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Genetic Inheritance
MENDEL & MUTATIONS
Father of Genetics
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Monk and teacher.
Experimented with purebred tall and short
peas.
Discovered some of the basic laws of
heredity.
Studied seven purebred traits in peas.
Called the stronger hereditary factor
dominant.
Called the weaker hereditary factor
recessive.
Presentation to the Science Society
in1866 went unnoticed.
He died in 1884 with his work still
unnoticed.
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His work rediscovered in 1900.
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Known as the “Father of Genetics”.
Mendel’s Observations
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He noticed that peas are easy to breed for
pure traits and he called the pure strains
purebreds.
He developed pure strains of peas for seven
different traits (i.e. tall or short, round or
wrinkled, yellow or green, etc.)
He crossed these pure strains to produce
hybrids.
He crossed thousands of plants and kept
careful records for eight years.
Mendel’s Peas
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In peas many traits appear in two forms (i.e. tall
or short, round or wrinkled, yellow or green.)
The flower is the reproductive organ and the
male and female are both in the same flower.
He crossed pure strains by putting the pollen
(male gamete) from one purebred pea plant on
the pistil (female sex organ) of another purebred
pea plant to form a hybrid or crossbred.
Analyzing Mendel’s Results
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Analyses using Punnett squares
demonstrate that Mendel’s results reflect
independent segregation of gametes.
The Testcross:
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Can be used to determine the genotype of an
individual when two genes are involved.
MENDEL’S LAWS OF HEREDITY
WHY MENDEL SUCCEEDED
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Gregor Mendol – father of genetics
1st studies of heredity – the passing of
characteristics to offspring
Genetics – study of heredity
The characteristics passed on called traits
PHENOTYPES & GENOTYPES
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PHENOTYPE – THE WAY AN ORGANISM
LOOKS AND BEHAVES – ITS PHYSICAL
CHARACTERISTICS (i.e. – TALL, GREEN,
BROWN HAIR, BLUE EYES, ETC.)
GENOTYPE – THE GENE COMBONATION
(ALLELIC COMBINATION) OF AN
ORGANISM – (i.e. – TT, Tt, tt, ETC.)
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HOMOZYGOUS – 2 ALLELES ARE THE SAME
HETEROZYGOUS – 2 ALLELES DIFFERENT
From Genotype to Phenotype
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Multiple Alleles:
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Sometimes more than two alleles (multiple alleles)
exist for a given trait in a population.
EX. ABO blood designation.
A and B are codominant.
Rh Blood group:
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Rh is a cell surface marker on red blood cells
About 85% of the population is Rh+ (have the marker)
Problems: Mother is Rh negative has an Rh+ fetus.
MENDEL CHOSE HIS SUBJECT
CAREFULLY
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Used garden peas to study
Have male & female gametes (sex cells)
Male & female same flower
Know what pollination & fertilization mean
He could control the fertilization process
Not many traits to keep track of
PUNNETT SQUARES
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A QUICK WAY TO FIND THE GENOTYPES
IN UPCOMING GENERATIONS
1ST DRAW A BIG SQUARE AND DIVIDE IT
IN 4’S
PUNNETT SQUARE
CROSS T T X Tt
CONT’D
TT X Tt
T
T
T
T
T
T
T
t
T
t
T
t
MENDEL WAS A CAREFUL
RESEARCHER
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USED CAREFULLY CONTROLLED
EXPERIMENTS
STUDIED ONE TRAIT AT A TIME
KEPT DETAILED DATA
MENDEL’S MONOHYBRID
CROSSES
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MENDEL STUDIED 7 TRAITS CAREFULLY
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11.1
Mendel crossed plants w/ diff. traits to see
what traits the offspring would have
These offspring are called hybrids –
offspring of parents w/ different traits
A monohybrid cross is one that looks at
only one trait (let’s look at plant height –
tall or short)
THE 1ST GENERATION
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Mendel crossed two plants – 1 tall & 1
short (they came from tall & short
populations)
These plants are called the parental
generation (P generation)
The offspring were all called the 1st filial
generation (F1 generation)
All the offspring were tall (the short plants
were totally excluded)
THE 2ND GENERATION
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Next, Mendel crossed two plants from the
F1 generation
The offspring from this cross are called
the 2nd filial generation (F2 GENERATION)
Mendel found that ¾ of the offspring
were tall & ¼ were short (the short plants
reappeared!!!!!!)
Mendel Proposes a Theory
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By convention, genetic traits are assigned a
letter symbol referring to their more common
form
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dominant traits are represented by uppercase letters,
and lower-case letters are used for recessive traits
for example, flower color in peas is represented as
follows
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P signifies purple
p signifies white
Mendel Proposes a Theory
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The results from a cross between a true-breeding, whiteflowered plant (pp) and a true breeding, purple-flowered
plant (PP) can be visualized with a Punnett square
A Punnett square lists the possible gametes from one
individual on one side of the square and the possible
gametes from the other individual on the opposite side
The genotypes of potential offspring are represented
within the square
A Punnett square analysis
How Mendel analyzed flower color
TO GO ANY FURTHER, WE
MUST UNDERSTAND ALLELES,
DOMINANCE, & SEGREGATION
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Genes – a section of DNA that codes for
one protein
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These genes are what control & produce traits
The genes Mendel studied came in two
forms (tall/short - round/wrinkled
yellow/green…….etc.)
Alternate forms of a gene are called alleles
Alleles are represented by a one or two
letter symbol (e.g. T for tall, t for short)
ALLELES CONT’D
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THESE 2 ALLELS ARE NOW KNOWN TO BE
FOUND ON COPIES OF CHROMOSOMES –
ONE FROM EACH PARENT
THE RULE OF DOMINANCE
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A dominant trait is the trait that will always be
expressed if at least one dominant allele is
present
The dominant allele is always represented by
a capital letter
A recessive trait will only be expressed if both
alleles are recessive
Recessive traits are represented by a lower
case letter
DOMINANCE CONT’D
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LET’S USE TALL & SHORT PEA PLANTS
FOR AN EXAMPLE
WHICH OF THESE WILL SHOW THE
DOMINANT & RECESSIVE TRAIT?
TT
Tt
DOMINANT TRAIT
tt
RECESSIVE TRAIT
THE LAW OF SEGREGATION
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MENDEL ASKED HIMSELF……..”HOW DID
THE RECESSIVE SHORT PLANTS
REAPPEAR IN THE F2 GENERATION?”
HE CONCLUDED THAT EACH TALL PLANT
FROM THE F1 GENERATION CARRIED
TWO ALLELES, 1 DOMINANT TALL ALLELE
& ONE RECESSIVE SHORT ALLELE
SO ALL WERE Tt
SEGREGATION CONT’D
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HE ALSO CONCLUDED THAT ONLY ONE
ALLELE FROM EACH PARENT WENT TO
EACH OFFSPRING
HIS CORRECT HYPOTHESIS WAS THAT
SOMEHOW DURING FERTILIZATION, THE
ALLELES SEPARATED (SEGREGATED) &
COMBINED WITH ANOTHER ALLELE
FROM THE OTHER PARENT
The law of segregation states that during
gamete formation, the alleles separate to
different gametes
F1 GENERATION
TT
FATHER
MOTHER
Tt
T t
Tt
tt
F2 GENERATION
- the law of dominance explained the
heredity of the offspring of the f1
generation
- the law of segregation explained the
heredity of the f2 generation
DIHYBRID CROSS
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TOOK TWO TRUE BREEDING PLANTS FOR
2 DIFFERENT TRAITS (ROUND/WRINKLED
SEEDS ------- YELLOW/GREEN SEEDS)
1ST GENERATION
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WHAT WOULD HAPPEN IF HE CROSSED JUST
TRUE BREEDING ROUND W/ TRUE BREEDING
WRINKLED (ROUND IS DOMINANT)
ALL THE OFFSPRING ARE
ROUND
DIHYBRID CROSS – 1ST
GENERATION CONT’D
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SO WHAT DO YOU THINK HAPPENED
WHEN HE CROSSED TRUE BREEDING
ROUND/YELLOW SEEDS WITH TRUE
BREEDING WRINKLED/GREEN SEEDS
ALL THE F1 WERE ROUND
AND YELLOW
DIHYBRID CROSS – 2ND
GENERATION
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TOOK THE F1 PLANTS AND BRED THEM
TOGETHER (PHENOTYPE WAS
ROUND/YELLOW X ROUND/YELLOW)
2ND GENERATION
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FOUND ROUND/YELLOW
FOUND ROUND/GREEN
FOUND WRINKLED/YELLOW
FOUND WRINKLED/GREEN
( 9 : 3 : 3 : 1 RATIO)
-9
-3
-3
-1
EXPLANATION OF 2ND
GENERATION
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MENDEL CAME UP W/ 2ND LAW – THE
LAW OF INDEPENDENT ASSORTMENT
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GENES FOR DIFFERENT TRAITS ARE
INHERITED INDEPENDENTLY FROM EACH
OTHER
THIS IS WHY MENDEL FOUND ALL THE
DIFFERNENT COMBONATIONS OF TRAITS
DIHYBRID CROSSES
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A LITTLE DIFFERENT
HhGg X HhGg
MUST FIND OUT ALL THE POSSIBLE
ALLELIC COMBONATIONS
USE THE FOIL METHOD LIKE IN MATH
FOIL – FIRST, OUTSIDE, INSIDE, LAST
H hGg X HhGg
1. HG
2. Hg
3. hG
4. hg
BOTH PARENTS
ARE THE SAME
NOW LET’S DO A DIHYBRID
CROSS
HhGg X HhGg
HG
Hg
hG
HG
HHGG
HHGg
HhGG
HhGg
Hg
HHGg
HHgg
HhGg
Hhgg
hG
HhGG
HhGg
hhGG
hhGg
hg
HhGg
Hhgg
hhGg
hhgg
hg
WHAT ARE THE PHENOTYPIC
RATIO’S?
HhGg X HhGg
HG
Hg
hG
HG
HHGG
HHGg
HhGG
HhGg
Hg
HHGg
HHgg
HhGg
Hhgg
hG
HhGG
HhGg
hhGG
hhGg
hg
HhGg
Hhgg
hhGg
hhgg
hg
Analysis of a dihybrid cross
PROBABILITY
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WILL REAL LIFE FOLLOW THE RESULTS
FROM A PUNNETT SQUARE?
NO!!!!!! – A PUNNETT SQUARE ONLY
SHOWS WHAT WILL PROBABLY OCCUR
IT’S A LOT LIKE FLIPPING A COIN – YOU
CAN ESTIMATE YOUR CHANCES OF
GETTING HEADS, BUT REALITY DOESN’T
ALWAYS FOLLOW PROBABILITY
MEIOSIS
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GENES, CHROMOSOMES, AND NUMBERS
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CHROMOSOMES HAVE 100’S OR 1000’S OF
GENES
GENES FOUND ON CHROMOSOMES
DIPLOID & HAPLOID CELLS
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ALL BODY CELLS
(SOMATIC CELLS)
HAVE
CHROMOSOMES
IN PAIRS
BODY CELLS ARE
CALLED DIPLOID
CELLS (2n)
HUMANS HAVE
THE 2n # OF
CHROMOSOMES
DIPLOID AND HAPLOID CELLS
CONT’D
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HAPLOID CELLS
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ONLY HAVE 1 OF EACH TYPE OF
CHROMOSOME (DIPLOID CELLS HAVE 2 OF
EACH TYPE)
SYMBOL IS (n)
SEX CELLS HAVE THE n # OF
CHROMOSOMES
HOMOLOGOUS CHROMOSOMES
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HOMOLOGOUS CHROMOSOMES ARE THE
PAIRED CHROMOSOMES THAT CONTAIN THE
SAME TYPE OF GENTIC INFORMATION, SAME
BANDING PATTERNS, SAME CENTROMERE
LOCATION, ETC.
THEY MAY HAVE DIFFERENT ALLELES, SO NOT
PERFECTLY IDENTICAL
WHY DO THEY HAVE DIFFERENT ALLELES?
CAME FROM DIFFERENT
PARENTS
IMPORTANT THINGS TO KNOW
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CROSSING OVER – OCCURS DURING
PROPHASE I
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CREATES GENETIC VARIABILITY (RECOMBINATION
OF GENES)
IN MEIOSIS I, HOMOLOGOUS CHROMOSOMES
SEPARATE (ANAPHASE I)
IN MEIOSIS II, SISTER CHROMATIDS SEPARATE
TETRAD – WHAT THE HOMOLOGOUS
CHROMOSOMES ARE CALLED WHEN THEY PAIR
UP DURING PROPHASE I
The journey from DNA to phenotype
Why Some Traits Don’t Show
Mendelian Inheritance
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Often the expression of phenotype is not
straightforward
Continuous variation
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characters can show a range of small
differences when multiple genes act jointly to
influence a character
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this type of inheritance is called polygenic
Height is a continuously varying
character
Why Some Traits Don’t Show
Mendelian Inheritance
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Pleiotropic effects
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an allele that has more than one effect on the
phenotype is considered pleiotropic: one
gene affects many characters
these effects are characteristic of many
inherited disorders, such as cystic fibrosis and
sickle-cell anemia
Figure 11.13 Pleiotropic effects of
the cystic fibrosis gene, cf
Why Some Traits Don’t Show
Mendelian Inheritance
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Incomplete dominance
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not all alternative alleles are either fully
dominant or fully recessive in heterozygotes
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in such cases, the alleles exhibit incomplete
dominance and produce a heterozygous
phenotype that is intermediate between those of
the parents
Incomplete dominance
Why Some Traits Don’t Show
Mendelian Inheritance
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Environmental effects
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the degree to which many alleles are
expressed depends on the environment
for example, some alleles are heat-sensitive
arctic foxes only produce fur pigment when
temperatures are warm
 the ch allele in Himalayan rabbits and Siamese
cats encodes a heat-sensitive enzyme, called
tyrosinase, that controls pigment production
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tyrosinase is inactive at high temperatures
Environmental effects on an allele
Why Some Traits Don’t Show
Mendelian Inheritance
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Epistasis
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in some situations, two or more genes interact with
each other, such that one gene contributes to or
masks the expression of the other gene
in epistasis, one gene modifies the phenotypic
expression produced by the other
for example, in corn, to produce and deposit pigment,
a plant must possess at least one functional copy of
each of two genes
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one gene controls pigment deposition
the other gene controls pigment production
How epistasis affects kernel color
Why is coat color in Labrador
retrievers an example of epistasis?
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E gene determines if dark pigment will be deposited in
fur or not
genotype ee, no pigment will be deposited in the fur,
and it will be yellow
genotype E_, pigment will be deposited in the fur
A second gene, the B gene, determines how dark the
pigment will be
Yellow dogs with the genotype eebb will have brown
pigment on their nose, lips, and eye rims, while yellow
dogs with the genotype eeB_ will have black pigment in
these areas.
The effect of epistatic interactions
on coat color in dogs
Why Some Traits Don’t Show
Mendelian Inheritance
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Codominance
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a gene may have more than two alleles in a
population
often, in heterozygotes, there is not a dominant
allele but, instead, both alleles are expressed
 these alleles are said to be codominant
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ABO Blood types
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They were discovered in 1900 and 1901 at
the University of Vienna by Karl
Landsteiner in the process of trying to
learn why blood transfusions sometimes
cause death and at other times save a
patient. In 1930, he belatedly received
the Nobel Prize for this discovery.
Why Some Traits Don’t Show
Mendelian Inheritance
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The gene that determines ABO blood type in
humans exhibits more than one dominant allele
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the gene encodes an enzyme that adds sugars to
lipids on the membranes of red blood cells
these sugars act as recognition markers for cells in
the immune system
the gene that encodes the enzyme, designated I, has
three alleles: IA,IB, and i
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different combinations of the three alleles produce four
different phenotypes, or bloodtypes (A, B, AB, and O)
both IA and IB are dominant over i and also codominant
Multiple alleles controlling the ABO
blood groups
Inheritance of Blood Type
63
Rh blood group system
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The Rh blood group system (including the Rh
factor) is one of the currently 30 human blood
group systems.
It is clinically the most important blood group
system after ABO.
The Rh blood group system currently consists of
50 defined blood-group antigens, among which
the 5 antigens D, C, c, E, and e are the most
important ones.
The commonly-used terms Rh factor, Rh positive
and Rh negative refer to the D antigen only.
Human Chromosomes
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Nondisjunction may also affect the sex
chromosomes
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nondisjunction of the X chromosome creates
three possible viable conditions
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XXX female
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XXY male (Klinefelter syndrome)
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usually taller than average but other symptoms vary
sterile male with many female characteristics and
diminished mental capacity
XO female (Turner syndrome)
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sterile female with webbed neck and diminished stature
Nondisjunction of the X
chromosome
The Role of Mutations in Human
Heredity
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Accidental changes in genes are called
mutations
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mutations occur only rarely and almost always
result in recessive alleles
not eliminated from the population because they
are not usually expressed in most individuals
(heterozygotes)
 in some cases, particular mutant alleles have
become more common in human populations and
produce harmful effects called genetic disorders
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Some Important Genetic Disorders
The Role of Mutations in Human
Heredity
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To study human heredity, scientists
examine crosses that have already been
made
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they identify which relatives exhibit a trait by
looking at family trees or pedigrees
often one can determine whether a trait is
sex-linked or autosomal and whether the
trait’s phenotype is dominant or recessive
Figure 11.27 A general pedigree
The Role of Mutations in Human
Heredity
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Sickle-cell anemia is a recessive
hereditary disorder
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affected individuals are homozygous recessive
and carry a mutated gene that produces a
defective version of hemoglobin
the hemoglobin sticks together inappropriately and
produces a stiff red blood cell with a sickle-shape
 the cells cannot move through the blood vessels
easily and tend to form clots
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this causes sufferers to have intermittent illness and
shortened life spans
Inheritance of sickle-cell anemia
11.9 The Role of Mutations in
Human Heredity
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The sickle-cell mutation to hemoglobin affects
the stickiness of the hemoglobin protein surface
but not its oxygen-binding ability
In heterozygous individuals, only some of their
red blood cells become sickled when oxygen
levels become low
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this may explain why the sickle-cell allele is so
frequent among people of African descent
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the presence of the allele increases resistance to malaria
infection
The sickle-cell allele
confers resistance to
malaria