Download Genetics Intro

Introduction to
Genetics
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The Work of Mendel
• Objectives:
• Describe how Mendel studied
inheritance in peas
• Summarize Mendel’s conclusion
about inheritance
• Explain the principle of dominance
• Describe what happens during
segregation
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Genetics:
• the scientific study of
heredity. Heredity is
the passing of
characteristics from
parent to offspring
through the genes.
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Gregor Mendel
• An Austrian monk born in 1822,
instrumental in developing ideas about
heredity. He worked at a monastery
teaching school and managing the
monastery garden. While at the
monastery he experimented with pea
plants and developed his ideas.
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Some quick facts about the
reproduction of peas:
Plants reproduce sexually. The
reproductive organ of a plant is the
flower. Flowers can have male and
female parts.
• Male parts: The stamen, which
contains the filament and the anther.
The anther contains the male sex cell
or gamete called pollen.
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Female parts:
• The carpel, which is composed of the
stigma, the style and the ovary. The
ovary contains the female sex cell or
gamete called the ovule or egg.
• When the pollen fertilizes the egg, a
seed is formed. The seed will then
give rise to a new plant.
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Mendel’s Peas
• Mendel worked with true breeding
plants. True breeding plants are
plants which have been allowed to self
pollinate, thus producing offspring
which are genetically identical to the
parent.
• By doing this he developed a number of
separate stocks, each representing one of 7
different traits.
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Plant Anatomy
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Mendel
• In order to avoid unwanted pollination
of his plants, Mendel removed the
stamens from all of the flowers (ouch),
collected their pollen and pollinated
them himself using a paint brush.
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Why peas??
Mendel chose peas as an experimental
subject for a number of reasons:
• They have contrasting traits that
usually come in only two forms.
• They reproduce sexually which allows
for diversity.
• Their reproductive cycle is short
allowing for production of a number of
generations in a short time.
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Why peas?, cont.
•
•
•
Their crosses (breeding) can be
controlled.
They produce a large number of
offspring.
They are easy to care for in the lab.
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Mendel’s Experiments
1. Principle of dominance – some alleles
(different forms of a gene) are
dominant and others are recessive.
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The Experiment
• Mendel crossed true breeding plants
with contrasting traits. (For example a
true breeding tall plant to a true
breeding short plant)
• These plants were referred to as the P
or parental generation.
• The offspring of these plants are
referred to as the F1 or first filial
generation.
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The Experiment, cont.
• Because the offspring inherited traits
from both parents they were called
hybrids.
• Instead of coming out half tall and half
short or all medium, the F1 generation
came out ALL TALL.
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Conclusions
•
Mendel came up with two
conclusions as a result of this
experiment.
Biological inheritance is determined
by factors that are passed on from
parent to offspring. We now call
those factors genes. Different forms
of a gene are called alleles.
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Conclusions
a. He came up with the principle of
dominance. An organism that has
the dominant allele for a trait will
always have that trait. The only time
the recessive form will show up is if
two recessive alleles are inherited for
that trait.
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• The dominant allele is represented by
a capital letter (usually the first letter
of the dominant trait). The recessive
allele is represented by a lower case
letter (also the first letter of the
dominant trait). Ex. T = tall, t = short
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2. Principle of segregation:
•
•
When gametes (sex cells) are formed
the two alleles of a pair are
segregated from one another so that
each gamete carries only a single
copy of the gene.
The question: Had the recessive
alleles in the F1 generation
disappeared or were they still
present?
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The Experiment
• Mendel allowed the F1 plants to self-
pollinate, resulting in an F2 or second
filial generation.
• Mendel expected tall plants to result in
tall offspring but this is not what
happened.
• 3/4 of the offspring were tall, but ¼ of
them were short!
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The Conclusions
• The allele for shortness had separated
from the allele for tallness. But when?
• He believed that the alleles separated
during the formation of gametes
(pollen and egg).
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Some terms to be familiar
with:
• Homozygous: Organisms that have
identical alleles for a trait. Ex. TT or tt
• Heterozygous: Organisms that have
two different alleles for a trait. Ex. Tt
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New Terms, cont.
• Phenotype: The physical
characteristic of a trait. Ex. Tall, Short
• Genotype: The genetic make-up of a
trait (the letters). Ex. TT, Tt, tt
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Independent Assortment
• Questions: Does the segregation
of one pair of alleles affect the
segregation of another pair of
alleles? (Ex. Does the gene that
determines hair color have
anything to do with height?)
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The Experiment
• Mendel crossed a plant that was
homozygous for both round seeds and
yellow color (RRYY) with one that was
homozygous for both wrinkled seeds
and green color (rryy).
• The gametes produced by the RRYY
parent were all RY, and the gametes
produced by the rryy parent were all
ry.
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The Experiment, cont
• The offspring that resulted from this
cross all had a genotype of RrYy and
were heterozygous round and yellow.
• Mendel wanted to know if the R’s
would always stay with the Y’s and the
r’s with the y’s so he allowed the RrYy
offspring to self-pollinate.
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• The gametes produced by these
plants were RY, Ry, rY, and ry.
When crossed the following
phenotypes were produced:
• 9/16 round and yellow
• 3/16 round and green
• 3/16 wrinkled and yellow
• 1/16 wrinkled and green
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The Conclusions
• Because some offspring have
trait combinations not found in
either parent Mendel concluded
that alleles for seed shape
segregate independently from
alleles for seed color.
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The Conclusions, cont
• The principle of independent
assortment: genes for different
traits can segregate
independently from each other
during the formation of gametes.
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