Transcript Notes 5-1 & 5-2

Mendel and Punnett Squares
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Mendel was a geneticist who studied pea
plants
He began his experiments by crossing 2
purebred organisms
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Gregor Mendel crossed pea plants that had
different traits. The illustrations show how
he did this.
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In all of Mendel’s crosses, only one form of the
trait appeared in the F1 generation. However, in
the F2 generation, the “lost” form of the trait
always reappeared in about one fourth of the
plants.
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Mendel studied several traits in pea plants.
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Genes are factors that control a trait
Each gene has 2 contrasting forms called alleles
Some alleles are dominant (represented by a
capital letter)
Some alleles are recessive (represented by a
lower case letter)
If a gene has both dominant alleles, it is
homozygous dominant (TT)
If a gene has both recessive alleles, it is
homozygous recessive (tt)
If a gene has 1 dominant and 1 recessive allele,
it is heterozygous (Tt)
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In this cross, both parents are heterozygous
for the trait of seed shape. R represents the
dominant round allele, and r represents the
recessive wrinkled allele.
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In a genetic cross, the allele that each parent
will pass on to its offspring is based on
probability.
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An organism’s
phenotype is its
physical appearance,
or visible traits. An
organism’s genotype
is its genetic makeup,
or allele
combinations.
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Mendel allowed
several F1 pea plants
with yellow seeds to
self-pollinate. The
graph shows the
approximate numbers
of the F2 offspring
with yellow seeds and
with green seeds.
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Reading Graphs:
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How many F2 offspring
had yellow seeds? How
many had green seeds?
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Yellow–6,000; green–
2,000
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Calculating:
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Use the information in
the graph to calculate the
total number of offspring
that resulted from this
cross. Then calculate the
percentage of the
offspring with yellow
peas, and the percentage
with green peas.
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8,000; 75% have yellow
peas and 25% have green
peas.
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Inferring:
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Use the answers to
Question 2 to infer the
probable genotypes of the
parent plants. (Hint:
Construct Punnett squares
with the possible genotypes
of the parents.)
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Both parents probably
had the genotype Bb.
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In codominance,
the alleles are
neither dominant
nor recessive. As
a result, both
alleles are
expressed in the
offspring.