Heredity and Reproduction Chapter 4 Reference Text: PCI LIFE
SCIENCE
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Chapter 4 - Section 1 TRAITS
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TRAITS Have you ever heard someone say something like, "You
have your mother's eyes, or "You have your father's chin"? Why do
you resemble your parents?
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TRAITS The color of your eyes is a trait. Traits are the
physical characteristics that an organism possesses. Parents pass
on their traits to their children (offspring). This is called
heredity.
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TRAITS What determines which traits you inherit from each
parent?
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Mendels Pea Plants In the 1800s, a scientist named Gregor
Mendel studied the passing of traits from parents to offspring. He
wanted to know why certain patterns showed up in living
things.
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Mendels Pea Plants Mendel had noticed that a certain trait
would appear in one generation but not in the next. Then, the trait
would show up again in the next generation. For example, a man
would have curly hair, his child would have straight hair, and his
grandchild would have curly hair.
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Mendels Pea Plants Mendel wanted to know why this happened. He
wondered how traits passed from parents to offspring.
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Mendels Pea Plants Mendel did some experiments to find out. He
decided to cross-pollinate pea plants and see what happened.
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Mendels Pea Plants Cross-pollination happens when pollen from
one is transferred to the reproductive structure of another plant.
The parent plants will produce a new plant.
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Mendels Pea Plants Cross-pollination can happen when an insect
sits on a flower and pollen sticks to the insect. When the insect
flies to another flower, it deposits the pollen from the first
flower onto the reproductive structure of the second flower.
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Mendels Pea Plants
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Mendel conducted his experiments with pea plants because they
grow quickly and there are a lot of different kinds of pea
plants.
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Mendels Pea Plants For example, pea plants can be short or
tall, can have round or wrinkled seeds, and can have purple or
white flowers.
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Mendels Pea Plants In Mendel's experiments, he cross-
pollinated plants that had different traits to see what the
offspring would look like. For example, Mendel wanted to know what
would happen if a tall plant and a short plant had offspring. Would
the offspring be tall, short, or medium? What do you think?
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Mendels Pea Plants If a plant with purple flowers is
cross-pollinated with a plant with white flowers, what color will
the offspring's flowers be? Mendel studied seven characteristics of
pea plants. He wanted answers to the following questions: If a
plant with yellow seeds is cross-pollinated with a plant with green
seeds, what color will the offspring's seeds be? If a plant with
round seeds is cross-pollinated with a plant with wrinkled seeds,
what shape will the offspring's seeds be? If a plant with green
pods is cross-pollinated with a plant with yellow pods, what color
will the offspring's pods be?...and so on
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Chapter 4 - Section 2 Dominant and Recessive Traits
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Mendels Pea Plants Gregor Mendel cross-pollinated pea plants to
study the characteristics of their offspring, For example, he
cross- pollinated tall plants with short plants. These plants are
called the parent generation (P). Their offspring are called
first-generation (F1) plants.
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Mendels Pea Plants In Mendel's experiment, all of the
first-generation plants were tall. What happened to the short
trait? Would all the offspring always be tall?
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Dominant & Recessive Traits The same results happened in
every experiment. One trait, like being tall, was always there in
the first generation. The other trait, like being short, seemed to
go away.
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Dominant & Recessive Traits He described the trait that
went away in the first generation as the recessive trait. Mendel
described the trait that was there in the first generation as the
dominant trait.
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Dominant & Recessive Traits A dominant trait is a trait
that is most likely to appear in offspring and a recessive trait is
least likely to appear in offspring.
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Dominant & Recessive Traits If green pod color is the
dominant trait and yellow pod color is the recessive trait, what
will the pod color of the F1 generation offspring have? If you said
GREEN, you are correct!
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Dominant & Recessive Traits Mendel wanted to know what
happened to the recessive traits. So, he watched what happened when
the first- generation plants reproduced. Their offspring were
called second-generation plants. (F2)
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Dominant & Recessive Traits Mendel noticed that the
recessive traits came back in some of the offspring. Most of the
second-generation plants were tall, but some were short.
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Dominant & Recessive Traits This happened with every set of
traits that Mendel studied. Each recessive trait reappeared in some
of the second-generation plants.
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Dominant & Recessive Traits Mendel figured out that each
plant had two sets of instructions for each characteristic.
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Dominant & Recessive Traits These instructions are called
genes. Genes are the parts of DNA that give instructions for what
traits offspring will have.
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Dominant & Recessive Traits Each parent gives one set of
genes to the offspring. The offspring has two forms of the same
gene for each characteristic. Each member of a gene pair is called
an allele.
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Genotype and Phenotype The combination of alleles in an
organism controls what traits the organism will have. For example,
there is one allele for shortness in pea plants and one allele for
tallness. If the offspring's gene for height is made up of two
alleles for tallness, the plant will be tall. If the offspring's
gene for height is made up of two alleles for shortness, the plant
will be short
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Genotype and Phenotype This combination of genes passed on to
offspring is called genotype. Dominant alleles are written as
uppercase letters. So, the genotype for a plant with two dominant
alleles for height is TT. Recessive alleles are written as
lowercase letters. So, the genotype for a plant with two recessive
alleles for height is tt. TT tt
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Genotype and Phenotype If an organism has one dominant allele
for height and one recessive allele for height, its genotype is
Tt.
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Genotype and Phenotype Phenotype is the way an organism looks.
An organism's phenotype is decided by its genotype.
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Genotype and Phenotype If a dominant allele is present in the
organism's genotype, that trait will appear in the organism. If a
plant has one dominant allele for height, the plant will be tall. A
recessive allele is hidden if a dominant allele is present.
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Genotype and Phenotype An organism will only display a
recessive trait when two recessive alleles are present and there
are NO dominant alleles.
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Genotype and Phenotype Imagine that two pea plants are cross-
pollinated. One pea plant has the genotype TT, so it is tall. The
other pea plant has the genotype tt, so it is short.
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Genotype and Phenotype Because the allele for tallness (T) is
dominant, a plant with the genotype TT or Tt will be tall.
Remember, a recessive allele is hidden if a dominant allele is
present. Offspring with the genotype tt have two recessive alleles.
These offspring will be short.
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Punnett Squares Biologists use a diagram called a Punnett
square to figure out the probability of an offspring expressing a
certain genotype. Probability is the mathematical chance that
something will happen.
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Punnett Squares Look at the Punnett square below. One parent's
genotype for height is recorded on the top row (TT). The other
parent's genotype for height is recorded in the left column (tt).
TT t t
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Punnett Squares Remember that a child gets one allele from each
parent. MOMDAD TT t t
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Punnett Squares The four examples in the Punnett square show
what will happen if the two parents have offspring. In this case,
every combination of alleles from the two parents results in the
genotype Tt. So, 100 percent of the offspring will be tall. TT t t
T t
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Punnett Squares What happens when one of the parents has a
dominant allele and a recessive allele? Tt t t
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Punnett Squares Look at the Punnett square below. In this case,
two offspring have the genotype Tt and two have the genotype tt.
Two offspring will be tall, and two will be short. Tt t t T t t T t
t
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Punnett Squares What happens when both parents have a dominant
allele and a recessive allele? Tt T t
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Punnett Squares Look at the Punnett square below. In this case,
one offspring has the genotype TT, two have the genotype Tt, and
one has the genotype tt. How many of the offspring will be tall,
and how many will be short? Tt T t T T t t
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Traits in Humans A combination of alleles makes you look the
way you do. You get traits from your parents, just like pea plants
do.
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Traits in Humans For example, the trait of earlobe shape in
humans is inherited. There is an allele for free earlobes and an
allele for attached earlobes. The allele for free earlobes is
dominant, and the allele for attached earlobes is recessive. Free
Earlobes Attached Earlobes
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Traits in Humans If your gene for earlobes is made up of two
alleles for free earlobes, you will have free earlobes. If you have
two alleles for attached earlobes, you will have attached earlobes.
If you have one allele for attached earlobes and one allele for
free earlobes, what will your earlobes look like?
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Traits in Humans The characteristics of all organisms are
controlled by the alleles that make up their genes. Have you ever
looked at a newborn baby and noticed that it resembles its parents?
You are observing the effects of alleles.
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Traits in Humans Alleles determine who has a cleft chin, curly
hair, blue eyes, and many other physical characteristics.