Transcript Mendel & Heredity

Essential Question: How do organisms
reproduce more organisms like themselves?
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All organisms are made out of cells
The DNA inside of cells contains instructions to
build proteins which determine the type of
organism that will develop, how it will be built,
and how it will function
DNA is passed down from parents to offspring
during reproduction
In sexually reproducing organisms (ex. humans),
the DNA comes from 2 parents and is passed
down through special cells called gametes
NOTES –
Genetics:
Mendel &
Heredity
What is genetics?
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Genetics – the study of heredity (how
traits are passed from parents to offspring)
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Trait – a characteristic
of an organism that
can be inherited
(passed from parents
to offspring)
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Traits are controlled by genes
EX. Eye color in humans is a trait controlled
by genes which are passed from parents to
offspring
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Alleles – Different
versions of the same
gene
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EX. A certain eye
color gene in humans
has two alleles:
1. Pigmented iris (nonblue eyes)
2. Non-pigmented iris
(blue eyes)
Who was Gregor Mendel and
why was he important?
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Gregor Mendel – Mendel
was a Czechoslovakian
monk who is considered
the “father of genetics” for
figuring out the basic rules
of how traits are inherited
by doing experiments with
pea plants back in the
1850’s
Mendel’s Experiments
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Mendel did breeding experiments called
test crosses
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A test cross is when you choose 2 parents
(P1 generation) and use them to create
offspring (F1 generation) to see how a
specific trait(s) is passed down
Mendel Studied Pea Plants
Mendel’s Famous Experiment
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Mendel noticed that pea
plants had either tall
stems or short stems
Mendel noticed that some
tall plants, when selffertilized, always
produced tall offspring
and he noticed the same
about certain short plants
He called these plants
true-breeding
Mendel took a truebreeding tall stem plant
and crossed it with a
true-breeding short stem
plant
What happened?
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All the F1 offspring were tall – what happened to
the short trait?
Mendel then allowed the F1 offspring to crosspollinate
In the next generation (F2), 75% of the offspring
were tall, but 25% of the offspring were short –
the short trait reappeared
How can you get short plants from tall plants?
What Mendel Figured Out
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The Principle of Unit Factors
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Sexually reproducing organisms have 2 copies
of each gene
One gene comes from the sperm cell and one
gene comes from the egg cell
The genes can have different versions called
alleles
EX. For stem length, a pea plant can have 2
tall alleles, 1 tall allele and 1 short allele, or 2
short alleles
What Mendel Figured Out
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The Principle of Dominance and Recessiveness
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When an organism has 2 different alleles (like
1 tall and 1 short), the trait that shows up is
said to be dominant
The allele that does not show up is said to be
recessive
What Mendel Figured Out
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The Principle of Segregation
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The two alleles that an organism has for a
particular trait separate during gamete
formation (meiosis)
A gamete contains 1 allele for each trait
How can we predict what traits will
appear in the offspring of parents?
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Thanks to Mendel’s discoveries, we can
predict what traits offspring are likely to
have as long as we know the traits of the
parents
Punnett Square
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Punnet Square – a chart
used to predict the traits of
offspring
 Capitol letters are used to represent dominant
alleles
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EX. T for tall allele in pea plants
Lower case letters are used to represent recessive
alleles
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EX. t for short allele in pea plants
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Phenotype – The
trait the organism
shows
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EX. tall pea plant or
short pea plant
Genotype – The 2
alleles present in the
organism for a trait
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EX. T T or T t or t t
Genotypes can be homozygous or
heterozygous
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Homozygous – The 2 alleles for a trait
are the same
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EX. T T or t t
Heterozygous – The 2 alleles for a trait
are different
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EX. T t
Mendel’s Experiment Revisited
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Here is another look at Mendel’s
experiment using some new terms
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Mendel examined the height gene in pea
plants
The height gene in pea plants has 2 alleles,
tall (T) and short (t)
The tall allele is dominant and the short allele
is recessive
Mendel’s First Cross
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Mendel crossed a
homozygous tall plant
with a homozygous
short plant
P1 = T T x t t
Mendel crosses a homozygous tall plant
(TT) with a homozygous short plant (tt)
The tall parent passes on either its first
tall allele (T)…
…or its second tall allele (T) – either
way, it always passes a tall allele
The short parent passes either its first
short allele (t)…
…or its second short allele (t) – either
way it always passes a short allele
Possible offspring 1 – the tall parent’s
first tall allele with the short parent’s
first short allele (T t)
Possible offspring 2 – the tall parent’s
second tall allele with the short parent’s
first short allele (T t)
Possible offspring 3 – the tall parent’s
first tall allele with the short parent’s
second short allele (T t)
Possible offspring 4 – the tall parent’s
second tall allele with the short parent’s
second short allele (T t)
Genotypic Percentage – 100% of the
offspring are heterozygous (T t)
Phenotypic Percentage – 100% of the
offspring are tall plants
Mendel’s First
Cross
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Each parent only
passes 1 of its alleles,
so the tall parent
always passed on a T
and the short parent
always passed on a t
All the offspring had the genotype T t, because
they received a T from the tall parent and a t from
the short parent
Because the tall allele is dominant, all the offspring
were tall
Mendel’s Second Cross
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Mendel allowed the F1
generation, which
were all
heterozygous, to selfpollinate
P2 = T t x T t
Mendel crosses a heterozygous tall plant
(Tt) with a heterozygous tall plant (Tt)
The first tall parent passes on
either a tall allele (T)…
…or a short allele (t)
The second tall parent passes on
either a tall allele (T)…
…or a short allele (t)
Possible offspring 1 – the first tall
parent’s tall allele with the second tall
parent’s tall allele (T T)
Possible offspring 2 – the first tall
parent’s short allele with the second tall
parent’s tall allele (T t)
Possible offspring 3 – the first tall
parent’s tall allele with the second tall
parent’s short allele (T t)
Possible offspring 1 – the first tall
parent’s short allele with the second tall
parent’s short allele (t t)
Genotypic Percentage – 25% of the
offspring are homozygous dominant (T T)
Genotypic Percentage – 50% of the
offspring are heterozygous (T t)
Genotypic Percentage – 25% of the
offspring are homozygous recessive (t t)
Phenotypic Percentage – 75% of
the offspring are tall plants
Phenotypic Percentage – 25% of
the offspring are short plants
Mendel’s Second Cross
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Each parent only
passes 1 of its alleles,
but because each
parent has 1 of each
type of allele, they can
pass either a T or a t
Some of the offspring received two T alleles, some
received one T and one t, and some received two t
alleles
The plants with genotype T T were tall, the plants
with genotype T t were tall, and the plants with
genotype t t were short
Check for
Understanding
1. A one-eyed purple
people eater is crossed
with a two eyed purple
people eater. All of their
offspring have two
eyes. Which trait is
dominant?
2. If you use the letter E
for this gene. What is
the genotype of the
offspring?
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Are these offspring the F1
or
F2 generation?
3. If you crossed the
offspring
with each other?
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How many of the new
offspring would you
expect to have two eyes?