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Mendel and the Gene
Idea
Chapter 14
Biology – Campbell • Reece
Gregor Mendel
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1822-1884
A monk from Austria
Taught school
Began breeding
garden peas in the
abbey garden to
study patterns of
inheritance
Why Pea Plants?
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They are available in many varieties
They grow quickly and produce many
offspring
Have several “either-or” traits
• Flower color, flower position, seed color, seed
shape, pod shape, pod color, stem length
Mendel’s Experiments
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Mendel decided to cross plants with
opposite traits
He always started with varieties that
were true-breeding
The crossing of two true-breeding
varieties is called hybridization
Mendel’s Experiments
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P generation –
parental generation
F1 generation – first
filial generation
F2 generation –
second filial
generation
Mendel’s Experiments
Mendel’s Conclusions
1.
2.
Alternative version of genes (different
alleles) account for variations in
inherited characters.
For each character, an organism
inherits two alleles, one from each
parent
Mendel’s Conclusions
3.
4.
If the two alleles differ, then one, the
dominant allele (P), is fully expressed
in the organism’s appearance; the
other, the recessive allele (p), has no
noticeable effect on the organism’s
appearance
The two alleles for each character
segregate (separate) during gamete
production (law of segregation)
Genetics Vocabulary
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Homozygous – two identical alleles for a trait
(PP or pp)
Heterozygous – one of each allele for a trait
(Pp)
Phenotype – the physical appearance of the
trait (purple flowers)
Genotype – the genetic makeup (PP, pp or Pp)
Testcross – the breeding of a homozygous
recessive with an organism of unknown
genotype
Genotype vs. Phenotype
Punnett Squares
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Used to predict a cross
Monohybrid (one trait)
Dihybrid (two traits)
• Law of independent assortment
Rules of Probability
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The rule of multiplication – to predict the
chance of two or more independent
events occurring simultaneously, multiply
the individual probabilities
The rule of addition – the probability of
an event that can occur in two or more
different ways is the sum of the
individual probabilities
Rules of Probability
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What is the probability of getting at least
two recessive traits in the offspring of the
following cross: PpYyRr x Ppyyrr?
• ppyyRr = ¼ x ½ x ½ = 1/16
• ppYyrr = ¼ x ½ x ½ = 1/16
• Ppyyrr = ½ x ½ x ½ = 2/16
• PPyyrr = ¼ x ½ x ½ = 1/16
• ppyyrr = ¼ x ½ x ½ = 1/16
• Total = 6/16 or 3/8
Incomplete Dominance
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Where the F1
hybrids have an
appearance
somewhere between
the phenotypes of
the two parent
varieties
Red snapdragons x
white snapdragons
What is a dominant allele?
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Complete dominance – one allele is
completely dominant over the other
(purple vs. white flowers)
Incomplete dominance – a sort of
blending of the two traits (red, white, and
pink snapdragons
Codominance – both alleles “show” (roan
coloring in cattle)
The dominant trait is not necessarily more
common in a population
Multiple Alleles
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Three or more alleles determine a trait
Blood type:
• 4 types: A, B, AB, or O
• Based on the presence of a certain
•
•
carbohydrate on the surface of the cell (A or B)
May have one substance (type A or B), both
(type AB), or neither (type O)
A and B are codominant
Multiple Alleles
Pleiotropy
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The ability of a
gene to affect an
organism in many
ways
For example:
sickle-cell disease
causes multiple
symptoms
Epistasis
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A gene at one locus
(location on the
chromosome) alters
the phenotypic
expression of a gene
at a second locus
For example: fur
color in mice
Polygenic Inheritance
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The characters vary
in the population
along a continuum
An additive effect of
two or more genes
on a single
phenotypic character
For example: skin
color & height
Pedigrees
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An analysis of the results of matings that have
already occurred
The information is assembled into a family tree
Pedigrees
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Circle – female
Square – male
Shaded – has the trait
Unshaded – does not have the trait
Sometimes you will see a half-shaded
symbol which represents a carrier of the
trait
Recessive Disorders
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An allele that causes a genetic disorder
codes either for a malfunctional protein
or for no protein at all
Heterozygotes will be “normal” (they are
carriers of the trait)
Homozygous recessive individuals will
have the disorder
Recessive Disorders
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Cystic fibrosis – missing protein for
chloride ion transport, results in build up
of mucus in the pancreas, lungs,
digestive tract, and other organs (most
will die before their 5th birthday)
Tay-Sachs – dysfunctional enzyme that
fails to break down lipids in the brain
(child dies within a few years)
Recessive Disorders
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Sickle-cell – a substitution of a single
amino acid in the hemoglobin protein
• When blood oxygen is low, crystals form,
•
causing the blood cells to become sickle
shaped
Carriers have increased resistance to malaria
Dominant Disorders
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If a lethal dominant allele kills an
offspring before it can reproduce, the
allele will not be passed on
Achondroplasia – a form of dwarfism
Huntington’s disease – a degenerative
disease of the nervous system
Multifactorial diseases
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Diseases that have a genetic component
and are influenced by environmental
factors.
Include: heart disease, diabetes, cancer,
alcoholism, and certain mental illnesses
such as schizophrenia and manicdepressive disorder
Hereditary component is often polygenic
Fetal Testing
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Tests are available for Tay-Sachs,
sickle-cell & cystic fibrosis, to determine
if the parents are carriers
Amniocentesis – amniotic fluid can be
drawn and analyzed to find if there are
genetic disorders
Chorionic villus sampling (CVS) – fetal
tissue is taken from the placenta
Fetal Testing
Fetal Testing
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Ultrasound – uses sound waves to
produce an image of the fetus
Fetoscopy – a needle-thin tube
containing a viewing scope and fiber
optics is inserted into the uterus
(provides a 3-dimensional image)
In 1% of cases, amniocentesis or
fetoscopy causes complications
Newborn Screening
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Some genetic disorders can be detected
at birth
Phenylketonuria (PKU) – a recessive
disorder in which children cannot break
down the amino acid phenylalanine
• Toxic levels can cause mental retardation
• A special diet can prevent retardation