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Chapter 3
Part 2
Sections 7-12
SECTION 3-7 What are
incomplete dominance and
codominance?
Key terms!
• Incomplete dominance- pattern of
inheritance in which alleles from both
parents are blended
• Codominance- patter of inheritance in
which both alleles of a gene are
expressed
Incomplete Dominance
• In simple dominance, one allele is
dominant over another.
• There are special cases, however, when
there is no completely dominant trait.
This is called incomplete dominance.
– When this happens offspring will show a
blending of traits from each parent.
Incomplete Dominance
• Incomplete dominance can been seen is
many organisms.
• We can still complete a Punnett square
for these cases, to determine the
probability of what the offspring will be
like.
• Now, instead of having a capital and a
lowercase letter. We will have two
capital letters for both alleles!
For Example
• There is a flower called the four o’clock
flower, which gets it’s name for the
time of day that the flowers open,
shows incomplete dominance.
• RR- are red plants
• WW- are white plants
• RW- are pink
It looks like this
Codominance
• A different type of dominance is
codominance.
– This happens when both alleles of a certain
trait appear in the offspring.
• As with incomplete dominance,
codominance neither dominant allele is
dominant over each other.
• IT IS NOT THE SAME AS
INCOMPLETE DOMINANCE!!!
Codominance in Chickens
• In some chickens there is a gene for
black feathers (B) and there is a gene
for white feathers (W).
• A chicken that has:
– Black feathers is BB
– White feathers is WW
– Both black and white is BW*
• *This is the heterozygous form.
Heterozygous Crosses
• When crossing two organisms showing
incomplete dominance you will have 3
categories that the offspring fall into.
• It looks like this:
3-8 How is gender
determined?
Key Terms
• Gender – sex of a person or other
organism
X and Y
• In people, there are 23 chromosomes.
The 23rd chromosome determines a
person’s gender, or sex.
• Males are X and Y
• Females are X and X
A Karyotype
Gametes
• Gametes (or sex cells) are different in
males and females.
• Male gametes have an XY.
– During meiosis, a sperm cell can carry and X or
Y.
• Female gametes have an XX
– During meiosis, an egg cell can carry only an X.
• Males will determine the sex of a their
offspring
Gender of Offspring
• The gender of the offspring is
determined by the sperm cell
– If they get an X or a Y
• XX – female
• XY – male
• The chances of someone being male or
female is 50%, ALWAYS!
3-9 What are sex-linked
traits
Key Terms
• Sex-linked traits- traits that are
controlled by the sex chromosomes
• Carrier- an organism that has a
recessive gene for a trait but does not
show the trait (they are heterozygous)
• Pedigree- chart that shows inheritance
of certain traits over several
generations.
Inherited traits
• Some traits are inherited along with
gender.
• These traits are controlled by the X and Y
gender chromosomes.
• Traits that are inherited along with gender
are called sex linked traits.
• The X chromosome carries more traits
than the Y.
– So most of the sex-linked traits are found o
the X chromosome.
Sex-linked Disorders
• Some hereditary disorders are caused
by sex-linked traits.
• 2 examples are color blindness and
hemophilia.
– Hemophilia is when the blood does not clot
properly.
– Color blindness is where a person cannot
see the different between colors.
Sex-linked disorders
• They are found more commonly in men
than women.
• This is because the genes for most sexlinked disorders are recessive.
Carriers
• Women who have one normal gene and
one recessive gene (heterozygous) are
said to be carriers of the disorder.
– This means they carry the disorder and can
pass it on, but do not have it themselves.
If we did a Punnett Square…
X
XC
X
Normal
XX
Carrier
XXC
Y
Normal
XY
Color blind
XCY
Practice Problem #1
• Dave is color blind, his wife Anna isn’t color
blind, but her father is. What is the
chances that they will have a color blind
son?
Practice Problem #2
• Susan is a carrier for color-blindness, her
husband, Alex, isn’t color blind. What is the
chance that they will have a color blind son? A
daughter that is a carrier?
Pedigrees
• Scientists often study the traits of
past generations in order to predict the
traits of offspring. One way to do this
is to use a pedigree.
• Pedigrees are very similar to a family
tree. It shows how a trait is passed
from one generation to the next.
• Pedigrees can identify carriers of a
trait, and people who have a trait.
How to read a pedigree
• A pedigree uses symbols to represent males,
females, carriers, those who are normal, and
those who have the trait that we are examining.
• All this information will be found in a key.
• A male will be represented by a square
• A female by a circle.
• Those who are affected by the trait will have
their circle or square filled in.
• Those who carry it, will have half of it filled in.
• And those who are normal, or don’t have it all, will
have an empty or not colored in circle.
Left handed- ness
Key:
Another Example
3-10 What are some
inherited diseases?
Key Term
• Inherited disease- disease caused by an
inherited gene
Inherited disease
• Sometimes a gene is defective or
abnormal. The abnormal gene stops the
body from working correctly and then
causes a disease
• An abnormal gene can be passed from
parents to their offspring.
• This is called an inherited disease
Examples of Inherited Diseases
• Sickle-cell anemia- a disease that affects the
shape of red blood cells. They are shaped like a
crescent moon.
• PKU (phenylketonuria)- people who have this
disease cannot eat foods that are protein rich
such as fish, milk and eggs.
• Tay-Sachs Disease- is a disease that affects
those who are mostly of Jewish decent. It
affects children at an early age by preventing
them from breaking down fat. As a result fat
gathers in the child’s brain and causes death at
an early age.
• Huntington’s Disease- This one is
carried by a dominant gene and it
affects males in their later 30’s to 40’s
after they have already had children. It
breaks down the ability to control your
muscles, and results in death at an
earlier age than normal.
People in Science
• Genetic Counselor
– Are people who study family histories and
interpret test results in order to predict
the possible traits of offspring.
3-11 How does the environment
affect inherited traits?
Key Terms
• Mutations- change in a gene
Conditions in the environment
• Genes control many of your traits.
– This is true for all living organisms
• However sometimes, other factors can
affect traits.
• The environment affects the traits of
living things in many different ways.
– Ex.) Green plants need sunlight to live. If
they don’t have the greatest sunlight they
won’t grow as tall.
Environment and Traits
• An example of the environment
affecting trait is the divi-divi tree that
lives on the island of Aruba. It grows
side ways because of the strong winds,
NOT because of genetics.
Environment and Genes
• Plants that are grown in poor soil are small
in size. They will produce only a few fruits.
• Adding nutrients to the soil changes the
environment, which means that the plant
can now grow taller and produce more
fruit.
• In this example the genes are not changed
or affected, only the appearance of the
plant.
Genetic Mutations
• A mutation is a change in the genetic
material of an organism.
– This can be harmful if it decreases the ability
of the organism to function properly.
• If the mutation occurs in the gametes, it
can be passed on to the next generation.
• If it only affects the body cells, it will
NOT be passed to the next generation.
• Mutations can affect a single gene, or an
entire chromosome.
Causes of Mutations
• Mutations occur randomly in nature.
• However, they can be increased by factors
in the environment.
• Factors that increase mutations are called
mutagens.
– Ex.) ultraviolet light and some chemicals;
radiation from X-rays or nuclear power plants.
• Some mutagens are known to cause cancer
and are called carcinogens.
3-12 How is genetics used to
improve things?
Key Terms
• Controlled breeding- mating organisms
to produce offspring with certain traits
• Mass selection- crossing plants with
desirable traits
• Hybridization- mating two different
kinds of organisms
• Inbreeding- mating closely related
organisms.
Controlled breeding
• People have been raising and breeding
crops and animals for thousands of years.
• Over the centuries they have learned that
some crops produced better plants than
others, and that some animals had better
traits than others.
• They began to mate those with desirable
traits together, this is called controlled
breeding.
Mass selection
• Plant growers use a process called mass
selection.
– This is where plants with desirable traits are
crossed.
– Seeds from these plants are collected and
then planted.
– The seeds from those offspring with the
desirable traits are then kept to keep the
process going.
• An example of this is the corn that we eat.
Hybridization
• Sometimes 2 organisms with different
kinds of genes are crossed.
• The offspring with show traits from
both parents and is called a hybrid.
• The process of breeding 2 different
organisms with different genes is called
hybridization.
– Ex.) mule- mother is a horse, father is a
donkey.
Inbreeding
• Inbreeding is the mating of closely
related organisms. The offspring have
genes that are very similar to BOTH
parents.
• This is done with racehorses, and a lot
of plants.