Transcript Mendel and Heredity

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The Sphynx cat, is a
rare breed. It is
sometimes known as
the Canadian Hairless,
originating in Ontario,
Canada in 1966, when
a black and white
house cat gave birth
to a male hairless
kitten, similar to the
Mexican Hairless.
Nature periodically
produces naturally
hairless cats.
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In 1930, Dr Clyde
Keller developed
Himalayan Cat breed
by crossing between
Siamese and Persian
Chapter 8
Mendel and Heredity
Quick Review p. 161
DNA from the beginning
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Origins of Genetics
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Heredity – the passing of
characters from parents
to offspring – was one of
the greatest mysteries of
science
Gregor Johann Mendel,
Austrian monk, who
carried out experiments
on garden pea – first to
develop rules that
accurately predict
patterns of heredity
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Genetics - the branch of
biology that focuses on
heredity
Mendel repeated
experiments of a British
farmer, T.A. Knight
Cross – refers to the
mating or breeding two
individuals
Knight used garden peas
Mendel counted the
number of each kind of
offspring & analyzed the
data
Useful Features in Peas
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Several characters
of the garden pea
(Pisum sativum)
exist in two clearly
different forms
Character – used to
mean inherited
characteristics,
flower color
Trait – to a single
form of a character,
purple flower
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Male & female
reproductive
parts of garden
peas are
enclosed within
the same flower
– helping control
mating
Self-fertilization
– fertilize itself
Cross
fertilizationtransfer the
pollen from
flower to
another
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Stamens – male
reproductive
organs that
produce pollen
Pistil – female
reproductive part
that produces
eggs
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Garden pea is
small, grows
easily,
matures
quickly, and
produces
many
offspring
Results can be
obtained
quickly, and
there are
plenty of
subjects to
count
Traits Expressed as Simple Ratios
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Mendel initial experiments
were monohybrid crosses
Monohybrid cross – a cross
that involves one pair of
contrasting traits
His experiments had 3 steps:
Step 1: allowed each
variety to self-pollinate for
several generations
True-breeding – all the
offspring would display only
one form of the character
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P generation (parental
generation) – the first
two individuals that are
crossed in a breeding
experiment
Step 2 - Mendel crosspollinated two P
generation plants that
had contrasting traits
Offspring were called
filial generation or F1
generation
Examined and recorded
the number of F1 plants
expressing each trait
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Step 3 – Mendel
allowed the F1 to
self-pollinate
Called the offspring
of the F1 generation
the second filial
generation or F2
generation
Each F2 generation
was characterized
and counted
Mendel’s Results
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The F1 generation
showed only one form
of the trait
When F1 generation was
allowed to selfpollinate, the missing
trait reappeared in
some of the plants
A ratio is a comparison
of two numbers and
can be written as a
fraction (705/224) or
with a colon 705:224
Homework
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Write the questions with the answers
P. 165 section 1 review 1-6
P. 184 1,6,7
Section 2 Mendel’s Theory
A Theory of Heredity
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Before Mendel, many
people thought
offspring were a blend
of the traits of their
parents
Mendel correctly
concluded that each
pea has two separate
“heritable factors”
Gametes fuse during
fertilization, the
offspring has two
factors for each
character, one from
each parent
Mendel’s Hypotheses
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Four Hypotheses Mendel
developed – foundation
of genetics
1) For each inherited
character, an individual
has two copies of the
gene – one from each
parent
2) There are alternative
versions of genes – now
called alleles – different
versions of a gene
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3) When two different
alleles occur together,
one of them may be
completely expressed,
while the other may
have no observable
effect on the organism’
appearance
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Dominant – the
expressed form of the
character –
Recessive – the trait that
was not expressed when
the dominant form was
present –
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4) When gametes are
formed, the alleles for
each gene in an
individual separate
independently of one
another. Thus, gametes
carry only one allele for
each inherited character.
When gametes unite
during fertilization, each
gamete contributes one
allele.
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Because of the way
gametes are produced
during meiosis
Mendel’s Findings in Modern Terms
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Letters are often
used to represent
alleles
Dominant alleles are
indicated by writing
the first letter of the
character as capital
letter
Recessive alleles are
also indicated by
writing the first
letter of the
dominant trait, but
the letter is
lowercase
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Homozygous- two
alleles of a
particular gene
present in an
individual are the
same – recessive
trait is always
homozygous, but
a dominant may
not be
ex. TT - tall
Heterozygous –
the alleles of a
particular gene
present in an
individual are
different Tt - tall
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Since only the
dominant allele is
expressed, you can
not tell if an organism
is homozygous or
heterozygous
Genotype – the set of
alleles that an
individual has for a
character
Phenotype – the
physical appearance
of a character;
determined by which
alleles (genotype) are
present
The Laws of Heredity
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Law of
Segregation –
the two
alleles for a
character
segregate
(separate)
when
gametes are
formed
Law of Independent Assortment
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Mendel conducted
dihybrid crosses to study
how different pairs of
genes are inherited
Dihybrid cross – a cross
that considers two pairs
of contrasting characters
Mendel found that for
the characters he
studied, the inheritance
of one character did not
influence the inheritance
of any other character
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Law of
Independent
assortment – that
the alleles of
different genes
separate
independently of
one another during
gamete formation
This law applies
only to genes that
are located on
different
chromosomes or
are far apart on the
same chromosome
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Mendel called
the traits
“factors”
Today we call
them genes –
units of
heredity are
portions of
DNA, which
are found on
the
chromosomes
that are
inherited from
its parents
Homework
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Section 2 Review p. 169 restate or rewrite
the questions 1-4,6
Chapter Review p. 184 & 185
5,8,9, 13, 16
Section 3
Studying Heredity
Punnett Squares
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Breeders must be able to
predict how often a trait
will appear
“ Expected” results of
genotypes or
phenotypes in a cross is
to use a Punnett square
Punnett square- a
diagram that predicts
the outcome of genetic
cross by considering all
possible combinations of
gametes in the cross
Named for Reginald
Punnett
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The possible gametes that
one parent can produce are
written along the top of the
square
The possible gametes that
the other parent can produce
are written along the left
side of the square
Each box inside the square is
filled in with two letters by
combining the allele from the
top and side
The letters in the boxes
indicate the possible
genotypes of the offspring
One pair of Contrasting Traits
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Monohybrid cross – a
cross that considers one
pair of contrasting traits
between two individuals
Punnett square can be
used to predict outcome
between a homozygous
purple flower with a
homozygous white
flower (P generation)
All F1 generation is
purple and
heterozygous
Punnett square can also
predict the results of a
cross of two
heterozygous F1
generations
 Genotype: ¼
homozygous (RR); ½
heterozygous (Rr); ¼
homozygous (rr)
 Ratio would be 1:2:1
 Phenotype: ¾ purple;
¼ white
Ratio would be: 3:1
Punnett square allow direct
and simple predictions
about outcomes
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Synpolydactyly HOH gene defect
semidominant
Determining Unknown Genotypes
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Breeders need to know if
a breeding organism is
homozygous or
heterozygous
Test Cross – crossing an
individual whose
phenotype is dominant,
but whose genotype is
not known, with a
homozygous recessive
individual
B = black
b = white
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If all the offspring are
purple, then the
genotype of the
unknown individual is PP
and all the offspring will
be Pp
If ½ of the offspring are
purple and ½ are white
then the genotype of
unknown individual is Pp
and ½ of the offspring
will be Pp and ½ will be
pp
Outcomes of Crosses
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Probability – the likelihood that a specific event will
occur
Can be expressed in words, as decimals, as
percentages, or as fractions; we’ll use fractions or
ratios
Probability = Number of one kind of possible outcome
total number of all possible outcomes
Racing Game with One Die
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To find the probability of two independent
events that occur in sequence, find the
probability of each event occurring
separately, and then multiply the
probabilities. This multiplication rule is
defined symbolically below. Note that
multiplication is represented by AND.
Multiplication Rule 1:
When two events, A and B, are
independent, the probability of both
occurring is:
P(A and B) = P(A) · P(B)
Probability of a specific Allele in a
Gamete
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Same formula can be
used to predict the
probability of an allele
being present in a
gamete
If the alleles are the
homozygous, then the
probability would be ___
If the alleles are
heterozygous, then the
probability would be ___
Probability of the Outcome of a Cross
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Both parents must be
considered when calculating
the probability of the
outcome of a genetic cross
Tossing two coins at the
same time
To find the probability that a
combination of two
independent events will
occur multiply the separate
probabilities of the two
events
½x½=¼
Inheritance of Traits
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Pedigree – a family history that shows how a
trait is inherited over several generations
Helpful if a trait causes a genetic disorder
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Carriers are individuals who are
heterozygous for an inherited disorder,
but do not show symptoms of the disorder
Carriers can pass the allele for the
disorder to their offspring
Autosomal or Sex-linked
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If a gene is autosomal, it
will appear in both sexes
equally
If a trait is sex-linked, its
effects are usually seen
only in males
Sex-linked gene’s allele
is located only on the X
or Y chromosome
Most sex-linked genes
are carried on the X
chromosome and are
recessive
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Sex-linked traits
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Red-green color
blindness
Fragile X
syndrome
Hemophilia
Duchenne
muscular
dystrophy
Boys inherit from
mother
Dominant or Recessive?
Heterozygous or Homozygous?
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Autosomal dominant,
every individual with the
condition will have a
parent with that
condition
Homozygous or
heterozygous dominant,
the trait will show up
Noonan Syndrome –
genetic disorder that
prevents normal
development of various
parts of the body
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Recessive Autosomal
Traits
Cystic fibrosis –
Caucasean population
 Sickle cell anemia –
Black population
 Tay-Sachs – Eastern
European AshkenaziJewish population
 Alpha thalassemia –
oriental population
Everybody carries 6-8
recessive genes which
don’t function well
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Your Genes Your Health
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Homework
Section review p. 176 1-5 rewrite or
restate questions
 Chapter review p. 184 -185 rewrite or
restate the questions
3,10,11,14
Standardized Test Prep (STP) 1-3
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Section 4
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Complex Patterns of Heredity
Complex Patterns of Heredity
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Polygenic inheritance
– when several
genes influence a
character
Eye color, skin color,
height, weight, hair
color
Incomplete
dominance – an
individual displaying
a phenotype that is
intermediate
between the two
parents
Characters Controlled by Genes with
Three or More Alleles
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Multiple alleles – genes
with three or more
alleles ex blood type
which is determined by
three alleles IA,IB,i
A & B are dominant over
O, but not dominant
over each other, they
are codominant
Four different blood
types: A, B, AB, O
Characters with Two Forms Displayed
at the Same Time
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When two
dominant alleles
are expressed
at the same
time –
codominance
Different from
incomplete
dominance
because both
traits are
displayed
Characters Influenced by the
Environment
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AN individual’s
phenotype often
depends on conditions in
the environment
Hydrangea - acid soilblue flowers; neutral to
basic soil-pink flowers
Temperature affects the
arctic fox, Siamese cats,
and gender of crocodile
Humans internal &
external influences
Genetic Disorders
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The harmful effects
produced by inherited
mutations are called
genetic disorders
Sickle Cell Anemia –
caused by a mutated
allele that produces a
defective form of the
protein hemoglobin
Recessive allele helps
protect the cells of
heterozygous individuals
from the effects of
malaria
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Cystic Fibrosis (CF) most
common, fatal,
hereditary, recessive
disorder among
Caucasians
One in 25 Caucasian
individuals has at least
one copy of a defective
gene
1 in 2500 Caucasian
infants in the US is
homozygous for the cf
allele
Hemophilia
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Impairs the blood’s
ability to clot
Sex-linked trait
Occurs more in males
Huntington’s Disease (HD)
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A genetic disorder caused by
a dominant allele located on
an autosome
First symptoms – mild
forgetfulness and irritability –
thirties or forties
Causes loss of muscle
control, uncontrollable
physical spasms, severe
mental illness, and
eventually death
The disease is unknowingly
passed on from one
generation to the next
Treating Genetic Disorders
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Genetic counseling - a
form of medical
guidance that informs
people about genetic
problems that could
effect them or their
offspring
PKU - required testing of
all newborns – can cause
severe mental
retardation if not treated
Gene therapy – modify
genetic material of living
cells for therapeutic
purposes
Homework
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Section 4 review p. 182 rewrite or restate
questions 1-6
Chapter review p. 184-185 rewrite or
restate questions 4,12,17