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Chapter 14.
Beyond Mendel’s Lawsof Inheritance
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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 each gene has only 2 alleles, 1 of which
is completely dominant to the other The relationship between genotype &
phenotype is rarely simple
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Incomplete dominance Heterozygotes show an intermediate
phenotype RR = red flowers rr = white flowers Rr = pink flowers
make 50% less color
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Incomplete dominancetrue-breeding
red flowerstrue-breeding white flowers
X
100%100% pink flowers
F1generation(hybrids)
25%white
F2generation
25%red 1:2:1
P
self-pollinate50%pink
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Incomplete dominanceCRCW x CRCW
CR CWmale / sperm
CR
CW
fem
ale
/ egg
s CRCR
CRCW CWCW
CRCW
25%
1:2:1
25%
50%
25%
1:2:1
%genotype
%phenotype
CRCR
CRCW
CRCW
CWCW
25%
50%
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Co-dominance 2 alleles affect the phenotype in
separate, distinguishable ways ABO blood groups 3 alleles = IA, IB, and i
both the IA & IB alleles are dominant to the iallele
IA & IB alleles are codominant to each other determines presences of
oligosaccharides on the surface of redblood cells
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Blood type
universaldonor
universalrecipient
__
__
status
no oligosaccharideson surface of RBC
both type A & type Boligosaccharides onsurface of RBC
type Boligosaccharides onsurface of RBC
type Aoligosaccharides onsurface of RBC
phenotype
type Oi i
type ABIA IB
type BIB IB IB i
type AIA IA IA i
phenotypegenotype
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Blood compatibility Matching compatible blood groups is critical
for blood transfusions A person produces antibodies against
foreign blood factors = oligosaccharides if donor’s blood has an A or B oligosaccharide
that is foreign to the recipient, antibodies inthe recipient’s blood will bind to the foreignmolecules
cause the donated blood cells to clumptogether & can kill the recipient
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Blood donation
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Pleiotropy Most genes are pleiotropic
one gene affects more than onephenotypic character wide-ranging effects due to a single gene: dwarfism (achondroplasia) gigantism (acromegaly)
The genes that we have covered so far affect only one phenotypiccharacter, but most genes are pleiotropic
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Acromegaly: André the Giant
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Pleiotropy It is not surprising that a gene can
affect a number of organism’scharacteristics consider the intricate molecular &
cellular interactions responsible for anorganism’s development cystic fibrosis sickle cell anemia Timothy disease
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Epistasis 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
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Polygenic inheritance Some phenotypes determined by
additive effects of 2 or more genes ona single character phenotypes on a continuum human traits
skin color height weight eye color intelligence behaviors
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AlbinismJohnny & Edgar Winter
albinoAfricans
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Nature vs. nurture Phenotype is controlled by both
environment & genes
Color of Hydrangea flowersis influenced by soil pH
Human skin coloris influenced byboth genetics &environmentalconditions
• The relative importance of genes & the environment in influencinghuman characteristics is a very old & hotly contested debate
• a single tree has leaves that vary in size, shape & color, dependingon exposure to wind & sun
• for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performanceon intelligence tests
• even identical twins — genetic equals — accumulate phenotypicdifferences as a result of their unique experiences
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Chromosome theory of inheritance experimental evidence from improved
microscopy & animal breeding led us toa better understanding of chromosomes& genesbeyond Mendel Drosophila studies
It all started with a fly…
A. H. Sturtevantin the Drosophila
stockroom
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Chromosome theory of inheritance Experimental evidence from improved
microscopy & animal breeding led usto a better understanding ofchromosomes & genes beyond MendelDrosophila studies
A. H. Sturtevantin the Drosophila
stockroom
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Thomas Hunt Morgan Morgan was an embryologist at
Columbia University 1st to associate a specific gene with a
specific chromosomeDrosophila breeding
prolific 2 week generations 4 pairs of chromosomes XX=female, XY=male
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Morgan’s first mutant… Wild type fly = red eyes Morgan discovered a mutant white-eyed
male trace a gene for eye color to a specific
chromosome
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Discovery of sex linkagered eyefemale
white eyemalex
allred eye
offspring
75%red eyefemale
25%white eye
malex
How is this possible?Sex-linked trait!
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Sex-linked traits Although differences between women &
men are many, the chromosomal basis ofsex is rather simple
In humans & other mammals, there are 2sex chromosomes: X & Y 2 X chromosomes develops as
a female: XX redundancy
an X & Y chromosome develops asa male: XY no redundancy
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Sex chromosomes
autosomal
chromosomes
sexchromosomes
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Genes on sex chromosomes Y chromosome
SRY: sex-determining region master regulator for maleness turns on genes for production of male
hormonespleiotropy!
X chromosome other traits beyond sex determination
hemophilia Duchenne muscular dystrophy color-blind
Duchenne muscular dystrophy affects one in 3,500 males born inthe United States.
• Affected individuals rarely live past their early 20s.
• This disorder is due to the absence of an X-linked gene for akey muscle protein, called dystrophin.
• The disease is characterized by a progressive weakening of themuscles and loss of coordination.
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Map of the Y chromosome?
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Sex-linked traits
Hh x HH
XH Ymale / sperm
XH
Xh
fem
ale
/ egg
s XHXH
XHY
XHXh
XHXh
XHY
XhY
XHXhXH
Xh
XHYY
XHXHXH XHY
XHXh XhY
sex-linked recessive
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Sex-linked traits summary X-linked
follow the X chromosomes males get their X from their mother trait is never passed from father to son
Y-linked very few traits only 26 genes trait is only passed from father to son females cannot inherit trait
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Hemophilia is a sex-linked recessive trait defined by the absence ofone or more clotting factors.
• These proteins normally slow and then stop bleeding.Individuals with hemophilia have prolonged bleeding because a firmclot forms slowly.
• Bleeding in muscles and joints can be painful and lead toserious damage.
Individuals can be treated with intravenous injections of the missingprotein.
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X-inactivation Female mammals inherit two X
chromosomes one X becomes inactivated during
embryonic development condenses into compact object = Barr body
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X-inactivation & tortoise shell cat 2 different cell lines in cat
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Male pattern baldness Sex influenced trait
autosomal trait influenced by sex hormones age effect as well: onset after 30 years old
dominant in males & recessive in females B– = bald in males; bb = bald in females
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Mechanisms of inheritance What causes the differences in alleles
of a trait? yellow vs. green color smooth vs. wrinkled seeds dark vs. light skin Tay sachs disease vs. no disease Sickle cell anemia vs. no disease
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Mechanisms of inheritance What causes dominance vs. recessive?
genes code for polypeptides polypeptides are processed into proteins proteins function as…
enzymes structural proteins hormones
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How does dominance work: enzyme= allele coding for
functional enzyme= allele coding for
non-functional enzyme
= 100% non-functional enzyme• normal trait is not exhibited aa
= 50% functional enzyme• sufficient enzyme present• normal trait is exhibited
Aacarrier
= 100% functional enzyme• normal trait is exhibited AA
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AA
How does dominance work: structure= allele coding for
functional structuralprotein
= allele coding fornon-functional structuralprotein
= 100% non-functional structure • normal trait is not exhibited
= 50% functional structure• 50% proteins malformed• normal trait is not exhibited
Aa
= 100% functional structure • normal trait is exhibited aa
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Prevalence of dominance Because an allele is dominant
does not mean… it is better it is more common
Polydactyly:dominant allele
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Polydactylyindividuals are born withextra fingers or toes
dominant to recessiveallele for 5 digits
recessive allele far morecommon than dominant→399 individuals out of 400
have only 5 digits→most people are homozygous
recessive (aa)
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Hound Dog Taylor