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Chapter 3: Reproduction and Heredity
Lesson 1: How do organisms Reproduce and grow?
(Pages A78-A85)
Main Idea: Living things pass their genetic traits to
offspring through asexual or sexual reproduction.
Every species lives on by reproducing. To reproduce is to
make more of one’s own kind.
Asexual Reproduction: reproduction in which ONE
parent produces offspring that are identical to the
Example: Bacteria through bacteria fission.
Hydra through budding.
Some plants through growing from roots, buds, or
Sexual Reproduction: reproduction from the union of
TWO parents that each pass on traits to the offspring.
Each organism in every generation gets a unique
combination of traits.
Example: Humans
Some plants through forming seeds.
In flowering plants, the flowers produce both eggs and
sperm. Pollen is the source of the sperm cells. The egg
cell is located deep in the flower, inside the ovule. After
fertilization, the ovule develops into a seed. The seed can
then germinate and grow into a plant. This new plant has
the genetic traits inherited from its parents, but it’s
genetically different from them.
Fertilization: the process in which a male gamete joins a
female gamete to produce a new cell that develops into
an organism.
Gametes: specialized reproductive cells. They are the
end product of a process that divides a parent cell twice.
A male gamete is called sperm.
A female gamete is called an egg.
Large organisms grow from one fertilized egg cell into an
adult with perhaps trillions of cells. This happens because
cells divide over and over again, often becoming more
specialized during the process.
Different types of cells include: skin cells, red blood cells,
white blood cells, muscle cells, & nerve cells.
Each cell contains a huge amount of genetic information.
When cells divide, they pass the genetic plan for the
entire organism to the new cells. Different parts of this
plan are expressed in the different cells.
The genetic information is held in molecules called DNA.
A cell’s DNA is stored in the nucleus in structures called
Mitosis: the stage in the cell cycle during which the
nucleus divides.
The Cell Cycle: All body cells grow and divide in a regular
cycle called the cell cycle.
STAGE 1: The longest part of the cell cycle is the
Interphase. During Interphase, the cell spends much of
its time growing. It also makes a copy of each of its
STAGE 2: Mitosis, the division of the nucleus. During
Mitosis, the cell gets ready to divide. It begins organizing
the pairs of chromosomes in its nucleus. Then, the pairs
separate into two nuclei. Each new nucleus receives one
copy of each chromosome, which is why the daughter
cells are identical to their parent.
STAGE 3: (FINAL STAGE): Cytokinesis. In this stage, a new
cell wall or cell membrane divides into two daughter
cells. The daughter cells then enter their own cell cycles.
The genetic information in a cell controls the cell’s
growth, development, and reproduction, creating genetic
Genetic Traits: characteristics of living things; those that
genes control.
In species that reproduce sexually, gametes typically
combine only with gametes of the same species. Over
time, a species develops variations. These are the
different forms of a trait. For example, blue and brown
are variations of eye color.
Chromosomes come in pairs. Each member of a pair of
chromosomes carries genes from one parent. One
chromosome came from the father and the other
chromosome came from the mother.
Dominant Trait: is produced by a variation of a gene that
is always expressed.
Recessive Trait: is one that will not appear if the gene for
the dominant trait is present.
Selective Breeding: two individuals with the desired traits
are mated to each other.
Lesson 2: How do organisms Inherit Traits? (Pages A86A97)
Main Idea: An organism’s genes determine its inherited
Gregor Mendel – honored as the father of genetics.
Mendel wondered why the pea plants he grew in his
garden had different physical traits. Some were tall,
some were short, some produced yellow seeds, and
others produced green seeds. Mendel also noticed that
new pea plants often looked similar to their parents, but
not always. Sometimes offspring looked different.
Mendel chose to study pea plants because they grow
quickly and have many traits that are easy to observe.
Mendel experimented to find out what would result from
crossing or mating two plants with different traits.
Mendel crossed two plants, one was tall and one was
short. Mendel called the parent plants the “parental
generation” or the “P generation.” He called the first
generation of offspring the “first filial generation” or “F1
generation.” Mendel predicted that some of the
offspring would be tall and some would be short.
Mendel’s prediction was wrong. All the offspring grew to
be tall.
What happened to the shortness trait?
To find out, Mendel let the plants in the F1 generation
produce lots of offspring. He called the offspring of the
F1 plants the “F2 generation.” Again, Mendel was
surprised. The plants in the F2 generation were a mix of
short and tall even though their parents (the F1 plants)
were all tall. Mendel found that ¾ of the F2 plants were
tall and ¼ were short. Put another way, for every 3 tall
plants, there was 1 short plant. That is a 3:1 ratio.
Mendel expanded his experiment to include other traits,
such as color and shape. In all his tests, he got the same
3:1 ratio of results in the F2 generation. ¾ of the F2
plants had one trait and ¼ had the other trait.
When Mendel studied peas, genes hadn’t been
discovered yet. Mendel proposed that “factors” control
traits. Mendel reasoned that these factors must come in
pairs, one from the male and one from the female. Based
on his observations, Mendel inferred that one factor
covers up the other. For example, all the F1 plants were
tall, so tallness covers up shortness. Mendel’s
conclusions proved correct. Scientists later found that his
“factors” are genes which often come in different forms.
Allele: one of different forms of a gene for a trait.
Each pea plant inherits two alleles for a given gene, such
as the gene for height. Individual alleles control the
plant’s traits. So, it’s really alleles that are inherited, not
In Mendel’s F1 pea plants, the allele for tallness covered
up the allele for shortness. When this happens, scientists
say that one allele is dominant and the other allele is
A dominant allele is expressed even if it is paired with a
recessive allele. On the other hand, a recessive allele is
expressed only if the organism has two recessive alleles.
In other words, a dominant allele always covers up a
recessive allele.
If tallness is a dominant trait, why did short plants appear
in Mendel’s F2 generation?
The answer lies in the alleles. Even though all the F1
plants had a dominant trait, each plant still carried an
allele for the recessive trait. Thus, some pea plants in the
F2 generation got 2 recessive alleles, one from each
parent. As a result, these F2 pea plants were short.
Genetic traits shorthand: Instead of writing the
description of a trait, scientists use a letter to represent
the gene for a trait such as height.
A capital letter is used for the dominant allele. Example:
for tallness, T
A lowercase letter is used for a recessive allele. Example:
for shortness, t
If a plant inherits 2 alleles for shortness, the letters tt
represent the alleles.
If a plant inherits 2 alleles for tallness, the letters TT
represent the alleles.
If a plant inherits an allele for tallness and an allele for
shortness, the letters Tt represent the alleles.
Purebred strains: TT or tt; meaning their parents were
either tall or short exclusively.
By crossing the purebred strains, each plant in the F1
generation received one allele for tallness and one for
shortness. Such plants are called hybrids.
Hybrid: an organism that has two different alleles of the
same gene.
A hybrid receives a dominant allele from one parent and
a recessive allele from the other parent.
Which allele is recessive and which is dominant in the
abbreviation Bb?
Predicting traits:
Probability is the chance that a certain event will happen.
In genetic studies, probability is usually stated as a ratio.
It can also be written as a fraction or as a percent.
If we toss a coin, the coin has 2 sides, before we toss it,
you do not know whether it will land on one side or the
other. When you call “heads” or “tails” you are making a
Because a coin has 2 possible outcomes, the probability
of either outcome is 1 out of 2.
Written as a ratio, the probability as 1:2.
Written as a fraction, the probability is one half (½).
Written as a percent, the probability is 50%.
All of the values represent the same ratio.
The probability of receiving an allele from a parent is a
lot like flipping a coin. Each parent has 2 alleles for a
gene, so the probability of receiving either allele is 1/2.
From two parents, the probability of receiving a specific
pair of alleles is ½ x ½, or ¼.
For example, in Mendel’s F2 cross of pea plants, each
offspring had a ½ probability of receiving the shortness
(t) allele from one parent, and a ½ probability of
receiving the same allele from the other parent. The
result is that ¼ of the offspring received both shortness
Punnett Squares
One way of finding the probability of each result is to use
a chart called a Punnett Square. A Punnett Square helps a
researcher predict each possible combination of alleles
that may occur in the offspring of a cross.
The allele that each parent will pass on is decided by
chance. The different alleles that can come from each
parent are shown above and beside the square. Each box
receives the letter above it and the letters beside it. Each
box in the Punnett Square represents an offspring.
Look at the Punnett Squares. In one, all the offspring are
gray. This is because eac one got one dominant allele (G)
and one recessive allele (g).
In the second Punnett Square, notice that 3 of the boxes
have gray rabbits and 1 of the boxes has a white rabbit.
The probability of having gray fur is 3:4 and the
probability of having white fur is 1:4.
These ratios are the same ones that Mendel saw. In the
F2 generation, 3 in 4 pea plants were tall and one in four
was short.
Inheritance Patterns:
Many alleles are either dominant or recessive, but that is
not always the case. In certain species of chickens, for
example, feather color is inherited differently. When a
black chicken is crossed with a white chicken, some of
their chicks have both black and white feathers. This is an
example of codominance.
Codominant: result of two alleles both being expressed.
In other cases, a dominant allele is only partly expressed.
Incomplete dominance: result of a dominant allele being
only partly expressed.
For example, the pink color of some four o’clock flowers
comes from a dominant allele for red that is only partly
How can you determine the probable ratio of traits in
Phenotype: the physical appearance an organism
presents to observers.
Genotype: the sum total of all the genes that an
organism inherits.
Heterogeneous: different in kind; mixed unevenly.
Homogeneous: the same; uniform in composition; mixed
In organisms that reproduce asexually, offspring receive
the same genetic information as the parent. Because
there is only one parent, there are no different alleles to
In organisms that reproduce sexually, however, each of
two parents gives only half their chromosomes to the
offspring. Gametes (eggs & sperm) each have half the
number of chromosomes as the body cells in the same
organism. The process that produces gametes is called
Meiosis: cell division that reduces the number of
chromosomes by half.
During meiosis, the parent cell divides twice and the
gametes have copies of half the parent’s chromosomes.
When the gametes unite, the new cell has two complete
sets of similar chromosomes. One set came from the
father and the other set came from the mother.
Scientists now understand that Mendel’s “factors” are
the alleles of genes. Genes are located on chromosomes,
and chromosomes are what cells pass to offspring.
What are genes made of?
Genes contain DNA.
DNA (deoxyribonucleic acid): the genetic material of all
living things.
DNA contains a code that can be copied and that allows
it to send “messages” to the cell and direct its activities.
What type of cells form by meiosis?
Human Genetics:
Humans have 46 chromosomes, which occur in 23 pairs.
Each chromosome contains a huge number of genes. In
fact, many scientists think that every cell has
approximately 40,000 genes. These genes are spread out
among 23 pairs.
In humans and many other animals, chromosomes are
classified into two groups. Two of the chromosomes are
called sex chromosomes. They are abbreviated X and Y.
Males are XY (one of each type of chromosomes in their
Females are XX (no Y’s).
Fraternal twins receive different sets of genes from their
parents. They are not identical. The differences include
different sex chromosomes. Thus, one twin is a girl and
the other is a boy.
All other chromosomes are called autosomes. A pair of
autosomes resemble each other much more than X and Y
chromosomes do.
The genes an individual receives determines many things
about his or her traits. They determine gender, eye and
hair color, and many other traits.
Some genes also put an individual at risk for health
problems. Studying how genes are passed in humans can
help doctors and scientists treat and counsel people for
these problems.
Your genetic traits are only part of what makes you who
you are. As you grow older, the knowledge and skills you
learn and the choices you make help change and define
What is one possible benefit of understanding human
Lesson 3: How is Genetic Information Used? (Pages A100A109)
Main Idea: DNA is the material that makes up genes and
determines traits. The structure of DNA is important to
the way living things use and pass on genetic
In 1953, scientists James Watson and Francis Crick built
the first model of DNA and proposed an explanation of
how it worked (their model relied partly on the work of
Rosalind Franklin and Maurice Wilkins). Their model
showed, DNA has the shape of a double helix. This looks
like a spiral staircase or twisted ladder. The sides of the
ladder are made up of sugars and phosphates. The steps
are the base pairs. Base pairs contain the coded
information that DNA holds.
DNA uses 4 different bases:
Adenine (A); thymine (T); guanine (G), and cytosine (C).
When they pair up, they ALWAYS pair up in the same
Adenine pairs with thymine, and guanine pairs with
Adenine (A) - Thymine (T)
Guanine (G) - Cytosine (C)
A single gene on a chromosome may have anywhere
from a hundred base pairs to a million base pairs.
You can read a single strand of DNA as a very long word.
In fact, the word would be billions of letters long! A
single strand of DNA is made of only four letters: A, T, G,
and C. The arrangement of these letters forms the long
coded message that DNA contains.
Scientists have studied the DNA of a huge variety of
Earth’s living things, from the largest animals and plants,
to the smallest protists and bacteria. With the exception
of some very ancient and unusual species, every
organism uses DNA in the same way. All use DNA of the
same shape, the same 4 bases, and the same four bases,
and the same type of coded information.
Who contributed to the discovery of DNA?
When cells copy their DNA to pass it on to offspring we
call this process replication.
Replication: the process of making identical copies of
How does this happen? The answer depends on 2 of the
DNA molecule’s features:
1. Its double helix
2. The way its bases pair up
Replication begins when the two sides of the double
helix “ladder” separate, much like a zipper coming
unzipped. This breaks the bonds that hold two base
pairs together.
Remember, adenine pairs only with thymine, and
guanine pairs only with cytosine. On each side of the
ladder, the exposed single bases find a new partner
from unpaired bases in fluids around them. As the bases
pair up, a new sugar-phosphate side forms and a new
ladder results.
In this way, each side of an original DNA molecule is used
as a template to make a new DNA molecule. The new
molecules have exactly the same sequence of base pairs
as the original. Each new DNA molecule may become
part of a new cell or even a new organism.
DNA stores instructions for cell structure and function.
In order for these instructions to be carried out, the DNA
must be transcribed, or decoded, into RNA. The
information can then be used to build a cell structure or
carry out a cell function.
RNA (ribonucleic acid) is similar to DNA, with 2
important differences:
1. RNA has the base uracil instead of thymine. Uracil
pairs with adenine.
2. RNA is a single strand, not a double helix.
The RNA made by transcription is used to make
proteins. These proteins determine the structures of a
cell and the functions they will perform. Insulin, for
example, is a protein that helps keep the level of sugar in
the blood fairly constant.
Making Proteins:
Proteins are made outside of the nucleus of a cell on
structures called ribosomes.
Ribosome: a cell structure where proteins are
Before the process can begin, the cell needs a
messenger to take the genetic code from the DNA
inside the nucleus to the ribosomes. The messenger is a
kind of RNA, called messenger RNA, or mRNA.
In the nucleus, DNA serves as a pattern from which
mRNA is made. Again, the DNA molecule’s “ladder”
separates between the base pairs. This time, RNA bases
pair up with them to form a single strand of mRNA. The
strand of mRNA then separates from the DNA.
Information from the DNA has been transferred to the
mRNA strand.
After mRNA leaves the nucleus, it attaches to a
ribosome. The ribosome holds an mRNA strand so that
three bases at a time are in position to bind to another
form of RNA.
This other form of RNA is called transfer RNA, or tRNA.
Transfer RNA molecules have an amino acid attached to
them. Amino acids are building blocks of proteins. There
are about 20 different kinds of tRNA molecules. Each
carries one of the 20 different kinds of amino acids.
Each tRNA unit binds to an mRNA strand by linking three
of its base to three bases in the mRNA. The order of the
three mRNA bases is a code for one amino acid. One
tRNA with its amino acid binds to every three bases along
the mRNA strand. In this process, the amino acids with
the tRNA line up next to each other.
Bound together, a chain of amino acids forms a protein
or part of one. The chain grows as building blocks are
added until a three-base code that means “stop” is
reached. Then, the ribosome releases a new protein.
How do the roles of messenger RNA and transfer RNA
Changes in DNA:
Imagine that instead of base G, the base A is added to
the DNA molecule. This will cause a permanent change in
the sequence of bases in the DNA. When such a change
occurs, a cell’s genetic information may change. This
change may affect only one gene or perhaps many genes.
Any permanent change in a gene or a chromosome is
called a mutation.
Mutation: any change in a genome or a chromosome.
Gene mutation changes one gene or a few.
Chromosomal mutation can affect a large amount of
genetic information.
Sometimes a mutation may only affect a certain cell.
Other times, a mutation happens in a cell that divides to
make gametes. In this case, the mutation may be passed
on to an offspring. A mutation can cause the proteins
that an organism makes to be different. With different
proteins, some of the organism’s physical traits will be
Mutations can be harmful or beneficial. Some mutations
change not just the protein itself, but also how much
protein is made. The mutation may also change where
the protein is made. These changes in DNA can result in
too much or too little protein being made, or they can
result in proteins being made in the wrong cell at the
wrong time.
Scientists now know that many disorders in humans are
caused by mutations. Some forms of cancer are caused
by mutations. For example, exposure to ultraviolet
causes mutations in skin cells. This may lead to skin
Sickle-cell anemia is a genetic disorder caused by a
mutation in the gene for hemoglobin. Hemoglobin is a
protein that helps red blood cells carry oxygen through
the body. The mutation causes the red blood cells to be
sickle-shaped. These cells can get stuck on blood vessels,
causing pain and tissue damage.
Some mutations are very helpful. For instance, one
chromosomal mutation causes strawberry plants to
make very large fruits. The cells of these strawberries
have extra sets of chromosomes.
Beneficial mutations can make a living thing more likely
to survive and reproduce. One example is the resistance
of bacteria to antibiotics. Gene mutations help some of
the cells become immune to the deadly effect of an
How is a chromosomal mutation different from a gene
Studying DNA:
No two people have the same fingerprints. Unless you
have an identical twin, no one has the same DNA
pattern as you. Investigators of crimes often take DNA
samples of skin and hair and use them to identify a
person. A DNA fingerprint is made to show the unique
patterns in an individual’s DNA.
Scientists who study DNA have also been working to
figure out the entire human genome. A genome is all the
genetic information that is found in the members of a
species. Figuring out the entire human genome would
help doctors and scientists better understand how our
bodies develop and work. It will also lead to a new
understanding of disease and help scientists develop new
medical treatments.
How is DNA like a fingerprint?