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Unit 6 Notes
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In 1851, Gregor Mendel (a priest from Europe) taught
high school and maintained the monastery’s garden
In the garden, Mendel grew hundreds of pea plants
and began noticing that they had different physical
characteristics (traits)
¤  Some
pea plants were short, others tall
¤  Some pea plants produced green seeds, others yellow
¨ 
Mendel observed that the pea plant’s traits were
similar to those of their parents
¤  Heredity
= the passing of traits from parents to offspring
¤  Genetics = the scientific study of heredity
n  Mendel
is known as the “Father of Genetics”
¨ 
Mendel’s Peas
¤  A
new organism begins to form when egg and sperm
are joined in the process of fertilization
n  When
¤  Mendel
plants fertilize themselves, it is called self-pollination
developed a method by which he could crosspollinate his pea plants, in order to conduct his
experiments
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Mendel’s Experiments
¤  Mendel
started his experiments with purebred plants
n  Purebred
= plant that always produces offspring with the
same form of a trait as the parent
¤  Mendel’s
First Experiment
n  Mendel
crossed 1 purebred tall plant with 1 purebred short
plant (P1 generation)
n  The offspring of the P1 cross were called the first filial
generation (F1 generation)
n  The offspring of the F1 cross were called the second filial
generation (F2 generation)
n  See results in Figure 2, p. 78
n 
Note: the F2 offspring are ¼ short, ¾ tall
- Mendel’s Work
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Other Traits (see Figure 3, p. 79)
¤  Mendel
n  Seed
observed seven characteristics in total:
shape – round or wrinkled
n  Seed color – yellow or green
n  Seed coat color – gray or white
n  Pod shape – smooth or pinched
n  Pod color – green or yellow
n  Flower position – side or end
n  Stem height – tall or short
- Mendel’s Work
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Dominant and Recessive Alleles
¤  Mendel’s
experiments taught him that individual genes
must control the inheritance of traits in peas
n  Alleles
n 
= the different forms of a gene
Example: stem height gene has a tall allele and a short allele
n  The
female parent gives one allele, the male parent gives
one allele
¤  Mendel
also learned that one allele can mask (hide) the
other allele
n  Example:
the tall allele masked the short allele in the F1
generation (Figure 2, p. 78)
¤  Individual
alleles control the inheritance of traits
n  Dominant
allele = one whose trait always shows up in the
organism when the allele is present
n  Recessive allele = one whose trait is covered up whenever
the dominant allele is present
n  Examples:
If we cross two tall P1 plants, can we have a short F1 plant?
n  If we cross one tall P1 plant and one short P1 plant, can we have
a short F1 plant?
n  Offspring are hybrids (they have two different alleles for
the same trait)
n  If we cross two short P1 plants, can we have a short F1 plant?
n 
¨ 
Using Symbols in Genetics
¤  Scientists
n  A
use letters to represent alleles in genetics
dominant allele is represented by a capitol letter
n 
Example: dominant tall stem height = T
n  A
recessive allele is represented by the lowercase version of
the dominant trait’s letter
n 
Example: recessive short stem height = t
two dominant parents produce offspring à TT
¤  When one dominant and one recessive parent produce
offspring à Tt (hybrid)
¤  When two recessive parents produce offspring à tt
¤  When
Chapter 3-2
Probability = the likelihood that a particular event
will occur
¨  Principles of Probability
¨ 
¤  If
you tossed a coin…
n  What
is the probability that the coin would land heads up?
n  What is the probability that the coin would land tails up?
n  In twenty tosses, how many would you predict would land
heads up?
¤  The
laws of probability predict what is likely to occur –
not necessarily what will occur
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Probability and Genetics
¤  Mendel
was the first scientist to recognize that the
principles of probability can be used to predict the
results of genetic crosses
n  Mendel
counted the offspring of every cross he carried out
n  Example: Mendel crossed two plants hybrid for stem height
(Tt x Tt) – ¾ of the F1 offspring had tall stems, ¼ had short
stems
n 
Therefore, the probability of producing long-stemmed offspring
is 3 in 4, and the probability of producing short-stemmed
offspring is 1 in 4
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Punnett Squares
¤  Punnett
square = chart that shows all the possible
combinations of alleles that can result from genetic
crosses
n  Punnett
squares can also predict the probability of a
particular outcome
¤  Phenotype
n  Examples:
¤  Genotype
is present)
= physical appearance (what it looks like)
tall, green
= genetic makeup (what allele combination
n  Examples:
TT, Gg
- Probability and Heredity
- Probability and Heredity
n  Homozygous
= an organism that has identical alleles for a
trait
n 
Examples: TT, tt, GG, gg
n  Heterozygous
= an organism that has two different alleles
for a trait (hybrid)
n 
¤  For
Examples: Tt, Gg
the following examples, use these abbreviations:
n  Homozygous
dominant tall = TT
n  Heterozygous (hybrid) tall = Tt
n  Homozygous recessive short = tt
¤  Example:
n  P1:
Purebred tall x Purebred tall
n  F1 result à all tall
n 
100% chance of being tall
T
T
T TT TT
T TT TT
¤  Example:
n  P1:
Purebred tall x hybrid tall
n  F1 result à all tall
n 
100 % chance of being tall
T
T
t
T
TT TT
Tt Tt
¤  Example:
Purebred tall x short
n  F1 result à all tall
T
n  P1:
n 
100% chance of being tall
T
t
Tt Tt
t
Tt Tt
¤  Example:
hybrid tall x hybrid tall
n  F1 result à 3 tall, 1 short
T
n  P1:
75% chance of being tall
n  25% chance of being short
n 
T
t
t
TT Tt
Tt $
¤  Example:
hybrid tall x short
n  F1 result à 2 tall, 2 short
T
n  P1:
50% chance of being tall
n  50% chance of being short
n 
t
t
Tt $
t
Tt $
¤  Example:
short x short
n  F1 result à all short
t
n  P1:
n 
100% chance of being short
t
t
$ $
t
$ $
- Probability and Heredity
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Codominance
¤  Sometimes,
a dominant allele and a recessive allele do
not exist
¤  Codominance = alleles are neither dominant nor
recessive
n  Examples:
Chickens in Figure 10, p. 89
n  Labrador retrievers (yellow, black, chocolate)
n 
- Probability and Heredity
Chapter 3-3
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Chromosomes and Inheritance
¤  In
1903, Walter Sutton studied sex cells in
grasshoppers
¤  Chromosome theory of inheritance = genes are
carried from parents to their offspring on chromosomes
n  Sex
cells (eggs and sperm) contain half the number of
chromosomes of body cells
¤  Meiosis
= the process by which the number of
chromosomes is reduced by half to form sex cells (eggs
and sperm)
n  See
“Meiosis” on p. 94-95
- The Cell and Inheritance
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Meiosis and Punnett Squares
¤  See
Figure 14, p. 95
¤  Also, a Punnett square can be used to determine the
probability of the gender of offspring
n  Example:
n  P1:
XY (male) x XX (female)
n  F1 results à 2 females, 2 males
n  50% chance of being male
n  50% chance of being female
X
X
X
Y
- The Cell and Inheritance
- The Cell and Inheritance
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Chromosomes
¤  Organisms
can vary greatly in the number of chromosomes
in their body cells
n  Humans
have 46 chromosomes (23 pairs) per body cell
n  Dogs have 78 chromosomes per body cell
n  Goldfish have 94 chromosomes per body cell
n  Note: larger organisms do not necessarily have more
chromosomes!
¤  Although
your body may only have 23 pairs of
chromosomes, your body cells contain between 30,000 and
35,000 genes – each controlling a particular trait
n  That
is why no two people are exactly alike!
n  See Figure 15, p. 96
Chapter 3-4
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The Genetic Code
¤  The
main function of genes is to control the production of
proteins in the organism’s cells
n  Proteins
help determine the size, shape, and many other traits of
an organism
¤  Chromosomes
are composed mostly of DNA
¤  DNA is composed of four different nitrogen bases (adenine,
thymine, guanine, cytosine)
single “rung” on the DNA “ladder” contains hundreds of
millions of nitrogen bases
n  The nitrogen bases are arranged in a specific order
n  A
n 
Example: ATGACGTAC
¤  The
order of the nitrogen bases along a gene forms a
genetic code that specifies what type of protein will be
produced
n  Groups
of three nitrogen bases result in the production of a
specific amino acid
n  Amino acids combine to make proteins
¤  Think
of the following analogy:
n  Nitrogen
bases = letters
n  Amino acids = words
n  Protein = sentence
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How Cells Make Proteins
¤  Protein
synthesis = the production of proteins
n  The
cell uses information from a gene on a chromosome to
produce a specific protein
n  Takes place on the ribosomes in the cytoplasm of the cell
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The Role of RNA
¤  RNA
and DNA are similar, but differ in important ways
looks like only one side of the “ladder”
n  RNA contains a different sugar than DNA
n  RNA has the nitrogen bases adenine, guanine, and cytosine,
but has uracil (U) instead of thymine
n  RNA
¤  Types
of RNA
n  Messenger
RNA (mRNA) = copies the coded message from
the DNA in the nucleus and carries the message to the
cytoplasm
n  Transfer RNA (tRNA) = carries amino acids and adds them
to the growing protein
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Translating the Code
¤  See
“Protein Synthesis” p. 100-101
n  Know
steps one through four!
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Mutations
¤  Types
of Mutations
n  Single-base
substitution (Example: A attaches instead of G)
n  Chromosomes do not separate evenly during meiosis
(resulting in too many or too few chromosomes)
¤  The
Effects of Mutations
n  Helpful
mutations – Example: new, better-tasting potatoes
n  Harmful mutations – Example: cancerous tumor
n  Neither helpful nor harmful mutations – Example: albino
animal in captivity
- The DNA Connection
- The DNA Connection
- The DNA Connection
Chapter 4-1
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Traits Controlled by Single Genes
¤  Many
traits are controlled by a single gene with two
alleles
n  Often
one allele is dominant, and one allele is recessive
¤  Example:
n  P1
genotype: Ww x Ww
n  P1
n  F1
Figure 2, p. 111
phenotype: widow’s peak x widow’s peak
genotype: 1 WW, 2 Ww, 1 ww
n  F1
phenotype: ¾ widow’s peak, ¼ straight hair line
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Multiple Alleles
¤  Some
human traits are controlled by a single gene with
more than two alleles
¤  Multiple alleles = three or more forms of a gene that code
for a single trait
¤  Example: Human blood types (Figure 3, p. 112)
n  There
are four main blood types – A, B, AB, O
n  Three alleles control the inheritance of blood types
The allele for type A and the allele for type B are codominant
n  The allele for type O is recessive
n 
n  Note:
People with type O blood are “universal donors”
n  People with type AB blood are “universal recipients”
n 
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Traits Controlled by Many Genes
¤  Some
human traits show a large number of phenotypes
because the traits are controlled by many genes
n  Examples:
Height – controlled by at least four genes
n  Skin color – controlled by at least three genes
n 
¤  The
genes act together as a group to produce a single
trait
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Male or Female?
¤  The
sex of a baby is determined by genes on
chromosomes
¤  Each human body cell has 23 pairs of chromosomes
n  One
pair is made of two sex chromosomes
The sex chromosomes determine the baby’s gender
n  The sex chromosomes are the only pair of chromosomes that do
not always match
n  Remember from Chapter 3: XX (Female), XY (Male)
n  See Figure 5, p. 113
n 
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Sex-Linked Genes
¤  Sex-linked
genes = genes on the X and Y chromosome
¤  Because males have only one X chromosome, males are
more likely than females to have a sex-linked trait that is
controlled by a recessive allele
n  Example:
red-green colorblindness
See Figure 6, p. 114
n  See Figure 7, p. 115
n  It takes two recessive alleles to have a colorblind female
n 
§  Carrier = one who has a recessive allele, but does not have the trait
n 
But, it takes only one recessive allele to have a colorblind male
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The Effect of the Environment
¤  The
effects of genes are often altered by the
environment
¤  Examples:
n  Diet
n 
Due to better eating habits, the average height of adults in the
U.S. has increased by almost 10cm in the last one-hundred years
n  Medical
care
n  Living conditions
Chapter 4-2
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Genetic Disorders
¤  Genetic
disorder = an abnormal condition that a
person inherits through genes or chromosomes
¤  Genetic disorders are caused by mutations (changes in
a person’s DNA)
¤  Examples:
n  Cystic
fibrosis
n  Sickle-cell disease
n  Hemophilia
n  Down syndrome
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Cystic Fibrosis
¤  Cystic
fibrosis = genetic disorder in which the body
produces abnormally thick mucus in the lungs and
intestines
n  Bacteria
¤  The
grow in the mucus and cause infections
mutation that causes cystic fibrosis is carried on a
recessive allele
¤  Currently, there is no cure for cystic fibrosis
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Sickle-Cell Disease
¤  Sickle-cell
disease = genetic disorder that affects the
production of hemoglobin in blood
n  Hemoglobin
= protein in red blood cells that carries oxygen
n  When oxygen concentrations are low, red blood cells take on an
unusual shape
n 
Figure 9, p. 118
n  The
sickle-shaped cells cannot carry as much oxygen and block
blood vessels, resulting in pain and weakness
¤  The
mutation that causes sickle-cell disease is codominant
with the normal allele
¤  Currently, there is no cure for sickle-cell disease
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Hemophilia
¤  Hemophilia
= genetic disorder in which a person’s
blood clots very slowly or not at all
n  A
person with hemophilia can bleed to death from a minor
cut or scrape
¤  The
mutation that causes hemophilia is caused by a
recessive allele on the X chromosome (sex-linked
disorder)
¤  Currently, there is no cure for hemophilia
n  People
with hemophilia can live normal lives – they just have
to be careful
¨ 
Down Syndrome
¤  Down
syndrome = results when a person’s cells have
an extra copy of chromosome 21 (due to an error in
meiosis)
¤  People with Down syndrome have a distinctive physical
appearance and some degree of mental retardation
n  p.
117
¤  Heart
defects are common, but can be treated
¤  Despite limitations, people with Down syndrome can
lead full, active lives
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Pedigrees
= a chart or “family tree” that tracks which
members of a family have a particular trait
¤  Pedigree
n  Geneticists
humans
¤  See
use pedigrees to trace the inheritance of traits in
Figure 10, p. 119
n  Circle
= female
n  Square = male
n  Colored shape = person has trait
n  Half-colored shape = person is carrier for trait
n  No color in shape = person does not have trait
n  Horizontal line = connects two married people
n  Vertical line & bracket = connects parents to children
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Diagnosing Genetic Disorders
¤  Scientists
began diagnosing genetic disorders with Punnett
squares and pedigrees
¤  Today, scientists use tools such as amniocentesis and
karyotypes to help predict genetic disorders
n  Amniocentesis
= procedure done before a baby is born which
determines whether the baby will have some genetic disorders
n 
Cells are taken from the fluid surrounding the baby
n  Karyotype
n 
= picture of all the chromosomes in a cell
Made from the cells taken by amniocentesis
¤  Genetic
counseling = guidance for couples with family
histories of genetic disorders
Chapter 4-3
People have developed several ways to create
organisms with desirable traits
¨  Selective breeding
¨ 
¤  Selective
breeding = the process of selecting a few
organisms with desired traits to serve as parents for the
next generation
¤  Techniques
n  Inbreeding
= involves crossing two genetically similar
individuals
n  Hybridization = involves crossing two genetically different
individuals
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Cloning
¤  Clone
= an organism that is genetically identical to the
organism from which it was produced
¤  In plants, scientists grow new plants from cuttings (small
parts of the original plant)
¤  In animals, scientists remove an egg, replace the
nucleus, and implant the nucleus to develop
n  This
process takes three different animals of the same
species
n  This is controversial, since removing the nucleus can be
considered “killing” a life
¨ 
Genetic Engineering
¤  Genetic
engineering = genes from one organism are
transferred into the DNA of another organism
¤  Also called “gene splicing” because DNA is cut open
and genes are added
¤  Genetic engineering was first successful in bacteria
n  See
n 
n  We
“Genetic Engineering” on p. 126
We use genetically engineered bacteria to create insulin (a
drug to treat diabetes)
also use bacteria to create human growth hormone (a
protein controlling growth in children)
¤  Genetic
Engineering in Other Organisms
n  Bacteria
have been implanted into tomatoes, wheat, and
rice to enable them to:
Survive in colder temperatures
n  Grow in poor soil conditions
n  Resist insect pests
n 
n  Genes
have been inserted into animals, which then create
medicines for humans
n 
Example: cows can produce a protein that clots blood – helping
those with hemophilia
¤  Gene
therapy = process of using genetic engineering to try
to correct genetic disorders
n  Working
copies of a gene are inserted directly into a person’s
cells
n  Example: “engineered” viruses can be inserted into the lung cells
of people with cystic fibrosis, helping them breathe
¨ 
DNA Fingerprinting
¤  DNA
fingerprinting = identifying a person by their own
unique DNA
n  Scientists
have found ways gather DNA samples from hair, skin,
and blood at crime scenes
n  These techniques have put many criminals in jail
¨ 
The Human Genome Project
¤  Genome
= all the DNA in one cell of one organism
n  Researchers
estimate that there are 20,000-25,000 genes in
one cell’s DNA
¤  The
main goal of the Human Genome Project is to
identify the DNA sequence of every gene in the human
genome
n  This
would help us understand the following:
How humans develop
n  What makes our bodies work
n  What causes things to go wrong in our bodies
n  Potential treatments/cures for genetic disorders and disease
n