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Extending Mendelian genetics
§ Mendel worked with a simple system
peas are genetically simple
most traits are controlled by a
single gene
u each gene has only 2 alleles, 1 of which
is completely dominant to the other
The relationship between
genotype & phenotype
is rarely that simple
u
u
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§
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Incomplete dominance
§ Heterozygotes show an intermediate
Incomplete dominance
phenotype
u RR = red flowers
u WW = white flowers
u RW = pink flowers
X
true-breeding
red flowers
P
true-breeding
white flowers
100% pink flowers
F1
100%
generation
(hybrids)
§ make 50% less color
self-pollinate
25%
red
50%
pink
25%
white
1:2:1
F2
generation
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Incomplete dominance
R W
CC
R W
x CC
female / eggs
male / sperm
CR
CW
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CR
CW
CRCR
CRCW
CRCW
CW CW
%
%
genotype phenotype
R R
CC
CRCW
25%
25%
50%
50%
Codominance
§ 2 alleles affect the phenotype in
separate, distinguishable ways
u ABO blood groups
u 3 alleles
§ IA , IB , i
§ both IA & IB are dominant to i allele
CRCW
CW CW
§ IA & IB alleles are co-dominant to each other
25%
25%
1:2:1
1:2:1
u
determines presences of
oligosaccharides on the
surface of red blood cells
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1901 | 1930
Blood compatibility
§
§
Blood donation
Matching compatible blood groups
u critical for blood transfusions
A person produces antibodies against
oligosaccharides in foreign blood
u wrong blood type
Kar l Landsteiner
(1868-1943)
§ donor’s blood has A or B carbohydrate that
is foreign to recipient
§ antibodies in recipient’s blood bind to
foreign molecules
§ cause donated blood cells to clump together
§ can kill the recipient
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Codominance and Multiple alleles
§
§
§
A heterozygous type
“A” man is married to
a heterozygous type
“B” woman.
Can they have a type
O baby?
A type AB baby?
IB
i
IA
IA IB
IA i
i
I Bi
ii
Other DNA…..
Mitochondrial DNA can also
cause certain types of
disorders.
§ most of these genes affect electron
transport chain or ATP synthase
§
§
Diabetes mellitus with deafness
Leber's hereditary optic neuropathy
u visual loss beginning in young
adulthood
§
Leigh syndrome
u Seizures and ventilatory failure
Chloroplast DNA causes some
plant disorders
§ Tobacco Mosaic Virus
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Polygenic inheritance
§ Some phenotype s determined by
additive effects of 2 or more genes on a
single character
u phenotypes on a continuum
u human traits
Nature vs. nurture
§ Phenotype is controlled by
both environment & genes.
Human skin color is influenced
by both genetics &
environmental conditions
§ skin color
§ height
Coat color in arctic
fox influenced by
heat sensitive alleles
§ weight
§ eye color
§ intelligence
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§ behaviors
Color of Hydrangea flowers
APinfluenced
Biology
is
by soil pH
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It all started with a fly…
§ Chromosom e theory of inheritance
u
1910 | 1933
Thomas Hunt Morgan
§ embryologist at Columbia University
1st to associate a specific gene with a
specific chromosome
u Drosophila breeding
experimental evidence from improved
microscopy & animal breeding led us to
a better understanding of chromosomes
& genes
beyond Mendel
u
§ prolific
§ 2 week generations
§ Drosophila studies
§ 4 pairs of chromosomes
§ XX=female, XY=male
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A. H. Sturtevant in
the Drosophila
stockroom at
Columbia University
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Gene Linkage
§ When several genes of interest exist on
the same chromosome
§ Such genes form a linkage group
§ Tend to be inherited as a block
§ Fewer gamete possibilities because
fewer crossing over events
§ Genes are close together on
chromosome
§ Independe nt assortment does not apply
§No linkage if genes on separate
chromosomes :
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§ # of recombinants increase
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Mendelian Testcross no linkage
Recombinants have different allele combinations, thus
look different from parents
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Phenotypic ratio: 1 : 1 : 1 : 1
Frequency: 25% : 25%: 25% : 25%
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Morgan’s Testcross showing linkage:
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Chromosome Mapping
§ Since linked genes are only separated
by crossing over events, then the
higher the recombinant frequency of
offspring…the greater the likelihood of
there being a crossing over event
between them.
§ 1 map unit = 1% recombination
frequency
§ Greater % = greater distance
§ Lower % = closer distance
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Morgan’s first mutant…
§ Wild type fly = red eyes
§ Morgan discovered a mutant
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Discovery of sex linkage
red eye
female
white-eyed male
u traced the gene for eye color to
a specific chromosome
x
white eye
male
all
red eye
offspring
75%
red eye
female
x
25%
white eye
male
How is this possible?
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Biology
Sex-linked
trait!
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Sex-linked traits
§ Although differences between women &
§
Sex chromosomes
men are many, the chromosomal basis of
sex is rather simple
In humans & other mammals, there are 2
sex chromosomes: X & Y
u
autosomal
2 X chromosomes develops as
a female: XX
chromosomes
§ redundancy
u
an X & Y chromosome develops as
a male: XY
sex
chromosomes
§ no redundancy
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Sex-linked traits
H
XHHh
Xh x XHH
Y
sex-linked recessive
Sex-linked traits summary
§ X-linked
follow the X chromosomes
males get their X from their mother
u trait is never passed from father to son
u
XH
female / eggs
male / sperm
XH
XH
Y
XHXH
XHY
u
XHXh
Xh
§ Y-linked
very few traits
only 26 genes
u trait is only passed from father to son
u females cannot inherit trait
u
XH
Xh
XHXh
Xh Y
u
XHY
Y
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Phenotypes
§ When using sex-linked traits, you
always need to include the sex of the
individual.
§ EX: hemophilic male, hemophilic female,
normal male, normal female.
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X-inactivation
§ Female mammals inherit two X
chromosomes
u one X becomes inactivated during
embryonic development
(XIST- X-inactive specific transcript)
§ condenses into compact object = Barr body
§ Therefore, there are four phenotypes possible
for sex-linked traits.
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X-inactivation & tortoise shell cat
§ 2 different cell lines in cat
Mechanisms of inheritance
§ What causes dominance vs. recessive?
genes code for polypeptides
polypeptides are processed into proteins
u proteins function as…
u
u
§ enzymes
§ structural proteins
§ hormones
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How does dominance work: enzyme
= allele coding for
functional enzyme
= 50% functional enzyme
• sufficient enzyme present
• normal trait is exhibited
• NORMAL trait is DOMINANT
How does dominance work: structure
= allele coding for
= allele coding for
functional structural
non-functional structural
protein
protein
= allele coding for
non-functional enzyme
= 50% functional structure
• 50% proteins malformed
• normal trait is not exhibited
• MUTANT trait is DOMINANT
Aa
carrier
Aa
= 100% non-functional enzyme
• normal trait is not exhibited
aa
= 100% non-functional structure
• normal trait is not exhibited
AA
= 100% functional enzyme
• normal trait is exhibited
AA
= 100% functional structure
• normal trait is exhibited
aa
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Prevalence of dominance
§ Because an allele is dominant
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Polydactyly
individuals are born with
extra fingers or toes
does not mean…
u it is better
u it is more common
dominant to the recessive
allele for 5 digits
recessive allele far more
common than dominant
Polydactyly:
dominant allele
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® 399 individuals out of 400
have only 5 digits
® most people are homozygous
recessive (aa)
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You must be able to solve the following problems.
Monohybrid cross
Working Backwards
Test cross
Dihybrid cross
Incomplete dominance & codominance
Sex linked inheritance
Multiple Alleles (Blood Type)
Pedigree analysis
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