Unit 2 Lesson 4 Heredity
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Unit 2 Lesson 4 Heredity
Give Peas a Chance
What is heredity?
• Traits, such as hair color, result from the
information stored in genetic material.
• Heredity is the passing of genetic material from
parents to offspring.
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Unit 2 Lesson 4 Heredity
What did Gregor Mendel discover about
heredity?
• Gregor Mendel was an Austrian monk. In the
1800s, Mendel performed the first major
experiments in heredity.
• Mendel studied seven characteristics of pea
plants.
• A characteristic is a feature that has different
forms in a population.
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Unit 2 Lesson 4 Heredity
What did Gregor Mendel discover about
heredity?
• The seven different characteristics Mendel studied
were plant height, flower and pod position, seed
shape, seed color, pod shape, pod color, and
flower color.
• Each characteristic had two different forms. These
different forms are called traits.
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Unit 2 Lesson 4 Heredity
What did Gregor Mendel discover about
heredity?
• Mendel studied each characteristic separately,
always starting with plants that were truebreeding.
• True-breeding plants always produce offspring
with the same trait if allowed to self-pollinate
naturally.
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Unit 2 Lesson 4 Heredity
What did Gregor Mendel discover about
heredity?
• Mendel crossed plants that were true-breeding for
producing yellow seed pods with plants that were
true-breeding for green seed pods.
• All of the plants from the first generation
produced green seed pods.
• Mendel called the green seed pod the dominant
trait, and the yellow seed pod the recessive trait.
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Unit 2 Lesson 4 Heredity
What did Gregor Mendel discover about
heredity?
• Next, Mendel let the first generation plants selfpollinate.
• Out of the generation that resulted, called the
second generation, about three-fourths had green
seed pods and one-fourth had yellow pods.
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Unit 2 Lesson 4 Heredity
What did Gregor Mendel discover about
heredity?
• The recessive trait had seemed to disappear in the
first generation, but it reappeared in the second
generation.
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Unit 2 Lesson 4 Heredity
What did Gregor Mendel discover about
heredity?
• Mendel hypothesized that each plant must have
two heritable “factors” for each trait, one from
each parent.
• Some traits, such as yellow color, could only be
observed if a plant had two of the same factors.
• A plant with two different factors would show the
dominant factor but be able to pass on both
factors to its offspring.
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Unit 2 Lesson 4 Heredity
It’s in Your Genes!
How are traits inherited?
• Mendel’s ideas can be further explained by our
modern understanding of DNA.
• What Mendel called “factors” are actually
segments of DNA known as genes.
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Unit 2 Lesson 4 Heredity
How are traits inherited?
• Genes are segments of DNA. They give
instructions for producing a certain characteristic.
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Unit 2 Lesson 4 Heredity
How are traits inherited?
• The offspring has two versions of the same gene
for every characteristic—one from each parent.
• Different versions of a gene are known as alleles.
• Dominant alleles are shown with a capital letter,
and recessive alleles are shown with a lowercase
version of the same letter.
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Unit 2 Lesson 4 Heredity
How are traits inherited?
• An organism with one dominant and one recessive
allele for a gene is heterozygous for that gene.
• An organism with two of the same alleles for a
gene is homozygous for that gene.
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Unit 2 Lesson 4 Heredity
How are traits inherited?
• The combination of alleles that you inherited from
your parents is your genotype.
• Your observable traits make up your phenotype.
• The phenotypes of some traits follow patterns
similar to the ones Mendel discovered.
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Unit 2 Lesson 4 Heredity
How are traits inherited?
• The dominant allele contributes to the phenotype
if one or two copies are present in the genotype.
• The recessive allele contributes to the phenotype
only when two copies of it are present.
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Unit 2 Lesson 4 Heredity
How are traits inherited?
• If one chromosome in the pair contains a
dominant allele and the other contains a recessive
allele, the dominant allele determines the
phenotype.
• This is called complete dominance.
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Unit 2 Lesson 4 Heredity
How are traits inherited?
• Some characteristics are a result of several genes
acting together.
• Sometimes, one gene influences more than one
trait.
• For example, many genetic disorders, such as
sickle cell anemia, are linked to a single gene but
affect many traits.
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Unit 2 Lesson 4 Heredity
How are traits inherited?
• Sometimes, the environment can influence an
organism’s phenotype.
• Some traits are acquired only from one’s
environment and are not inherited.
• For example, your ability to read and write is an
acquired trait.
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Unit 2 Lesson 4 Heredity
Bending the Rules
What are the exceptions to complete
dominance?
• Some traits do not follow the pattern of complete
dominance.
• For traits that show incomplete dominance and
codominance, one trait is not completely dominant
over another.
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Unit 2 Lesson 4 Heredity
What are the exceptions to complete
dominance?
• In incomplete dominance, each allele in a
heterozygous individual influences the phenotype.
• The result of incomplete dominance is a
phenotype that is a blend of the phenotypes of the
parents.
• An example of this in humans is hair. A person
with one allele for straight hair and one allele for
curly hair will have wavy hair.
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Unit 2 Lesson 4 Heredity
What are the exceptions to complete
dominance?
• For a trait that shows codominance, both of the
alleles in a heterozygous individual contribute to
the phenotype.
• Heterozygous individuals have both of the traits
associated with their two alleles.
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Unit 2 Lesson 4 Heredity
What are the exceptions to complete
dominance?
• Human blood type is an example of codominance.
• Three alleles, called A, B, and O, play a role in
determining blood type.
• A person with an A allele and a B allele has type
AB blood.
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