Genetics Part 1

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Transcript Genetics Part 1

Chapter 11
Introduction to Genetics
Section 11.1
The Work of Gregor Mendel
• Essential Question: How does cellular
information pass from one generation to
another?
• Guiding Question: How does an organism pass
its characteristics on to its offspring?
• Objectives:
– Where does an organism get its unique
characteristics?
– How are different forms of a gene distributed to
offspring?
The Experiments of Gregor Mendel
• Every living thing has a set of
characteristics inherited from
its parent or parents
• Heredity – delivery of
characteristics from parent
to offspring
• Genetics – study of heredity
– Key to understanding what
makes each organism unique
The Experiments of Gregor Mendel
• Modern science of genetics
founded by an Austrian
monk named Gregor Mendel
– Born in 1822 in what is now
Czech Republic
– Became a priest
– Studied science and math at
University of Vienna
– 14 years teaching high school
& working in a monastery
– He was charged with the
monastery garden
The Experiments of Gregor Mendel
• Used ordinary garden peas b/c:
– Small
– easy to grow
– Single pea plant can produce hundreds
of offspring
• “model system” – convenient to study
and may tell us how other organisms
actually function
– would have taken decades/ centuries to do
with other large animals
– Impossible to do with humans…. WHY??
The Role of Fertilization
• Male part of flower makes pollen
– contains sperm – male
reproductive cells
• Female part of flower makes
eggs – female reproductive cells
• Fertilization – male and
female reproductive cells join
to form a new cell
• In peas: new cell develops into
a tiny embryo encased within a
seed
The Role of Fertilization
• Pea flowers are normally selfpollinating
– sperm cells fertilize egg cells
from within the same flower
• A plant grown from a seed
produced by self-pollination =
inherits all of its
characteristics from the
single plant that bore it
– It has a single parent
– It is a clone
The Role of Fertilization
• “true-breeding” – self-pollinating – produce
offspring identical to themselves
– The traits of each successive generation would be the
same
• Trait – specific characteristic of an individual
– Seed color or plant height
• Many traits vary from one individual to another
– One stock = only tall plants; another only short
– One line = only green seed; another only yellow
The Role of Fertilization
• To learn how traits were determined
 Mendel “crossed” his stocks of
true-breeding plants
– Caused one plant to reproduce with
another
– Had to prevent self-pollination: cut away
pollen-bearing male part of flower
– Dusted the pollen from a different plant
onto the female part of that flower
• Cross-pollination – described above
– produces a plant that has 2
different parents
– Allowed Mendel to breed plants with
traits different from those of their parents
then study the results
The Role of Fertilization
• Mendel studied seven different traits:
– Traits had 2 contrasting characteristics
• Green seed color v. yellow seed color
• Mendel crossed plants with each of the 7
contrasting characteristics
• Then studied their offspring (hybrids)
• Hybrids – crosses between parents
with different traits
Genes and Alleles
• Original pair of parents = P1 or parental
generation
• Offspring of original parents = F1 or first
filial generation
– (filius and filia = son and daughter)
Genes and Alleles
•
Mendel’s F1 hybrid plants:
Observations:
– all offspring had characteristics
of only one of its parents
– trait from other parent seemed
to have disappeared
•
Conclusions:
1. characteristics are determined
by factors passed from parents
to the next
•
Genes – factors that are
passed from parent to
offspring
Genes and Alleles
• traits are controlled by a
single gene that occurred in 2
contrasting varieties
• Variations produced different
expressions of each trait
• Allele – different forms of a
gene
– Example: gene for height occurred
in one form that produced tall
plants and in another form that
produced short plants
Dominant and Recessive Alleles
• Conclusion:
2. Principle of Dominance – some alleles are
dominant and others are recessive;
• at least 1 dominant allele for a particular trait will
exhibit that form of the trait
• recessive allele for a particular trait will only show
up if dominant allele for the trait is not present
Segregation
• Had the recessive alleles
simply disappeared, or
were they still present in
the new plants?
• To find out, Mendel
allowed all 7 kinds of F1
hybrids to self pollinate
– Crossed F1 generation with
itself to produce the F2
offspring
• F2 = offspring of an F1
cross (second filial
generation)
The F1 Cross
• Mendel compare F2 plants:
– Traits controlled by recessive alleles
reappeared in second generation
• ~1/4 of F2 plants showed trait controlled by
recessive allele
Explaining the F1 Cross
• At beginning, Mendel assumed that a
dominant allele had masked the
corresponding recessive allele in F1
generation
• Trait controlled by recessive allele did
not show up in some of the F2 plants
 indicated that, at some point, the
allele for shortness had separated
from allele for tallness
• Mendel suggested = alleles for
tallness and shortness in F1
plants must have segregated
during formation of the sex
cells
• Gametes – sex cells
The Formation of Gametes
• Assume: All F1 plants inherited an
allele for tallness from tall parent
and one for shortness from short
parent.
– Because tallness = dominant  all F1
plants are tall
• During gamete formation, alleles for
each gene segregate from each
other, so that each gamete carries
only one allele for each gene
– Each F1 plant produces 2 kinds of
gametes:
• Those with tall allele and those with short
allele
The Formation of Gametes
• Capital letter = dominant allele
• Lowercase letter = recessive allele
• Each F1 plant in Mendel’s cross produced
2 kinds of gametes:
– Those with allele for tallness
– Those with allele for shortness
• Whenever gamete that carried t allele
paired with the other gamete that carried
the t allele to produce and F2 plant  that
plant was short
• Every time one or both gametes of the
pairing carried the T allele  a tall plant
was produced
• F2 generation had new combinations of
genes
Section 11.2
Applying Mendel’s Principles
• Essential Question: How does cellular
information pass from one generation to
another?
• Guiding Question: How can you predict the
outcome of a genetic cross?
• Objectives:
– How can we use probability to predict traits?
– How do alleles segregate when more than one gene
is involved?
– What did Mendel contribute to our understanding of
genetics?
Probability and Punnett Squares
• During Mendel’s crosses with pea plants
he carefully categorized and counted the
offspring to create data
• Through analysis  principles of probability
can be used to explain the results of genetic
crosses
• Probability – likelihood that a particular
event will occur
– EX: flipping a coin – 2 possible outcomes (heads
or tails)  probability of either outcome is equal
for every flip (1/2 = 50%)
Probability
• Flip a coin  50% of either outcome
• Flip a coin 3 times  each flip is an
independent event with ½ probability of
landing heads up:
– Probability of flipping 3 heads in a row =
½ * ½ * ½ = 1/8
• Multiplying individual probabilities tells us:
– Past outcomes do not affect future
outcomes
Using Segregation to Predict Outcomes
• Allele segregation during
gamete formation is every bit as
random as a coin flip
– So principles of probability can
be used to predict the outcomes
of genetic crosses
• Mendel’s P1 Cross:
• True-breeding tall x truebreeding short
• Offspring: ????
Mendel’s F1 Cross
– Question 57: F1 plants were
both tall
– When crossed  produced a
mixture of tall and short
plants
– Each plant has 1 allele for tall
and 1 allele for short, so ½ of
their gametes would carry the
short allele
– The only way to produce a
short plant is for 2 gametes
carrying the t allele to
combine
Using Segregation to Predict Outcomes
• Each F2 gamete has ½ chance of carrying the t
allele
– There are 2 gametes, so probability =
½ * ½ = 1/4
– ~ ¼ of F2 offspring should be short, and remaining
¾ should be tall
• 3 dominant : 1 recessive showed up consistently
in Mendel’s experiments
– For each of his experiments: 1/4 showed traits
controlled by recessive allele & ¾ showed traits
controlled by dominant allele
• Segregation did occur according to Mendel’s model
Using Segregation to Predict Outcomes
• F2 generation – not all have
same characteristics or the
same combinations of
alleles
– TT vs. Tt – both tall, but only
1 has identical alleles
• Homozygous – has 2 identical
alleles for a particular gene
(TT) (tt)
• Heterozygous – has 2
different alleles for the same
gene (Tt)
Speed Bump – label as
heterozygous or homozygous
•
•
•
•
•
•
Pp
Jj
hh
ii
WW
Qq
Probabilities Predict Averages
• Probabilities predict average outcome of a
large number of events
– To get the expected 50:50 ratio, you may
have to flip the coin many times
• Larger number of offspring = results
closer to predicted values
– May need hundreds or thousands of
individuals for ratios to come very close to
matching the predictions
Genotype and Phenotype
• Every organism has a genetic
makeup as well as a set of
observable characteristics.
– Phenotype – physical traits
(tallness)
– Genotype – genetic makeup
(TT, Tt, tt)
• Genotype of an organism is inherited
• Phenotype is largely determined by
genotype
• 2 organisms may have same
phenotype but have different
genotypes
Speed Bump – label as
phenotype or genotype
•
•
•
•
•
•
•
•
Green
Tall
Pp
rr
Curly
Red
FF
Qq
Using Punnett Squares
• One of best ways to predict outcome of a
genetic cross is by using a diagram
• Punnett square – use mathematical
probability to help predict the genotype
and phenotype combinations in genetic
crosses
Constructing a Punnett Square
• Make a square
• to write all possible
combinations of alleles in the
gametes produced by one
parent at the top edge of the
square
• write the other parent’s
alleles along the left edge
• Every possible offspring
genotype is written into the
boxes within the square,
Independent Assortment
• Does the segregation of 1 pair of alleles affect
another pair?
– Does gene that determines shape of a seed affect the
gene for seed color?
• To find out, Mendel followed 2 different genes as
they passed from one generation to the next
• 2-factor = dihybrid = involves 2 different
genes
• 1-factor = monohybrid
The 2-Factor Cross: F1
• Mendel crossed true-breeding plants
that produced only round yellow
peas with plants that produced
wrinkled green peas
– Round yellow = RRYY
– Wrinkled green = rryy
• All F1 offspring produced round
yellow peas
– Alleles for round and yellow are
dominant
– Genotype in each of F1 plants = RrYy
• Heterozygous for both seed shape and
see color
• Cross did not indicate whether genes
assort (segregate) independently
• Cross provided hybrid plants needed
to breed F2 generation
The 2-Factor Cross: F2
• Mendel crossed F1 plants to produce
F2 offspring
• F1 plants formed by fusion of gamete w/
dominant RY alleles with another gamete
carrying the recessive ry alleles
– Did this mean that the 2 dominant alleles
would always stay together, or would they
segregate independently, so that any
combination of alleles was possible?
• F2 plants produced 556 seeds.
• Mendel compared their variation
– 315 = round and yellow (parental
phenotype)
– 32 = wrinkled and green (parental
phenotype)
– 209 = combinations of phenotypes 
combinations of alleles not found in
either parent
• Alleles for seed shape segregated
independently of those for seed color
– Genes that segregate independently do
not influence each other’s inheritance
F2 Conclusions
• Mendel’s experimental
results very close to
9:3:3:1 ratio predicted by
the Punnett square
• Principle of Independent
Assortment – genes for
different traits can
segregate independently
during the formation of
gametes.
• Independent assortment
helps account for many
genetic variations observed
in plants, animals, etc –
even when they have the
same parents
Summary of Mendel’s Principles
• Mendel’s principles of heredity, observed
through patterns of inheritance, form the
basis of modern genetics
• The inheritance of biological characteristics is
determined by individual units called genes,
which are passed from parents to offspring.
• Where 2 or more forms (alleles) of the gene
for a single trait exist, some alleles may be
dominant and others may be recessive.
• In most sexually reproducing organisms, each
adult has 2 copies of each gene – one from
each parent. These genes segregate from
each other when gametes are formed.
• Alleles for different genes usually segregate
independently of each other
Summary of Mendel’s Principles
• Don’t only apply to plants
• Thomas Hunt Morgan (early 1900s) – used fruit flies to
advance the study of genetics
– A single pair can produce hundreds of flies
– Tested all of Mendel’s principles
– Learned that they applied to flies and other organisms as well
• Mendel’s basic principles can be used to study the
inheritance of human traits and to calculate the
probability of certain traits appearing in the next
generation
II. Transmission of Human Traits
• Human genes follow the same Mendelian
patterns of inheritance as the genes of
other organisms
A. Dominant and Recessive
Alleles
• Many human traits follow a pattern of
simple dominance
– Hair color determined by MC1Rs
• Red hair = 2 recessive alleles (one from each
parent)
• Dominant alleles produce darker hair color
– Rhesus (Rh blood group)
• Allele comes in 2 forms Rh+ (dominant) and Rh(recessive)
– Heterozygous (Rh+/Rh-) = Rh positive blood
– Homozygous recessive (Rh-/Rh-) = Rh negative
blood
Genetic Disorders Caused by Individual
Genes
• A change in a single gene causes the
following genetic disorders:
– Sickle – cell
– Huntington’s Disease
– Cystic Fibrosis
Cystic Fibrosis
Caused by the deletion of 3 bases from 1 gene
One of the most common recessive genetic
disorders in Caucasians
Affects the mucus-producing glands, digestive
enzymes, and sweat glands
This mucus clogs ducts in the pancreas, interferes
with digestion, and blocks respiratory passageways
in the lungs.
Need 2 copies of the defective gene to have the
disorder
People with cystic fibrosis are less likely to die from
typhoid
Cystic Fibrosis
• Treatment includes
physical therapy,
medication (to thin the
mucus), special diet,
cleaning of the mucus,
and replacement
digestive enzymes.
These have greatly
expanded the life span
of those affected by this
disorder
Sickle –Cell Disease
• Caused by a flawed allele for protein
hemogloblin (which carries oxygen in the
blood)
• This abnormal proteins often stick
together, which changes the shape of the
red blood cells
• However, this same flaw creates immunity
to disease malaria caused by a parasite
mosquitoes carry. The parasite can not
survive in the sickle cells
Sickle - Cell
• This flaw changes
the shape of the red
blood cells which
gives the disorder its
name
• These cells are less
flexible, which can
cause them to get
stuck in tiny vessels
Huntington’s Disease
Caused by a dominant allele for a protein found in
brain cells
Affects the nervous system
1 in about 10,000 individuals has the disease
Symptoms of this disorder do not usually appear
in affected individuals between the ages of 30 and
50
Symptoms include a gradual loss in brain function,
uncontrollable movements, and emotional
disturbances
No cure, no treatment, and genetic tests can be
done to detect the gene
Albinism
 Is the absence of the skin
pigment melanin in hair and
nails
 Found in other animals as well
as humans
 Those with albinism have white
hair, very pale skin, and pink
pupils
 Absence of pigment in the eyes
can cause vision problems
 They need to be especially
careful in the sun because they
burn very easily
Achondroplasia
• Most common form of dwarfism
• Has a small body size and limbs are
comparatively short
• Average adult height is about 4 feet and a
normal life expectancy.
• 75% of individuals with achondroplasia
are born to parents of average size, which
means that is occurred due to a mutation
or genetic change
B. Codominant and Multiple Alleles
• The alleles for many human genes display
codominant inheritance
– Example: ABO blood group – determined by a
gene with 3 alleles: IA, IB, and i
• Alleles IA & IB are codominant
– They produce molecules know as antigens on the
surface of red blood cells
– Individuals with IA & IB alleles  produce both
antigens  blood type = AB
• i allele is recessive
– Individuals w/ IA IA or IA i  only produce the A
antigen  blood type A
– Individuals w/ IB IB or IB i  only produce the B
antigen  blood type B
– Homozygous for i allele (ii)  produce no antigen
 blood type O
• If a patient has AB-negative blood  has IA & IB
alleles from ABO gene and 2 Rh- alleles from
Rh gene
C. Sex-Linked Inheritance
• Because the X and y chromosomes
determine sex, the genes located on
them show a pattern of inheritance
called sex-linkage
• Sex-linked gene – gene located
on a sex chromosome
– Genes on Y chromosome are found
only in males and are passed
directly from father to son
`
– Genes on X chromosome are found in
both sexes
• Men have just one X chromosome leads to
some interesting consequences
– Example: 3 genes for color vision (all located on
X)
– Males = defective allele for any of these genes
results in colorblindness
» Red/green = most common = 1/12 males
– females = 1 in 200 affected b/c in order to be
expressed in females  must be present in 2
copies (one on each X)
• The recessive phenotype of a sex-linked
genetic disorder tends to be much more
common among males than among
females
Sex linked Trait – Red/Green
Colorblindness
• Is a recessive x
linked trait
• 8 % of males in the
U.S. have redgreen
colorblindness
• These individuals
see colors
differently than
those with normal
vision
Sex Linked Trait - Hemophilia
• Recessive x linked disorder
• More common in males than females
• Royal families have a high occurrence of
the disorder due to only being able to
marry with royal blood.
• With new treatments like clotting
disorders, hemophiliacs now live much
longer.
• They must be careful in daily life
Chromosomal Disorders
• These occur when meiosis does not occur
properly and each gamete does not
receive the correct number of
chromosomes (23)
• Usually occur when chromosomes do not
separate during division; this is known as
nondisjunction
• Nondisjunction results in trisomies (3 copies of
1 chromosome) or monosomies (1 copy of a
chromosome)
Examples of Chromosomal
Disorders
Downs Syndrome
Person has 3 copies of chromosome #21
Depending on individual, they will have varying levels of
mental disabilities and birth defects
Turner’s Syndrome
A female that only inherits 1 x chromosome
Causes developmental issues and they will be unable to
reproduce
Kleinfelter’s Syndrome
 A male that inherits an extra x chromosome
 Is unable to reproduce
If a woman is a carrier of an
X-linked recessive
allele for a disorder and her
mate does not have
it, their boys will have a 50%
chance of inheriting
the disorder. None of their
girls will have it, but
half of them are likely to be
carriers.
If a man has an X-linked
recessive disorder and his
mate does not carry the allele
for it, all of their girls
will be carriers. None of their
boys will inherit the
harmful allele. Only girls receive
X chromosomes
from their fathers.
D. X-Chromosome Inactivation
• In female cells, most of the genes in one
of the X chromosomes are randomly
switched off  form dense region in
nucleus (Barr Body)
– Not found in males because their single X
chromosome is still active
– Example: Cat coat color gene is on X
• One allele = orange
• One allele = black
– In cells in some parts of the body  one
chromosome is switched off
– In other parts of the body  the other X
chromosome is switched off
– Results – mixture of orange and black spots
» Male cats (1 X chromosome) can have
spots of only one color
» If the cat’s fur has 3 colors (white with
orange and black spots)  you can almost
be certain that the cat is female