Patterns of inheritance

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Patterns of inheritance

• Chapter 9

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• The first genetics research lab was a monks
  abbey garden

The same genetic defect that causes sickle-cell
disease can also protect you against malaria
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• In certain isolated communities, one out of 100
  boys is born with the genetic defect that causes
  Duchenne muscular dystrophy

Intermarriage caused the disease hemophilia to
spread through the royal families of Europe
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TESTING YOUR BABY

• Genetic testing

Allows expectant parents to test for
possibilities in their unborn child Includes
amniocentesis and CVS Has risks associated with it
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HERITABLE VARIATION AND PATTERNS OF INHERITANCE

• Wild type traits are traits most commonly found
  in nature

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• Various traits exist in organisms

These traits are usually inherited in particular
patterns
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Parents
Parents
Wild-type
Sky-blue
Wild-type
Wild-type
First- generation offspring
Wild-type
wild-type
Offspring
All
All
Matings
Second- generation offspring
(a) Offspring from the mating of two wild-type
birds
and
1/4
3/4
Sky-blue
Wild-type
(b) Two generations of offspring from the mating
of a wild-type with a sky-blue bird
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• Gregor Mendel

Was the first person to analyze patterns of
inheritance Deduced the fundamental principles of
genetics
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In an Abbey Garden

• Mendel studied garden peas

These plant are easily manipulated These plants
can self-fertilize
Stamen
Carpel
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• Mendel carried out some cross-fertilization

White
1
Removed stamens from purple flower
Stamens
2
Transferred pollen from stamens of white flower
to carpel of purple flower
Carpel
Purple
Parents (P)
3
Pollinated carpel matured into pod
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Planted seeds from pod
Offspring (F1)
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Mendel then crossed two different true-breeding
varieties, creating hybrids

• He also created true-breeding varieties of plants

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Mendels Principles of Segregation

• Mendel performed many experiments

He tracked several characteristics in pea plants
from which he formulated several hypotheses
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Dominant
Recessive
Dominant
Recessive
Pod shape
Constricted
Inflated
Flower color
White
Purple
Pod Color
Green
Yellow
Flower position
Axial
Terminal
Seed color
Yellow
Green
Stem length
Dwarf
Tall
Seed shape
Round
Wrinkled
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Monohybrid Crosses
P Generation (true-breeding parents)

• A monohybrid cross is a cross between parent
  plants that differ in only one characteristic

Purple flowers
Whiteflowers
All plants have purple flowers
F1 Generation
Fertilization among F1 plants (F1 ? F1)
F2 Generation
1/4 of plants have white flowers
3/4 of plants have purpleflowers
(a) Mendels crosses tracking one characteristic
(flower color)
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• Mendel developed four hypotheses from the
  monohybrid cross

There are alternative forms of genes, now called
alleles For each characteristic, each organism
has two genes Gametes carry only one allele for
each inherited characteristic Alleles can be
dominant or recessive
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Genetic makeup (alleles)

• An explanation of Mendels results, including a
  Punnett square

pp
PP
P plants
All
P
p
Gametes
All
F1 plants (hybrids)
All Pp
P
Gametes
1/2
1/2
p
P
P
Sperm
Eggs
F2 plants
PP
p
p
Pp
Pp
Phenotypic ratio 3 purple 1 white
pp
Genotypic ratio 1 PP 2 Pp 1 pp
(b) Explanation of the results in part (a)
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• Phenotype

An organisms physical traits Genotype An
organisms genetic makeup
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• Purple and white sweet pea flowers

Principle of segregation Pairs of alleles
segregate (separate) during gamete formation the
fusion of gametes during fertilization pairs
alleles once again
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Using a Punnett square derive the phenotypes and
genotypes in the F1 and F2 generations (from
self-fertilization of F1) of the following
crosses between true-bred plants

• PTall (TT) x Dwarf (tt)
• F1
• F2

• Round x Wrinkled
• seeds (RR) seeds (rr)
• F1
• F2

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Genetic Alleles and Homologous Chromosomes

• Homologous chromosomes

Have genes at specific loci Have alleles of a
gene at the same locus
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Homologous chromosomes
Gene loci
Dominant allele
a
P
B
a
b
P
Recessive allele
aa
Bb
PP
Genotype
Homozygous for the dominant allele
Heterozygous
Homozygous for the recessive allele
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• Homozygous

When an organism has identical alleles for a
gene Heterozygous When an organism has different
alleles for a gene
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Mendels Principle of Independent Assortment

• Two hypotheses for gene assortment in a dihybrid
  cross

Dependent assortment Independent assortment
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Hypothesis (a) Dependent assortment (b)
Independent assortment
RRYY
rryy
P Generation
rryy
RRYY
ry
Gametes
RY
ry
Gametes
RY
F1 Generation
RrYy
RrYy
1/4
RY
RY
1/4
Sperm
Eggs
Eggs
Sperm
1/4
rY
rY
1/2
1/4
RY
1/2
RY
RRYY
1/4
ry
1/2
1/2
Ry
Ry
ry
RrYY
RrYY
1/4
F2 Generation
ry
1/4
ry
RRYy
rrYY
RRYy
1/4
RrYy
RrYy
RrYy
RrYy
Yellow round
9/16
RRyy
Green round
rrYy
rrYy
3/16
Yellow wrinkled
Rryy
Actual results contradict hypothesis
Rryy
3/16
rryy
1/16
Green wrinkled
Actual results support hypothesis
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Mendels principle of independent assortment
Each pair of alleles segregates independently of
the other pairs during gamete formation
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Blind
Blind
Phenotypes
Chocolate coat, normal vision bbN_
Chocolate coat, blind (PRA) bbnn
Black coat, blind(PRA) B_nn
Black coat, normal vision B_N_
Genotypes
(a)
Mating of heterozygotes (black, normal vision)
BbNn
BbNn
Phenotypic ratio of offspring
1 chocolate coat, blind (PRA)
3 chocolate coat, normal vision
9 black coat, normal vision
3 black coat, blind (PRA)
(b)
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Using a Testcross to Determine an Unknown Genotype

• A testcross is a mating between

Testcross
Genotypes
An individual of unknown genotype and A
homozygous recessive individual
P_
pp
Two possibilities for the purple flower
PP
Pp
Gametes
P
p
P
P
p
Pp
pp
Pp
Offspring
1 purple 1 white
All purple
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The Rules of Probability

• The rule of multiplication

F1 Genotypes
B b female
B b male
Formation of eggs
Formation of sperm
The probability of a compound event is the
product of the separate probabilities of the
independent events
1/2
B
B
1/2
1/2
B
B
b
b
1/4
1/2
(1/2 ? 1/2)
b
B
B
b
1/4
1/4
b
b
F2 Genotypes
1/4
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What is the probability of a bb offspring from
a Bb x Bb cross? P Bb x
Bb Gametes B (½) b (½) B (½) b
(½) F1 bb ½ ½ ¼
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What are the chances of getting a Bb outcome
from a Bb x Bb cross? P Bb x Bb Gametes B
(½) b (½) B (½) b (½) F1 Bb
Bb ½½ ¼ and ½½ ¼ ¼ ¼ ½
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Two organisms have the genotype AaBbCc. What is
the probability that an offspring with genotype
aabbcc will be produced from their
cross? AaBbCc x AaBbCc ? aabbcc A
trihybrid cross is equivalent to three monohybrid
crosses Aa x Aa probability of aa
offspring ½ ½ ¼ Bb x Bb probability of
bb offspring ½ ½ ¼ Cc x Cc probability
of cc offspring ½ ½ ¼ Thus, probability
of an aabbcc offspring ¼¼¼ 1/64
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Family Pedigrees
Recessive Traits
Dominant Traits

• Mendels principles apply to the inheritance of
  many human traits

Freckles
No freckles
Straight hairline
Widows peak
Free earlobe
Attached earlobe
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• A family pedigree

Shows the history of a trait in a family Allows
researchers to analyze human traits
Female
Male
Deaf
Joshua Lambert
Abigail Linnell
John Eddy
Hepzibah Daggett
Hearing
Abigail Lambert
Jonathan Lambert
Elizabeth Eddy
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Female
Male
Deaf
Dd Joshua Lambert
Dd Abigail Linnell
D_ John Eddy
D_ Hepzibah Daggett
Hearing
D_ Abigail Lambert
dd Jonathan Lambert
Dd Elizabeth Eddy
Dd
Dd
dd
Dd
Dd
Dd
dd
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Human Disorders Controlled by a Single Gene

• Many human traits

Show simple inheritance patterns Are controlled
by genes on autosomes
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Recessive Disorders

• Most human genetic disorders are recessive

Individuals can be carriers of these diseases
Normal Dd
Normal Dd
Parents
D
D
Eggs
Sperm
DD Normal
d
d
Dd Normal (carrier)
Dd Normal (carrier)
Offspring
dd Deaf
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Dominant Disorders

• Some human genetic disorders are dominant

Achondroplasia is a form of dwarfism
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Fetal testing can spot many inherited disorders
early in pregnancy

• Karyotyping and biochemical tests of fetal cells
  and molecules can help people make reproductive
  decisions
• Fetal cells can be obtained through amniocentesis

Amnioticfluidwithdrawn
Centrifugation
Amnioticfluid
Fluid
Fetalcells
Fetus(14-20weeks)
Biochemicaltests
Placenta
Severalweeks later
Uterus
Cervix
Karyotyping
Cell culture
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• Chorionic villus sampling is another procedure
  that obtains fetal cells for karyotyping

Fetus(10-12weeks)
Several hourslater
Placenta
Suction
Karyotyping
Fetal cells(from chorionic villi)
Some biochemical tests
Chorionic villi
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• Examination of the fetus with ultrasound is
  another helpful technique

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Genetic testing can detect disease-causing alleles

• Genetic testing can be of value to those at risk
  of developing a genetic disorder or of passing it
  on to offspring

Dr. David Satcher, former U.S. surgeon general,
pioneered screening for sickle-cell disease
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BEYOND MENDEL

• Some patterns of genetic inheritance are not
  explained by Mendels principles

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Incomplete Dominance in Plants and People
P Generation

• In incomplete dominance F1 hybrids have an
  appearance in between the phenotypes of the two
  parents

Red RR
White rr
r
Gametes
R
F1 Generation
Pink Rr
1/2
1/2
r
R
Gametes
R
1/2
R
Sperm
1/2
Eggs
Red RR
1/2
r
r
1/2
Pink rR
Pink Rr
F2 Generation
White rr
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• Incomplete dominance in carnations
• red, pink, white

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• Hypercholesterolemia

Is a human trait that is incompletely dominant
LDL (carries cholesterol)
LDL receptor (mops up LDL)
Cell
Normal HH
Mild disease Hh
Severe disease hh
Genotypes
Heterozygous
Homozygous for ability to make LDL receptors
Homozygous for inability to make LDL receptors
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Multiple Alleles and Blood Type

• The ABO blood groups in humans are examples of
  multiple alleles
• Two of the human blood type alleles exhibit
  codominance
• Discovered by Karl Landsteiner (1930 Nobel
  Prize in Physiology and Medicine)

• Both alleles are expressed in the phenotype

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Blood Antibody Antigen Type Genotype
(serum) (RBC) AB IAIB - AgA,
AgB A IAIA, IAi B AgA - B IBIB,
IBi A - AgB O ii AB - -
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Blood group identification
Antibody ?
Blood grouping ?
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Pleiotropy and Sickle-Cell Disease

• Pleiotropy is the impact of a single gene on more
  than one characteristic

Sickle-cell disease is an example
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RBC normal and sickle shaped
(x 9,620)
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Individual homozygousfor sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes, causing red
blood cells to become sickle-shaped
Sickled cells
Clumping of cells and clogging of small blood
vessels
Accumulation of sickled cells in spleen
Breakdown of red blood cells
Damage to other organs
Heart failure
Brain damage
Physical weakness
Spleen damage
Pain and fever
Anemia
Impaired mental function
Pneumonia and other infections
Kidney failure
Paralysis
Rheumatism
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Polygenic Inheritance
P Generation
AABBCC (very dark)
aabbcc (very light)
F1 Generation
AaBbCc
AaBbCc

• Polygenic inheritance is the additive effects of
  two or more genes on a single phenotype

F2 Generation
Eggs
Sperm
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Inheritance of coat color in mice Cross
between heterozygous yellow mice give a 21 ratio
instead of a 31 ratio
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The Role of Environment

• Many human characteristics result from a
  combination of heredity and environment

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THE CHROMOSOMAL BASIS OF INHERITANCE

• The chromosome theory of inheritance states that

Genes are located on chromosomes The behavior of
chromosomes during meiosis and fertilization
accounts for inheritance patterns
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Green-wrinkled seeds (yyrr)
Yellow-round seeds (YYRR)
P Generation
Meiosis
Fertilization
Gametes
F1 Generation
All round yellow seeds (RrYy)
Principle of Independent Assortment Follow both
the Long and the short chromosomes.
Principle of Segregation Follow the long
chromosomes (carrying R and r) taking either the
left or right branch.
Meiosis
They are arranged in either of two equally
likely ways at metaphase I.
The R and r alleles segregate in anaphase I of
meiosis.
Metaphase I (alternative arrangements)
They assort independently, giving four gamete
types.
Only one long chromosome ends up in each gamete.
Metaphase II
Gametes
Fertilization recombines the r And R alleles at
random.
Fertilization results in the 9331
phenotypic ratio in the F2 generation.
Fertilization among the F1 plants
F2 Generation
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Gene Linkage

• In 1908, British biologists discovered an
  inheritance pattern inconsistent with Mendelian
  principles

Experiment
Purple flower
Long Pollen
PpLl
PpLl
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• This inheritance pattern was later explained by
  linked genes, which are

(b) Explanation Linked genes
Parental diploid cell PpLl
Meiosis
Most gametes
Genes located on the same chromosome Genes that
are typically inherited together
Fertilization
Sperm
Egg
Most offspring
3 purple long 1 red round Not accounted for
purple round and red long
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Genetic Recombination Crossing Over and Linkage
Maps

• Two linked genes

Can give rise to four different gamete
genotypes Can sometimes cross over
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(c) Explanation Crossing over
Tetrad
Crossing over
Parental-type gametes
Recombined gametes
Fertilization
Purple round
Red long
Among the offspring, some with recombinant
phenotypes
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• Early studies of crossing over were performed
  using the fruit fly
• Drosophila melanogaster

Wild type (gray body, long wings, red eyes)
Variant (black body, short wings, cinnabar eyes)
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• Studies using Drosophila

Developed a method for mapping gene loci Resulted
in linkage maps
Chromosome
Black body Cinnabar eyes Short wings
c
g
l
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9
9.5
Recombination frequencies
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Genes on the same chromosome tend to be inherited
together

• Certain genes are linked
• They tend to be inherited together because they
  reside close together on the same chromosome

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Geneticists use crossover data to map genes

• Crossing over is more likely to occur between
  genes that are farther apart
• Recombination frequencies can be used to map the
  relative positions of genes on chromosomes

Chromosome
g
c
l
17
9
9.5
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• Alfred H. Sturtevant, seen here at a party with
  T. H. Morgan and his students, used recombination
  data from Morgans fruit fly crosses to map genes

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• A partial genetic map of a fruit fly chromosome

Mutant phenotypes
Shortaristae
Black body (g)
Cinnabar eyes (c)
Vestigial wings (l)
Browneyes
Long aristae(appendageson head)
Gray body (G)
Red eyes (C)
Normal wings (L)
Redeyes
Wild-type phenotypes
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SEX CHROMOSOMES AND SEX-LINKED GENES

• Sex chromosomes

Influence the inheritance of certain traits
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Sex Determination Human and Fruit Flies
Male
Female
Somatic cells
44 XX
44 XY

• Sex chromosomes

Are designated X and Y Determine an individuals
sex
22 X
22 Y
22 X
Sperm
Egg
44 XY
44 XX
Male
Female
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• Other systems of sex determination exist in
  other animals and plants

The X-O system, where O stands for absence of sex
chromosome
The Z-W system, where sex is determined by the egg
Chromosome number
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Sex-linked genes exhibit a unique pattern of
inheritance

• All genes on the sex chromosomes are said to be
  sex-linked
• - In many organisms, the X chromosome carries
• many genes unrelated to sex
• - Were discovered in fruit flies, eye color is a
  sex-linked characteristic

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Male and female fruit flies Drosophila
Female with wild type eye
Male with mutant white eye
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• Their inheritance pattern reflects the fact that
  males have one X chromosome and females have two

These figures illustrate inheritance patterns for
white eye color (r) in the fruit fly, an X-linked
recessive trait
Female
Male
Female
Male
Female
Male
XrY
XRXR
XRXr
XRY
XRXr
XrY
XR
Xr
XR
XR
XR
Xr
Y
XRXr
XRXR
XRXr
Y
Y
Xr
Xr
XRY
XrXR
XRY
XrXr
XRY
XrY
XrY
R red-eye allele r white-eye allele
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• Inheritance patterns of a sex-linked gene

Male
Female
Male
Female
Male
Female
XRXR
XrY
XRXr
XRY
XRXr
XrY
All females inherit two X chromosomes, one from
each parent.
Female
Female
All males inherit one X chromosome, always from
the mother.
R red-eye allele r white-eye allele
Male
Male
(c) Heterozygous female? white-eyed male
(b) Heterozygous female ? red-eyed male
(a) Homozygous red-eyed female ? white-eyed male
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Sex-Linked Disorders in Humans

• A number of human conditions result from
  sex-linked (X-linked) genes

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• Red-green color blindness

Is characterized by a malfunction of
light-sensitive cells in the eyes
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Queen Victoria
Albert

• Hemophilia

is a blood-clotting disease
Louis
Alice
Czar Nicholas II of Russia
Alexandra
Alexis
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• Duchenne muscular dystrophy

Is characterized by a progressive weakening and
loss of muscle tissue
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EVOLUTION CONNECTIONTHE TELLTALE Y CHROMOSOME

• Sex chromosomes

Influence the inheritance of certain traits
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• The Y chromosome of human males is only about
  one-third the size of the X chromosome

Scientists believe that X and Y were once a fully
homologous pair Major episodes of change have
rearranged pieces of the Y chromosome
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• Researchers recently used comparisons of Y DNA to
  confirm that the Lemba tribe in Africa descended
  from ancient Jews

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SUMMARY OF KEY CONCEPTS

• Mendels Principle of Segregation

Alleles
Meiosis
Fertilization
Diploid zygote (contains paired alleles)
Diploid cell (contains paired alleles,
alternate forms of a gene)
Haploid gametes (allele pairs separate)
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• Using a Testcross to Determine an Unknown Genotype

Phenotype
Recessive pp
Dominant P?
Genotype
or
Phenotype
1 dominant 1 recessive
All dominant
Unknown parent was PP
Conclusion
Unknown parent was Pp
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• Incomplete Dominance in Plants and People

Recessive phenotype
Dominant phenotype
rr
RR
Intermediate phenotype (incomplete dominance)
Rr
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• Pleiotropy and Sickle-Cell Disease

Pleiotropy
Multiple traits
Single gene
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• Polygenic Inheritance

Polygenic inheritance
Single trait (e.g., skin color)
Multiple genes
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• Sex Determination in Humans and Fruit Flies

Female
Male
44 XY
44 XX
Somatic cells