Chapter 8
Gregor Mendel and Heredity
Sections 1-4
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Section 1: The origins of genetics.
Section 2: Mendel’s Theory
Section 3: Studying Heredity
Section 4: Complex Patterns of Heredity
Intro to Genetics
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The Origin of Genetics
• Heredity is the passing
of traits from parents to
offspring.
• Over 100 years ago, An
Austrian monk (Gregor
Mendel) carried out
experiments with pea
plants (Father of
Genetics)
• Genetics is the branch
of biology that focuses
on heredity
Gregor Mendel
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Why did Mendel Breed Pea
Plants?
•
Several pea traits
exist in two clearly
different forms
• Plants male/female
reproductive parts are
enclosed in same
flower (self
pollination)
• Pea is small, grows
easily, matures
quickly and produces
many offspring
Traits Expressed as Simple
Ratios
• Mendel’s initial experiments were
monohybrid crosses (1 pair of contrasting
traits)
• Mendel’s Process
– Step 1- Mendel allowed the plants to self pollinate
to ensure that they were true breeding. Purple
plants produce purple offspring.
– Step 2 - Mendel then cross pollinated two P
generation plants. Offspring called F1 generation
– Step 3 - Mendel allowed the F1 generation to self
pollinate. (F2 generation)
– He found a 3:1 ratio of offspring. Purple:White
Mendel’s Hypotheses
• For each inherited
trait, there 2 copies of
gene. (one from
mom/dad)
• There are alternative
version of genes
(alleles) purple/white)
• When 2 different
alleles are present,
the dominant one is
shown. (recessive is
hidden)
• Homozygous- 2 alleles
of a gene are the same
(PP or pp)
• Heterozygous- if the 2
alleles for a gene are
different (Pp)
• Genotype is the allele
combination
• Phenotype is the
physical appearance of
the trait
The Laws of Heredity
• Law of Segregation
states that the alleles
separate during
meiosis
• Law of Independent
Assortment states
that alleles of
different genes
separate
independently of one
another during
meiosis.
Punnett Squares
• Punnett squares are
diagrams used to
predict the outcome of a
genetic cross by
considering all possible
combinations
• Probability is the
likelihood that a specific
event will occur
Pedigree
• Pedigree is a family
history that shows how
a trait is inherited over
several generations.
• Using a pedigree
scientist can determine
– Autosomal/ sex linked
– Dominant recessive
– Hetero/homozygous
Complex Patterns of Heredity
• Polygenic Trait-- when several genes
influence a trait (height, weight, hair color)
• Incomplete Dominance- when one trait is not
totally dominant (red/white = pink)
• Multiple alleles- genes with three or more
alleles (ABO blood groups: A, B, AB, O)
• Codominance- two dominant alleles are
expressed (AB- both are dominant)
• Traits Influenced by Environment - artic fox
fur, hydrangea plant - acidic neutral, skin
color in humans, height is influenced by
nutrition
Genetic Disorders
Treating Genetic Disorders
• Genetic counseling is a form of medical
guidance that informs people about genetic
problems that could affect their offspring
• PKU - genetic disorder- person lacks an
enzyme that converts one amino acid into
another. Can be diagnosed soon after birth
and treated w/ diet.
• Gene Therapy- the correction of certain
recessive disorders by replacing defective
genes