, mutant is white.
Consider two independently isolated white
mutants, where wild type is dominant:
w
c
w
c
6
Cross them.
Two possible outcomes
Two possible outcomes
Allelic:
no complementation
Allelic:
no complementation
Different genes:
complementation
w
w
c W
c
c
Cw
Summary points
In a cross between two mutant lines
(with the same phenotype) ,
complementation results in a wild type
phenotype and indicates that two
mutations are in different genes. This
is called a complementation test.
V. Lethal alleles
Lethal alleles result in missing classes in a
genetic cross
Eg. 2:1 phenotypic ratio in a monohybrid
cross
VI. Non-Mendelian inheritance
A.
Maternal effects: influence of mother's
genotype on phenotype of offspring
B.
Cytoplasmic inheritance: extra-nuclear
genes that are found in chloroplasts and
mitochondria
7
Snail shell coiling
dextral (right hand)
sinistral (left hand)
Start with two true-breeding lines
Dextral female x sinistral male: all dextral
F1
Sinstral female x dextral male: all sinistral
F1
Give a hypothesis about how this trait is
controlled.
A. Maternal effects
Dextral female X sinistral male:
dextral F1
selfing produces all dextral
all
Sinstral female X dextral male:
all
sinistral F1
selfing produces all dextral
Genotype of mother determines cleavage
pattern in egg, and that determines direction
of coiling
She is sinistral
because her
mother was ss.
They are dextral
because their
mother was s+s.
What will their
offspring be?
Summary points
For traits determined by genetic
maternal effects, the genotype of the
mother (rather than of the gamete she
contributes) determines the phenotype
of the offspring through mRNA in the
egg.
8
B.
Cytoplasmic inheritance organelles or
particles
Mitochondria and chloroplasts have own
genome
Rules of extranuclear
inheritance
1.
2.
3.
1.
Mitochondrial genome
4.
Uniparental inheritance leads to
differences between reciprocal crosses.
Genes cannot be mapped to nuclear
chromosomes.
Ratios associated with Mendelian traits
cannot be found.
Extranuclear inheritance is persists
despite nuclear substitution.
Homoplasmic: all mitochondria identical
or
heteroplasmic: mitochondrial DNAs
have more than one sequence
Inheritance is uniparental; usually
maternal.
Summary points
For traits determined by cytoplasmic
inheritance, genes are encoded in the
cytoplasm (usually in the mitochondria
and chloroplast). These organelles
usually show uniparental inheritance.
V. Imprinting:
Phenotype of offspring depends upon
maternal or paternal source of altered
allele
i.e. which parent contributed the allele
Mutants act like a dominant, but only
through one parent.
9
Prader-Willi/Angelman syndrome
Results from deletion of 15q11-15q13.
But sometimes you got one phenotype
and sometimes another….
AS
PWS
Angelman syndrome: jerky, repetitive,
lurching body movements; seizures;
incoherent speech; loud bursts of
laughter; large mouths; red cheeks
Prader-Willi syndrome: mild to moderate
mental retardation; lack muscle tone;
insatiably hungry so become obese; tiny
hands and gonads.
Prader-Willi/Angelman syndrome
Results from deletion of 15q11-15q13.
Two different imprinted genes in the same
region.
Maternally inheritted deletion causes
Angelman’s
Paternal deletion causes Prader-Willi
Summary points
For traits determined by genomic
imprinting, only the allele inherited from
the parent of one sex is expressed.
VI. Anticipation: The severity increases or
the age of onset gets early when you look
across generations.
Eg: Grandfather age of onset: 50
Mother age of onset:
35
Child age of onset:
5
Ascertainment bias?
NO
10
Summary points
For traits that show anticipation, mutant
alleles are unstable and may change
over even one generation. They are
caused by trinucleotide repeats; more
repeats results in earlier and/or more
severe expression of the mutant
phenotype.
VII. Environmental effects on gene
expression
Mutations due to trinucleotide repeat
expansion: (CAG)n
Huntington’s
Several types of ataxia
Fragile X (non-coding)
Grandfather age of onset: 50 (CAG)25
Mother age of onset:
35 (CAG)50
Child age of onset:
5 (CAG)120
Summary points
Expression of many genes is modified
by the environment. This is known as a
gene x environment interaction.
This is so common as to be virtually
universal.
11