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Genetics
&
The Work of Mendel
AP Biology
2006-2007
Gregor Mendel
 Modern genetics began in the
mid-1800s in an abbey garden,
where a monk named Gregor
Mendel documented inheritance
in peas
used experimental method
 used quantitative analysis

 collected data & counted them

AP Biology
excellent example of scientific
method
Mendel’s work
 Bred pea plants

cross-pollinate True breeding parents (P)

(All offspring like the parents)
 P = parental

raised seeds & then observed traits (F1)
 F = filial
allowed offspring to self-pollinate &
observed next generation (F2)
 Used mathematical principals of probability
to interpret results
 (studies 2 generations)

AP Biology
Mendel collected data for 7 pea traits
AP Biology
Looking closer at Mendel’s work
P
F1
true-breeding
true-breeding
X
purple-flower peas
white-flower peas
100%
purple-flower peas
Where did
the white
flowers go?
100%
generation
(hybrids)
self-pollinate
F2
generation
AP Biology
75%
purple-flower peas
White
flowers came
back!
25%
white-flower peas
3:1
What did Mendel’s findings mean?
 Traits come in alternative versions
purple vs. white flower color
 Alternate versions of a gene called
alleles

 different alleles vary in the sequence of
nucleotides at the specific locus of a gene
 some difference in sequence of A, T, C, G
purple-flower allele &
white-flower allele are two DNA
variations at flower-color locus
different versions of gene at
same location on homologous
chromosomes
AP
Biology
Traits are inherited as discrete units
 For each characteristic, an organism
inherits 2 alleles, 1 from each parent

diploid organism inherits 1 set of
chromosomes from each parent
 homologous chromosomes
 Diploid = 2 sets of
chromosomes
 like having 2 editions
of encyclopedia
AP Biology
What are the
advantages of
being diploid?
What did Mendel’s findings mean?
 Some traits mask others

purple & white flower colors are
separate traits that do not blend
 purple x white ≠ light purple
 purple masked white

dominant allele
 functional protein
 masks other alleles
 Designated with an upper case letter

recessive allele

malfunctioning protein
 Effect only exerted in homozygous state
 Designated with a lower case letter
AP Biology
Genotype vs. phenotype
 Difference between how an organism
“looks” & its genetics

phenotype
 description of an organism’s trait

genotype
 description of an organism’s genetic
makeup
X
P
Explain Mendel’s results using
…dominant & recessive
purple
white
F1
AP Biology
all purple
Making crosses
 Can represent alleles as letters
flower color alleles  P or p
 true-breeding purple-flower peas  PP
 true-breeding white-flower peas  pp

AP Biology
Genotypes
 Homozygous = same alleles = PP, pp
 Heterozygous = different alleles = Pp
homozygous
dominant
heterozygous
homozygous
recessive
AP Biology
Phenotype vs. genotype
 2 organisms can have the same
phenotype but have different genotypes
Can’t tell
by lookin’
at ya!
How do you determine the
genotype of an individual with
with a dominant phenotype?
AP Biology
Test cross
 Breed the dominant phenotype —
the unknown genotype — with a
homozygous recessive (pp) to
determine the identity of the unknown
allele
How does
that work?
x
is it
PP or Pp?
AP Biology
pp
Mendel chose peas luckily
 Pea plants are good for genetic
research

Relatively simple genetically
 Most characters are controlled by a single
gene pair
 Each gene has only 2 alleles, one of which
is completely dominant to the other
AP Biology
Mendel chose peas luckily

Mendel had strict control over which
plants mated with which
 Each pea plant has male & female
structures
 Pea plants can self-pollinate
 Mendel could also cross-pollinate plants:
moving pollen from one plant to another
AP Biology
1st law
Mendel’s
of heredity
 Law of segregation
P
PP
P
When gametes (eggs and sperm) are
produced during meiosis during
meiosis, alleles segregate from each
other
pp
 each allele for a trait is packaged into
a separate gamete(egg or sperm)

p
p
P
Pp
p
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Law of Segregation
 Which stage of
meiosis creates the
law of segregation?
Whoa!
And Mendel
didn’t even know
DNA or genes
existed!
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Monohybrid cross
 Some of Mendel’s experiments followed
the inheritance of single characters

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Ex: flower height
 TT X tt
 Tt X Tt
 Tt X TT
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Dihybrid cross
 Other of Mendel’s
experiments followed
the inheritance of 2
different characters

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seed color and
seed shape
Mendel
was working out
many of the
genetic rules!
YYRR X yyrr
AP Biology
Mendel’s 2nd law of heredity
Can you think
of an exception
to this?
 Law of independent assortment

Each pair of alleles- for each trait- separate into
gametes independently
 non-homologous chromosomes align independently
 4 classes of gametes produced in equal amounts
 YR = Yr = yR = yr
 only true for genes on separate chromosomes or
on same chromosome but so far apart that crossing
over happens frequently
YyRr
YR
Yr
AP Biology
yR
yr
YR
Yr
yR
yr
Law of Independent Assortment
 Which stage of meiosis
creates the law of
independent assortment?
Remember
Mendel didn’t
even know DNA
—or genes—
existed!
AP Biology
Metaphase 1
EXCEPTION
 If genes are on same
chromosome & close together
 will usually be inherited
together
 rarely crossover separately
 “linked”
The
chromosomal
basis of Mendel’s
laws…
Trace the genetic
events through
meiosis, gamete
formation &
fertilization to
offspring
AP Biology
Probability & Genetics
AP Biology
2006-
Genetics & Probability
 Mendel’s laws:
segregation
 independent assortment

reflect same laws of
probability that apply to
tossing coins or rolling dice
AP Biology
Probability & genetics
 Calculating probability of
making a specific gamete
is just like calculating the
probability in flipping a
coin
probability of tossing
heads?
 probability making a B
gamete?
B
100%
BB
B

AP Biology
B
50%
Bb
b
Probability & genetics
 Outcome of 1 toss has no
impact on the outcome of the
next toss
probability of tossing heads
each time? 50%
 probability making a B gamete
each time? 50%

B
Bb
b
AP Biology
Rule of multiplication
 Chance that 2 or more independent
events will occur together

probability that 2 coins tossed at the
same time will land heads up
1/2 x 1/2 = 1/4

probability of Pp x Pp  pp
1/2 x 1/2 = 1/4
P
Pp
p
AP Biology
Calculating dihybrid probability
Use rule of multiplication to predict crosses
YyRr x YyRr
x
Yy
Yy
Rr
yyrr
x
?%
1/16
yy
AP Biology
rr
1/4
x
1/4
Rr
Apply the Rule of Multiplicationnot in lecture
AABbccDdEEFf
x
AaBbccDdeeFf
AabbccDdEeFF
Got it?
Try this!
AP Biology
AA x Aa  Aa
Bb x Bb  bb
cc x cc  cc
Dd x Dd  Dd
EE x ee  Ee
Ff x Ff  FF
1/2
1/4
1
1/2
1
1/4
1/64
Rule of addition
 Chance that an event can occur
2 or more different ways
 sum of the separate probabilities
 probability of Bb x Bb  BB or bb
sperm
egg
offspring
B
B
BB
1/2 x 1/2 =
b
b
1/2 x 1/2 =
AP Biology
1/4
bb
1/4
1/4
+ 1/4
1/2
Chi-square test
 Test to see if your data supports
your hypothesis
 Compare “observed” vs. “expected” data
is variance from expected due to
“random chance”?
 or is there another factor influencing data?

 null hypothesis
 degrees of freedom
 statistical significance
AP Biology
AP Biology
AP Biology
Beyond Mendel’s Laws
of Inheritance
AP Biology
2006-
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 that simple
AP Biology
Incomplete dominance
 Heterozygote shows an intermediate,
blended phenotype

example:
 RR = red flowers RR
 WW = white flowers
WW
 RW = pink flowers RW
 make 50% less color
AP Biology
RR
RW
WW
Incomplete dominance
P
X
true-breeding
red flowers
true-breeding
white flowers
100% pink flowers
F1
100%
generation
(hybrids)
self-pollinate
25%
red
F2
generation
AP Biology
50%
pink
25%
white
It’s like
flipping 2
pennies!
1:2:1
Co-dominance
 2 alleles affect the phenotype equally &
separately
not blended phenotype
 human ABO blood groups
 3 alleles

 IA, IB, i0
 IA & IB alleles are co-dominant
 glycoprotein antigens on RBC
 i0 allele recessive to both
AP Biology
Genetics of Blood type
phenogenotype
type
A
B
AB
O
AP Biology
antigen
on RBC
antibodies
in blood
donation
status
IA IA or IA i
type A antigens
on surface
of RBC
anti-B antibodies
__
IB IB or IB i
type B antigens
on surface
of RBC
anti-A antibodies
__
IA IB
both type A &
type B antigens
on surface
of RBC
no antibodies
universal
recipient
ii
no antigens
on surface
of RBC
anti-A & anti-B
antibodies
universal
donor
Blood compatibility
 Matching compatible blood groups is
critical for blood transfusions.
 A person produces antibodies
against foreign blood factors =
glycoproteins (donor blood).
If donor’s blood has an A or B
oligosaccharide that is foreign to the
recipient, antibodies in the recipient’s
blood will bind to the foreign molecules.
 Cause the donated blood cells to clump
AP Biology together & can kill the recipient.

Multiple Alleles
 Three or more alternative forms of a gene
(alleles) that can occupy the same locus.
However, only two of the alleles can be
present in a single organism.
For example: the
ABO system of
blood groups is
controlled by three
alleles, only two of
which are present in an individual.
AP Biology
Incomplete Penetrance:
 Offspring does not always show
dominant trait or the level of
expression.
 I.E. Polydactly
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Pleiotropy
 Most genes are pleiotropic

one gene affects more than one
phenotypic character
 1 gene affects more than 1 trait
 dwarfism (achondroplasia)
 gigantism (acromegaly)
AP Biology
Inheritance pattern of Achondroplasia
Aa
x aa
Aa
x Aa
dominant
inheritance
A
a
a
a
Aa
Aa
dwarf
dwarf
aa
aa
50% dwarf:50%
AP Biology
normal or 1:1
A
A
a
AA
Aa
lethal
a
Aa
aa
67% dwarf:33% normal or 2:1
Pleiotrophy Cont.
 Gigantism (acromegaly)
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Marfan Syndrome
 Mutated gene on chromosome 15

AP Biology
Disproportionately long arms, legs,
hands, feet, weakened aorta, poor
eyesight
Epistasis
 One gene completely masks another gene

coat color in mice = 2 separate genes
 C,c:
B_C_
bbC_
_ _cc
AP Biology
pigment (C) or
no pigment (c)
 B,b:
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!
Epistasis in Labrador retrievers
 2 genes: (E,e) & (B,b)


pigment (E) or no pigment (e)
pigment concentration: black (B) to brown (b)
eebb
AP Biology
eeB–
E–bb
E–B–
Polygenic inheritance
 Some phenotypes determined by
additive effects of 2 or more genes on a
single character
phenotypes on a continuum
 human traits

 skin color - dominant
alleles have a quantitative
effect…each adds to the effect
 height
 weight
 intelligence
AP Biology
 behaviors
Skin color: Albinism
Johnny & Edgar Winter
 However albinism can be
inherited as a single gene trait

aa = albino
albino
Africans
melanin = universal brown color
enzyme
tyrosine
AP Biology
melanin
albinism
OCA1 albino
AP Biology
Bianca Knowlton
Nature vs. Nurture
 Phenotype is controlled by both environment
& genes
 A single tree has leaves that vary in size,
shape & color, depending on exposure to
wind & sun
 For humans, nutrition influences height,
exercise alters build, sun tanning darkens the
skin, and experience improves performance
on intelligence tests
 Even identical twins — genetic equals —
accumulate phenotypic differences as a result
of their unique experiences
AP Biology
Pleiotrophy Cont.
 Gigantism (acromegaly)
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Multiple Alleles
 Three or more alternative forms of a gene
(alleles) that can occupy the same locus.
However, only two of the alleles can be
present in a single organism.
For example: the
ABO system of
blood groups is
controlled by three
alleles, only two of
which are present in an individual.
AP Biology
Sex linked traits
1910 | 1933
 Genes are on sex chromosomes
as opposed to autosomal chromosomes
 discovered by T.H. Morgan
 1st to associate a specific
gene with a specific
chromosome

AP Biology
Classes of chromosomes
autosomal
chromosomes
sex
chromosomes
AP Biology
Sex linked traits

Drosophila breeding - good genetic subject




AP Biology
1910 | 1933
Prolific, females mate once & lay hundreds of eggs
Short 2 week generations
Only 4 pairs of chromosomes
XX=female, XY=male (Like humans)
Discovery of sex linkage
P
true-breeding red-eye
female “Wild Type”
F1
X
true-breeding
Mutant white-eye male
100%
red eye offspring
Huh!
Sex matters?!
generation
(hybrids)
F2
generation
AP Biology
100%
red-eye female
50% red-eye male
50% white eye male
What’s up with Morgan’s flies?
x
RR
r
R
Rr
x
rr
Rr
r
Rr
Rr
R
R
r
RR
Rr
Rr
rr
Doesn’t work
that way!
R
AP Biology
Rr
Rr
100% red eyes
r
3 red : 1 white
Genetics of Sex
 In humans & other mammals, there are 2
sex chromosomes: X & Y

2 X chromosomes
 develop as a female: XX
 gene redundancy,
like autosomal chromosomes

an X & Y chromosome
X
Y
X
XX
XY
X
XX
XY
 develop as a male: XY
 no redundancy
AP Biology
50% female : 50% male
Let’s reconsider Morgan’s flies…
x
XR XR
Xr
XR
XR
AP Biology
XR Xr
XR Xr
x
XrY
XR Xr
Y
XRY
XRY
100% red eyes
XR
BINGO!
Xr
XRY
XR
Y
XR XR
XRY
XR Xr
X rY
100% red females
50% red males; 50% white males
Genes on sex chromosomes
 Y chromosome

few genes other than SRY
 sex-determining region
 master regulator for maleness
 turns on genes for production of male hormones
 many effects = pleiotropy!
 X chromosome

AP Biology
other genes/traits beyond sex
determination
Human X chromosome
 Sex-linked
Duchenne muscular dystrophy
Becker muscular dystrophy
usually
means
“X-linked”
 more than
60 diseases
traced to
genes on X
chromosome

Chronic granulomatous disease
Retinitis pigmentosa-3
Norrie disease
Retinitis pigmentosa-2
Hypophosphatemia
Aicardi syndrome
Hypomagnesemia, X-linked
Ocular albinism
Retinoschisis
Adrenal hypoplasia
Glycerol kinase deficiency
Ornithine transcarbamylase
deficiency
Incontinentia pigmenti
Wiskott-Aldrich syndrome
Menkes syndrome
Androgen insensitivity
Sideroblastic anemia
Aarskog-Scott syndrome
PGK deficiency hemolytic anemia
Anhidrotic ectodermal dysplasia
Agammaglobulinemia
Kennedy disease
Pelizaeus-Merzbacher disease
Alport syndrome
Fabry disease
Immunodeficiency, X-linked,
with hyper IgM
Lymphoproliferative syndrome
Albinism-deafness syndrome
Fragile-X syndrome
AP Biology
Ichthyosis, X-linked
Placental steroid sulfatase deficiency
Kallmann syndrome
Chondrodysplasia punctata,
X-linked recessive
Charcot-Marie-Tooth neuropathy
Choroideremia
Cleft palate, X-linked
Spastic paraplegia, X-linked,
uncomplicated
Deafness with stapes fixation
PRPS-related gout
Lowe syndrome
Lesch-Nyhan syndrome
HPRT-related gout
Hunter syndrome
Hemophilia B
Hemophilia A
G6PD deficiency: favism
Drug-sensitive anemia
Chronic hemolytic anemia
Manic-depressive illness, X-linked
Colorblindness, (several forms)
Dyskeratosis congenita
TKCR syndrome
Adrenoleukodystrophy
Adrenomyeloneuropathy
Emery-Dreifuss muscular dystrophy
Diabetes insipidus, renal
Myotubular myopathy, X-linked
Human X-Linked Disorders
1. More males have X-linked traits
because recessive alleles on the X
chromosome in males are expressed in males.
2. Color Blindness
 can be an X-linked recessive disorder involving
mutations of genes coding for green or red
sensitive cone cells, resulting in the inability to
perceive green or red, respectively; the pigment
for blue-sensitive protein is autosomal.
 About 8% of Caucasian men have red-green
color blindness..
AP Biology
Human X-Linked Disorders
Muscular Dystrophy
a. Duchenne muscular dystrophy is the most common
form and is characterized by wasting away of muscles,
eventually leading to death; it affects one out of every
3,600 male births.
b. This X-linked recessive disease involves a mutant gene
that fails to produce the protein dystrophin.
c. Signs and symptoms (e.g., waddling gait, toe walking,
frequent falls, difficulty in rising) soon appear.
D. Muscles weaken until the individual is confined to a
wheelchair; death usually occurs by age 20.
AP Biology
Human X-Linked Disorders
e. Affected males are rarely fathers; the gene passes
from carrier mother to carrier daughter.
f. Lack of dystrophin protein causes calcium ions to
leak into muscle cells; this promotes action of an
enzyme that dissolves muscle fibers.
g. As the body attempts to repair
tissue, fibrous tissue forms and cuts
off blood supply to the affected
muscles.
h. A test now detects carriers of
Duchenne muscular dystrophy;
treatments are being attempted.
AP Biology
sex-linked recessive
Hemophilia
H Xh x X
HY
HH
XHh
XH
female / eggs
male / sperm
XH
XH
Y
XH XH
XH Y
XH Xh
Xh
XH
Xh
AP Biology
XH Xh
XhY
carrier disease
XHY
Y
Human X-Linked Disorders
Hemophilia
a.About one in 10,000 males is a
hemophiliac with impaired ability of
blood to clot.
b. The two common types: Hemophilia A,
due to the absence of clotting factor IX;
Hemophilia B, due to the absence of
clotting factor VIII.
c. Hemophiliacs bleed externally after an
injury and also suffer internal bleeding
AP Biology
around joints.
Human X-Linked Disorders
 d. Hemorrhages stop with transfusions
of blood (or plasma) or concentrates of
clotting protein.
 e. Factor VIII is now available as a
genetically-engineered product.
 f. Of Queen Victoria’s 26 offspring,
five grandsons had hemophilia and four
granddaughters were carriers.
AP Biology
AP Biology
Pedigree Practice
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Pedigree Practice
AP Biology
Map of Human Y chromosome?
< 30 genes on
Y chromosome
Sex-determining Region Y (SRY)
Channel Flipping (FLP)
Catching & Throwing (BLZ-1)
Self confidence (BLZ-2)
Devotion to sports (BUD-E)
Addiction to death &
destruction movies (SAW-2)
note: not linked to ability gene
Air guitar (RIF)
Scratching (ITCH-E)
Spitting (P2E)
Inability to express
affection over phone (ME-2)
AP Biology
linked
Selective hearing loss (HUH)
Total lack of recall for dates (OOPS)
Sex-linked traits summary
 X-linked
Follow the X chromosomes
 Males get their X from their mother
(female carriers)
 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

AP Biology
 Chromosomal conditions involving the sex
chromosomes often affect sex determination
(whether a person has the sexual
characteristics of a male or a female), sexual
development, and the ability to have children
(fertility). The signs and symptoms of these
conditions vary widely and range from mild
to severe. They can be caused by missing or
extra copies of the sex chromosomes or by
structural changes in these chromosomes.
AP Biology
X-inactivation
 Female mammals inherit 2X chromosomes

one X becomes inactivated during
embryonic development
 condenses into compact object = Barr body
 which X becomes Barr body is random
 patchwork trait = “mosaic”
patches of black
XH 
XH Xh
tricolor cats
can only be
AP Biology
female
Xh
patches of orange
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
AP Biology
Environmental effects
 Phenotype is controlled by
both environment & genes
Human skin color is influenced
by both genetics &
environmental conditions
Coat color in arctic
fox influenced by
heat sensitive alleles
Color of Hydrangea flowers
APinfluenced
Biology
is
by soil pH
Any Questions?
AP Biology
2006-
Studying Inheritance in Humans
1
AP Biology
3
4
2
5
2006-2007
6
Pedigree analysis
 Pedigree analysis reveals Mendelian
patterns in human inheritance

= male
AP Biology
data mapped on a family tree
= female
= male w/ trait
= female w/ trait
Simple pedigree analysis
11
33
AP Biology
44
What’s the
likely inheritance
pattern?
22
55
66
Genetic counseling
 Pedigree can help us understand the past

& predict the future
Thousands of genetic disorders are
inherited as simple recessive traits

from benign conditions to deadly diseases
 albinism
 cystic fibrosis
 Tay sachs
 sickle cell anemia
 PKU
AP Biology
Genetic testing
CVS –Chorionic
Villus Sampling
sequence
individual genes
AP Biology
Recessive diseases
 The diseases are recessive because the
allele codes for either a malfunctioning
protein or no protein at all

Heterozygotes (Aa)
 carriers
 have a normal phenotype because one
“normal” allele produces enough of the
required protein
AP Biology
Heterozygote crosses
 Heterozygotes as carriers of recessive alleles
Aa x Aa
female / eggs
male / sperm
A
a
AP Biology
A
a
AA
AA
Aa
Aa
A
Aa
a
carrier
Aa
Aa
aa
carrier
disease
A
Aa
a
Cystic fibrosis (recessive)
 Primarily whites of
European descent

strikes 1 in 2500 births
 1 in 25 whites is a carrier (Aa)

normal lung tissue
normal allele codes for a membrane protein
that transports Cl- across cell membrane
 defective or absent channels limit transport of Cl- & H2O
across cell membrane
 thicker & stickier mucus coats around cells
 mucus build-up in the pancreas, lungs, digestive tract &
causes bacterial infections

AP Biology
without treatment children die before 5;
with treatment can live past their late 20s
Chloride channel
Effect on Lungs
normal lungs
airway
Cl–
transports salt through protein
channel out of cell
Osmosis: H2O follows Cl–
Cl– channel
H 2O
cells lining
lungs
cystic fibrosis
Cl–
H 2O
bacteria & mucus build up
thickened mucus
hard to secrete
AP Biology
mucus secreting glands
delta F508
loss of one
amino acid
AP Biology
Tay-Sachs (recessive)
 Primarily Jews of eastern European (Ashkenazi)
descent & Cajuns (Louisiana)
 strikes 1 in 3600 births
 100 times greater than incidence among
non-Jews

non-functional enzyme fails to breakdown lipids
in brain cells
 fats collect in cells destroying their function
 symptoms begin few months
after birth
 seizures, blindness &
degeneration of muscle &
mental performance
 child usually dies before 5yo
AP Biology
Sickle cell anemia (recessive)
 Primarily Africans

strikes 1 out of 400 African Americans
 high frequency
caused by substitution of a single amino
acid in hemoglobin
 when oxygen levels are low, sickle-cell
hemoglobin crystallizes into long rods

 deforms red blood cells into
sickle shape
 sickling creates pleiotropic
effects = cascade of other
symptoms
AP Biology
Sickle cell anemia
 Substitution of one amino acid in
polypeptide chain
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hydrophilic
amino acid
hydrophobic
amino acid
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Sickle cell phenotype
 2 alleles are codominant
both normal & mutant hemoglobins are
synthesized in heterozygote (Aa)
 50% cells sickle; 50% cells normal
 carriers usually healthy
 sickle-cell disease
triggered under blood
oxygen stress

 exercise
AP Biology
Heterozygote advantage
 Malaria

single-celled eukaryote parasite spends part of its
life cycle in red blood cells

Sickle cell frequency
 High frequency of heterozygotes is unusual for
allele with severe detrimental effects in
homozygotes.
 1 out of 400 African Americans
 Suggests some selective advantage of being
heterozygous.
AP Biology
Heterozygote advantage
 In tropical Africa, where malaria is common:



homozygous dominant individuals die of
malaria
homozygous recessive individuals die of
sickle cell anemia
heterozygote carriers are relatively free of both
 reproductive advantage
 High frequency of sickle
cell allele in African
Americans is vestige of
African roots•
AP Biology
Prevalence of Malaria
Prevalence of Sickle
Cell Anemia
AP Biology
Huntington’s chorea (dominant)
 Dominant inheritance

Testing…
Would you
want to
know?

1872
repeated mutation on end of
chromosome 4
 mutation = CAG repeats
 glutamine amino acid repeats in protein
 one of 1st genes to be identified
build up of “huntingtin” protein in brain causing cell
death
 memory loss
 muscle tremors, jerky movements
 “chorea”
 starts at age 30-50
 early death
 10-20 years after start
AP Biology
Genetics & culture
 Why do all cultures have a taboo against incest?

laws or cultural taboos forbidding marriages
between close relatives are fairly universal
 Fairly unlikely that 2 unrelated carriers of same
rare harmful recessive allele will meet & mate


AP Biology
but matings between close relatives increase risk
 “consanguineous” (same blood) matings
individuals who share a
recent common ancestor
are more likely to carry
same recessive alleles
A hidden disease reveals itself
Aa
x
Aa
male / sperm
male / sperm
A
A
A
a
A
AA
AA
A
AA
Aa
a
Aa
Aa
a
Aa
aa
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female / eggs
female / eggs
AA x Aa
• increase carriers in population
• hidden disease is revealed
Any questions?
AP Biology
2006-2007
Errors of Meiosis
Chromosomal Abnormalities
AP Biology
2006-2007
Chromosomal abnormalities
 Incorrect number of chromosomes

nondisjunction
 chromosomes don’t separate properly
during meiosis

breakage of chromosomes
 deletion
 duplication
 inversion
 translocation
AP Biology
Nondisjunction
 Problems with meiotic spindle cause errors in
daughter cells



2n
Tetrad chromosomes do not separate properly
during Meiosis 1
sister chromatids fail to separate during Meiosis 2
too many or too few chromosomes
n-1
n
n+1
n
AP Biology
Alteration of chromosome number
error in Meiosis 1
error in Meiosis 2
all with incorrect number
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1/2 with incorrect number
Nondisjunction
 Baby has wrong chromosome number

trisomy
 cells have 3 copies of a chromosome

n+1
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monosomy
 cells have only 1 copy of a chromosome
n-1
n
n
trisomy
monosomy
2n+1
2n-1
Human chromosome disorders
 High frequency in humans



most embryos are spontaneously aborted
alterations are too disastrous
developmental problems result from biochemical
imbalance
 Certain conditions are tolerated


AP Biology
upset the balance less = survivable
but characteristic set of symptoms = syndrome
Down syndrome
 Trisomy 21
3 copies of chromosome 21
 1 in 700 children born in U.S.

 Chromosome 21 is the
smallest human chromosome

but still severe effects
 Frequency of Down
syndrome correlates
with the age of the mother
AP Biology
Down syndrome & age of mother
Mother’s age
Incidence of
Down Syndrome
Under 30
<1 in 1000
30
1 in 900
35
1 in 400
36
1 in 300
37
1 in 230
38
1 in 180
39
1 in 135
40
1 in 105
42
1 in 60
44
1 in 35
46
1 in 20
48
1 in 16
49
1 in 12
AP Biology
Rate of miscarriage due to
amniocentesis:
 1970s data
0.5%, or 1 in 200 pregnancies
 2006 data
<0.1%, or 1 in 1600 pregnancies
Genetic testing
 Amniocentesis in 2nd trimester


sample of embryo cells
stain & photograph chromosomes
 Analysis of karyotype
 CVS – Chorionic Villus Sampling
AP Biology
Sex chromosomes abnormalities
 Human development more tolerant of
wrong numbers in sex chromosome
 But produces a variety of distinct
syndromes in humans




AP Biology
XXY = Klinefelter’s syndrome male
XXX = Trisomy X female
XYY = Jacob’s syndrome male
XO = Turner syndrome female
Klinefelter’s syndrome
 XXY male
one in every 2000 live births
 have male sex organs, but
are sterile
 feminine characteristics

 some breast development
 lack of facial hair
tall
 normal intelligence

AP Biology
Klinefelter’s syndrome
AP Biology
Jacob’s syndrome male
 XYY Males
1 in 1000 live male
births
 extra Y chromosome
 slightly taller than
average
 more active
 normal intelligence, slight learning disabilities
 delayed emotional maturity
 normal sexual development

AP Biology
Trisomy X
 XXX
1 in every 2000 live births
 produces healthy females

 Why?
 Barr bodies
 all but one X chromosome is inactivated
AP Biology
Turner syndrome
 Monosomy X or X0
1 in every 5000 births
 varied degree of effects
 webbed neck
 short stature
 sterile

AP Biology
error of
replication
Changes in chromosome structure
 deletion

 duplication

error of
crossing over
loss of a chromosomal segment
repeat a segment
 inversion

reverses a segment
 translocation

AP Biology
move segment from one chromosome
to another
Don’t hide…
Ask Questions!!
AP Biology
2006-2007