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Genetics
Biology 105
Laboratory 9
Chapter 20
Copyright © 2009 Pearson Education, Inc.
Note:
This information will be presented during lab
time but covered on the lecture exam.
You will be given a homework assignment with
genetics problems. The homework is due on
Tuesday, and similar content will be on the lab
exam.
Copyright © 2009 Pearson Education, Inc.
History – Gregor Mendel and his Pea Plants
When Mendel lived, no one knew about DNA
or meiosis.
It was known that offspring inherited traits
from the parents, but no one knew how…
It was thought that whatever the genetic
material was, it would be blended to produce
the offspring.
But nature did not seem to follow this rule!
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Problem!
If the genetic material was blending to
produce offspring, then why is the foal not
gray?
Even though people could see this in nature
and in agricultural breeding programs, they
still believed in the blending theory.
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Charles Darwin
Charles Darwin was one of the few scientists
who did not believe in the blending theory.
Darwin believed that individuals in a
population show variation.
The traits that give you an edge to survive will
be passed on to your offspring.
The traits are not blended, but instead the
traits will be seen more or less often
depending on how advantageous they are for
the individual.
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More About Mendel’s Work…
Just before Darwin presented his theory, Mendel
started to work on his experiments with peas.
Mendel was a monk and a scientist.
He studied both math and botany.
He was raised on a farm and was aware of
agricultural practices and research.
He was well-known for breeding new varieties of
fruits and vegetables.
Mendel believed that sperm and eggs contained
“units” of information (= traits).
He used pea plants to prove his theory.
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Pea Plant Characteristics
Pea shape – smooth or wrinkled
Seed color – yellow or green
Pod shape – smooth or wrinkled
Pod color – yellow or green
Flower color – purple or white
Flower position – low on stem or at tip
Stem length – tall or short
Mendel made three key decisions in his
experiments.
- use of purebred plants
- control over breeding
- observation of seven
“either-or” traits
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Terminology
Traits – distinguishing characteristics that
vary between individuals and are inherited.
Parental generation (P): the parents in
Mendel’s experiments.
These plants are “pure” – they only produce
plants with their own phenotype when they are
self-pollinated.
First filial generation (F1): the offspring of the
parental generation (= children).
Second filial generation (F2): the offspring of
the first filial generation (= grandchildren).
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• Mendel used pollen to fertilize selected pea plants.
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
– P generation crossed to produce F1 generation
– Interrupted the self-pollination process by
removing male flower parts
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• Mendel allowed the resulting plants to self-pollinate.
– Among the F1 generation, all plants had purple
flowers.
– F1 plants were all heterozygous.
– Among the F2 generation, some plants had
purple flowers and some had white.
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• Mendel observed patterns in the first and second
generations of his crosses.
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Results from Mendel’s Experiments
P generation were pure purple bred to pure white.
F1 generation were all purple flowers!
F2 generation had both purple and white flowers (3:1 ratio).
Somehow the white trait had “hidden” for a generation.
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Dominant and Recessive
The trait that is expressed is the dominant trait.
The trait that is hidden, or not expressed, is
recessive.
In this example, the purple trait is the dominant
trait over the recessive white trait.
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DNA and Genes
Gene: the specific part of DNA that tells a cell
how to make a specific protein.
There are pairs of chromosomes.
Pairs of homologous chromosomes contain the code
for the same proteins.
In a pair of homologous chromosomes, each one
contains the same genes.
These different forms of the same genes are called
alleles.
Allele = any alternative form of a gene occurring at a
specific place on a chromosome.
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Each chromosome in the pair of chromosomes
has an allele.
If both the chromosomes in the pair have the
same allele form, then they are homozygous
for that trait.
Just for that trait! Other alleles on the DNA can be
different.
If the chromosomes in the pair have different
alleles, they are heterozygous for this trait.
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Allele for
yellow is
“Y”
Allele for
green is
“y”
A yellow
pea can
be YY or
Yy
A green
pea is yy
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Dominant and Recessive
Dominant alleles are written with a capital
letter.
Recessive alleles are written with a lowercase
letter, and you always use the same letter that
you chose for dominant.
In Mendel’s peas, the allele for yellow peas is
written as “Y” and the allele for green peas is
written as “y”.
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Genotype and Phenotype
Phenotype – physical features of the organism.
Genotype – the genetic makeup that
determines the phenotype.
The phenotype for a yellow pea is yellow color,
and the genotype is YY or Yy.
The phenotype for a green pea is green color,
and the genotype is yy.
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Punnett squares illustrate genetic crosses.
The Punnett square is a grid system for
predicting all possible genotypes resulting
from a cross.
The axes represent
the possible gametes
of each parent.
The boxes show the
possible genotypes
of the offspring.
• The Punnett square
yields the ratio of
possible genotypes
and phenotypes.
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Punnett Square – Pea Example
Remember the yellow allele is dominant (Y)
and green allele is recessive (y).
In Mendel’s experiment, the parental
generation (P) self-fertilized and only produced
the same kind of plant.
So the plants with yellow peas are YY and the
plants with green peas are yy.
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When P generation were cross-fertilized
(yellow pea plants with green pea plants), they
produced only yellow plants.
The offspring of the P generation (= F1) were
all Yy genotype and yellow phenotype.
y y
Y Yy Yy
Y Yy Yy
Yello
w
Green
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Y y
Y YY Yy
y Yy yy
F2 Generation
The F1 peas were planted, and the plants
self-fertilized to produce the F2 generation.
Yellow
Yello
w
Phenotype of the
offspring: on
average, 3 peas
should be yellow
(YY or Yy) and 1
pea should be
green (yy).
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Q: You breed homozygous purple plants with homozygous
white plants (P generation), and the result is offspring that
are all purple (F1). Which allele is dominant?
1. Purple
2. White
Q: What are the genotypes of the P
generation?
1. PP and pp
2. Pp and pp
3. All Pp
4. All PP
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Q: What are the genotypes of the F1
generation?
1. PP and pp
2. Pp and pp
3. All Pp
4. All PP
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Practice Problems...
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X-Linked Inheritance in Humans
Remember that X and Y are the sex
chromosomes.
Genes that are located on the X
chromosomes are called X-linked or sex-
linked.
X-linked recessive traits are mainly seen in
males.
Why?
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X-Linked Inheritance in Humans
Males only have one X chromosome, so if
there is a trait on the X chromosome and they
inherit the recessive allele, then they will
express the trait.
If females inherit one recessive allele, they
have a chance to also inherit the dominant
allele on their other X chromosome.
There are many disorders that are X-linked:
color blindness, hemophilia, muscular
dystrophy
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X-Linked Color Blindness
Color-blind people can not distinguish between
certain shades of greens and reds.
In color blindness, the proteins in pigments that
absorb green and red light are controlled by
DNA on the X chromosome.
There are two alleles for this – the dominant
allele produces the proteins, and the recessive
allele does not.
This disorder is a recessive disorder, which
means that a person will only express it if they
are homozygous for the recessive allele.
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X-Linked Traits
When drawing Punnett Squares:
Normal X = X
X carrying the trait = Xb
Normal female: XX
Female carrier: XbX
Color-blind female: XbXb
Normal male: XY
Color-blind male: XbY
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Question!
Red-green color blindness is due to a sex-
linked recessive allele on the X chromosome.
Two parents with normal vision produce a
color-blind son.
Draw a Punnett square to show how this
happened:
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Q: What is the chance the couple could have
a color blind daughter?
1. 1/4
2. 2/4
3. 3/4
4. 0/4 (none)
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Autosomal Genetic Disorders
Sex-linked disorders are only those disorders
that are controlled by the X or Y
chromosomes.
All other chromosomes are autosomal (=
body) chromosomes – they control the
autosomal disorders.
There are autosomal recessive disorders, and
autosomal dominant disorders.
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Autosomal Recessive Disorders
Autosomal recessive disorders: disorders controlled by DNA in any of the autosomal chromosomes.
These are recessive disorders, so they are only expressed if the person is homozygous for the allele.
Examples include: sickle-cell anemia, cystic fibrosis, albinism, phenylketonuria
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Sickle Cell Anemia
The protein in blood
cells that carries the
oxygen is hemoglobin.
Sickle cell disorder is
due to a point mutation
in the DNA that controls
the formation of the
hemoglobin protein,
causing misshapen red
blood cells.
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A carrier is a person who does not express a
recessive disorder but carries the allele for the
disorder (heterozygous).
What is the chance that two carriers will
produce a child with sickle cell anemia?
To determine the chances that two carriers will
pass on the disorder, draw a Punnett square!
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The child will
have:
1 in 4 chance
(25%) of having
the disorder
50% chance of
being a carrier
25% chance of
being “normal”
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The Benefits of Sickle Cell
There is an
overlap of where
sickle cell anemia
is found in the
highest numbers
and where malaria
is found:
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Sickle Cell Anemia
People who are homozygous for the disorder
experience severe symptoms, but people who
are heterozygous (carriers) show few, if any,
symptoms.
But carriers of sickle cell anemia have greater
protection from malaria symptoms.
People who are heterozygous are able to
produce both the normal and abnormal
hemoglobin proteins.
The abnormal protein protects against the effects
of malaria.
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Autosomal Dominant Disorders
Autosomal dominant disorders are disorders controlled by the autosomal chromosomes.
These disorders will be expressed when the person has one or two alleles for the disorder.
The allele for the disorder is dominant over the normal allele.
Examples include: Huntington’s disease, cholesterolemia, ACHOO syndrome
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Huntington’s Disease
Huntington’s disease: degenerative disease that affects the cerebral cortex region of the brain.
Initial symptoms include abrupt, jerky movements.
These symptoms typically develop in middle age.
Dementia occurs late in the disease.
Caused by a repeat of just three bases of the DNA on chromosome 4.
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Huntington’s Disease
Huntington’s is an autosomal dominant disorder.
In a Punnett square, the Huntington’s allele is written “H” and the normal allele is “h”.
A person with the genotype Hh or HH will develop Huntington’s.
People with hh will not develop the disease.
People with Huntington’s do not usually show symptoms until after they have reproduced…
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Q: Would you want to know if you have the
Huntington’s allele?
1. Yes
2. No
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Practice Problems…
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Incomplete Dominance
In peas, everything worked out neatly!
In other plants like snapdragons it does not work
so cleanly...
If you cross a red snapdragon with a white
snapdragon, you get all pink snapdragons!
Red is dominant, but not completely dominant.
RR = red
rr = white
Rr = pink
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Co-Dominance
Another type of dominance is called co-
dominance.
Here, both the alleles are expressed in
heterozygous organisms.
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Blood Type Co-Dominance
What does your blood type refer to?
Refers to the type of proteins on the surface of the
blood cells.
There are two main proteins on blood cells: A
and B.
Neither type is dominant over the other.
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Type A has only A proteins
Type B has only B proteins
Type AB has A and B
proteins
O Type has no proteins
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Genotype Phenotype Blood Type
AB A & B proteins AB
AA A proteins A
BB B proteins B
OO No proteins O
BO B proteins B
AO A proteins A
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Q: If the parents have type A (homozygous) and type
O blood, what is the phenotype of the offspring?
1. Type A
2. Type B
3. Type AB
4. Type O
Q: If the parents have type A (homozygous) and type B
(homozygous) blood, what is the phenotype of the offspring?
1. Type A
2. Type B
3. Type AB
4. Type O
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Q: If the parents have type AB and type O blood, what
are the phenotypes of the offspring?
1. Type A
2. Type B
3. Type AB
4. Type O
5. Type A and B
6. Type AB, B and A
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Practice Problems…
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Genes and the Environment
The phenotype is not completely controlled by
genes.
Think about the height of a person:
Part of what determines the height of a person is
genetic, but diet also plays a roll.
The color of Hydrangea flowers depends on
the soil:
Acidic soil produces blue flowers, and basic soil
produces pink flowers .
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Aneuploidy
Aneuploidy are disorders where a person has
an abnormal number of chromosomes.
Everyone should have 23 pairs of
chromosomes (or 46 total chromosomes).
Gametes should have 23 total chromosomes,
so when two gametes join, the offspring will
have 46 chromosomes.
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Aneuploidy
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Down Syndrome
Down syndrome is caused by aneuploidy.
People with Down syndrome have three of the
21st chromosome, instead of the normal two
chromosomes.
Down syndrome results in mental retardation,
developmental delays, and other lifelong
health problems.
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Changes in the Chromosome Structure
During Meiosis I when crossing-over occurs,
or at other times during Meiosis, the
chromosomes may be damaged.
Examples of types of damage include:
deletion, inversion, translocation, and
duplication.
Causes can include: radiation, chemicals or
mere chance.
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Important Concepts for Lecture Exam
X-linked inheritance: how are traits passed to
offspring on the X chromosome?
Why are males more likely to be affected?
Example: color blindness
Autosomal disorders – differences between
recessive and dominant autosomal disorders,
and examples of recessive and dominant
autosomal disorders.
The benefits of sickle cell anemia.
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Important Concepts for Lecture Exam
What is aneuploidy and how does it happen?
Example: Down syndrome
What are the different types of chromosomal
structural changes (e.g. deletion, inversion,
translocation, and duplication) and what
happens to the chromosome in each of these
cases?
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Definitions
For both lecture and lab exams:
Phenotype, genotype, parental (P) generation,
first filial (F1) generation, X-linked, autosomal
disorder, autosomal chromosome, heterozygous,
homozygous, recessive, dominant
For lecture exam only: aneuploidy
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Important Concepts for Lab Exam
Given the genotype or phenotypes of the parents,
be able to determine the chance that: (1) the
offspring will inherit a disorder, (2) the offspring
will be a carrier, or (3) the offspring will not be
either.
You should be able to do this for “regular”, X-linked,
recessive autosomal, dominant autosomal,
incomplete dominance, and co-dominance.
You should be able to solve genetics
problems similar to those on the homework.
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Optional Information of Interest…
The following slides will not be tested on
either the lecture or lab exam!
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Genetic Counseling
What options do couples have to try to prevent having a baby with a birth defect?
The first option is going to a genetic counselor who will help them draw a family tree of genetic traits:
The counselor may recommend getting tested to see if they are carriers for certain traits (if the test is available).
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Genetic Screening
Genetic screening is the process of analyzing
blood or skin to search for a particular genotype.
In order to locate this faulty gene, scientists
search for variations in pieces of DNA called
"markers".
Markers have already been found for various
diseases, including Huntington’s disease, sickle
cell anemia, cystic fibrosis, Tay-Sachs disease,
Duchenne muscular dystrophy, and hemophilia.
With such markers it becomes theoretically
possible to screen individuals of every age, from
infants to adults, even babies before birth. Copyright © 2009 Pearson Education, Inc.
Types of Genetic Screening
1. Carrier screening:
Analysis of an adult to determine if they have a
gene or a chromosome abnormality that may cause
problems for offspring or the person screened.
2. Prenatal screening:
Performed when a fetus is at risk for various
identifiable genetic diseases or traits.
Examples: amniocentesis
3. Newborn screening:
Analyzes blood or tissue samples during early
infancy to detect genetic disorders for which early
intervention can avert serious health problems.
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Newborn Screening
Should all newborns be screened for all the
known diseases?
Who should pay for the tests?
Would it be worth it to test all babies in order
to save a few babies?
Would the health care money be better spent
on pre-natal care?
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4. Pre-Implantation Genetic Screening
What if you and your partner are both carriers
for a disorder, but you really want a child?
What is the chance that you will have a baby
with the disorder? Or a baby who will be a
carrier?
What if you (or your partner) are 43-years-old
and are having trouble conceiving, and want to
make sure your baby does not have Down
syndrome or another genetic disorder?
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Pre-Implantation Genetic Diagnosis
Many couples use in vitro fertilization
techniques to become pregnant.
The woman is given hormone injections so
she will produce more than one egg.
These eggs are collected and sperm is added
to the eggs outside the woman’s body.
The developing embryos are then placed into
the woman’s uterus to develop.
You can test the embryos for many disorders,
including Down syndrome.
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Things to Consider:
Not all the embryos produced in the clinics will be implanted.
What do you do with the surplus embryos?
If multiple embryos implant, would you keep them all?
Where would you draw the line?
What would you do with the embryos you do not use?
Which factors would you use to “select” your offspring (e.g. gender)?
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Ethical Dilemmas
Science gives us the tools to diagnose certain
disorders, but it is up to us to decide the ethical
use of these tools.
Couples need to decide what risks they are
willing to live with when conceiving, and what
to decide in the event of a bad outcome of a
prenatal test.
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Would you want to be told that you have the allele that is responsible for Huntington’s disease, or another disorder that has no cure?
Would you want to know if you are a carrier for an autosomal recessive disorder?
Would you want to have prenatal screening done, even if you know you will not abort the fetus no matter what the results?
What should the criteria be for deciding which children receive newborn screening?
Do you think that pre-implantation diagnosis is a good idea? What are some things you might want to consider before performing this procedure?