Download Introduction to Genetics

Introduction to Genetics
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For thousands of years
farmers and herders have
been selectively breeding their
plants and animals to produce
more useful hybrids.
It was somewhat of a hit or
miss process since the actual
mechanisms governing
inheritance were unknown.
Knowledge of these genetic
mechanisms finally came as a
result of careful laboratory
breeding experiments carried
out over the last century and a
half.
Intro. To Genetics
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By the 1890's, the
invention of better
microscopes allowed
biologists to discover the
basic facts of cell division
and sexual reproduction.
The focus of genetics
research then shifted to
understanding what really
happens in the
transmission of hereditary
traits from parents to
children.
Gregor Mendel
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A number of hypotheses
were suggested to explain
heredity, but Gregor Mendel,
a little known Central
European monk, was the
only one who got it more or
less right.
His ideas had been
published in 1866 but largely
went unrecognized until
1900, which was long after
his death
Gregor Mendel
His early adult life was
spent in relative obscurity
doing basic genetics
research and teaching high
school mathematics,
physics, and Greek in
Brünn (now in the Czech
Republic).
 In his later years, he
became the abbot (friar) of
his monastery and put
aside his scientific work.
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Gregor Mendel
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While Mendel's research
was with plants, the
basic underlying
principles of heredity
that he discovered also
apply to people and
other animals because
the mechanisms of
heredity are essentially
the same for all complex
life forms.
Mendel and Peas
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Through the selective crossbreeding of common pea
plants over many
generations, Mendel
discovered that certain traits
show up in offspring without
any blending of parent
characteristics.
For instance, the pea flowers
are either purple or white-intermediate colors do not
appear in the offspring of
cross-pollinated pea plants.
Mendel and Peas
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Mendel observed seven traits (specific
characteristic) that are easily recognized and
apparently only occur in one of two forms:
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flower color is purple or white
flower position is axial or terminal
stem length is long or short
seed shape is round or wrinkled
seed color is yellow or green
pod shape is inflated or constricted
pod color is yellow or green
Mendel’s Peas
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The pea plant was favorable organism for these
studies because it was self-fertilizing
When he made crosses, he followed only 1 or 2
(out of his 7) traits (characters) at a time
He employed a very consistent method:
- Opened flower & placed pollen from one type
onto the stigma
Mendel’s methods
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Mendel covered each
flower with little bag
When pods were ripe
harvested them and
planted seeds
He counted the
number of each type
of offspring and
carefully recorded all
of his data.
Mendel’s First Experiment
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Crossed (P1): Pure breeding Tall
x Pure breeding Short (Dwarf)
(P1) = parental generation (pure
breed)
Pure (true) breeding means that if
the plants were allowed to selfpollinate, they’re offspring would be
identical to themselves.
Predictions: The offspring would
be:
All tall
All short
All intermediate
Some would be tall and some short
Mendel’s 1st and 2nd
Experiment
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1st Exp (P1): Crossed
Pure Tall x Pure Short
All offspring (F1): All Tall
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These offspring of the
parental generation are
called hybrids, which are
offspring with different traits
than parents.
(F1) = first filia (son or
daugther)
2nd Exp: Bred F1
Results: Ratio of 787 tall to
277 short (3:1)
Mendel’s Principle of
Segregation
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Mendel assumed that the two “Factors” for each
trait must exist in the parental germ cells
producing the gametes (pollen / egg)
These “factors” are called alleles.
Each allele came from the parents and were
united in fertilization
In forming pollen and egg, the two alleles for any
trait must separate and go into different gametes
This became known as Mendel’s “Principle of
Segregation”
Mendel’s Third Experiment
Crossed one of the F1 tall
plants with its dwarf parent:
F1 Tall x Dwarf (P1)
 Possible Outcomes:
All would be tall
Mixture of Tall & Dwarf
All would be intermediate
 Experimental results —>
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50%
50%
Mendel’s Notation
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After the third experiment, Mendel formulated his
“Principles of Dominance” which states that some
alleles are dominant and some are recessive.
Used capital letter to denote what he called the
dominant form of the trait: T = tall
Used lower case letter to denote what he called the
recessive trait: t = short (dwarf)
Thus for the Tall and Dwarf crosses:
TT = Original pure-breeding tall parent
tt = Original pure-breeding short parent
Tt = Hybrid F1 offspring
Pure-breeding forms later called homozygous
Hybrids later called heterozygous
Mendel’s Experiment 3
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Mendel recognized that it
is not always possible to
tell what offspring will be
like by inspecting the
parent
Mendel could test if tall
plants were pure-breeds
(homozygotes) or hybrid
(heterozygotes) by the
“back-cross” or “testcross”
Test Cross: Crossing a
organism with the
dominant phenotype
with the same organism
with the recessive
phenotype to determine
the genotype of the
dominant organism.
Tt
TT
tt
tt
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Writing Mendel’s Crosses
Using His Notational System
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It was now possible
to account for the
3:1 ratio in the F1 in
Mendel’s second
experiment:
This method of
calculating traits of
offspring in each
generation is called
a Punnett square.
Punnett Squares
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The gene combination
that might result from a
genetic cross can be
determined by drawing
a Punnett square.
Punnett squares help
to determine
phenotypes and
genotypes.
Phenotype – Physical
characteristics (Purple)
Genotype – Genetic
make-up (BB or Bb)
Independent Assortment
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After showing alleles
segregated during the
formation of gametes,
Mendel wondered if they
did so independently.
In other words, does the
segregation of one pair of
alleles affect the
segregation of another set
of alleles?
Example: Does the gene
for seed color have
anything to do with the
gene for seed shape?
Independent Assortment
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To answer these questions,
Mendel performed an
experiment to follow two
different genes as they
passed from one
generation to the next.
This experiment is known
as the Two-Factor Cross
or Dihybrid cross.
Mendel Extended His
Analysis
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Mendel chose two different
characteristics of the pea:
- Seed Coat: Smooth
Wrinkled
- Seed color: Yellow
Green
Two-Factor (dihybrid) Cross
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First, Mendel crossed
pure-breeding plants
that produced only
round yellow peas
(RRYY) with plants that
produced wrinkled
green peas (rryy).
All of the F1 offspring
produced round yellow
peas (RrYy).
This proved round and
yellow must be
dominant alleles.
Try doing the Punnett
square.
RRYY
RY
RY
RY
RY
r
ry
RrYy
RrYy
RrYy
RrYy
r
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
y
y
Independent assortment
This cross does not
indicate whether
genes assorted
independently.
 This cross only
provided hybrids
plants (F1) needed to
produce an F2
generation that would
provide the answers
to Mendel's question.
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Results
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Mendel knew the
F1 had a genotype
of RrYy
(heterozygous).
How would the
alleles segregate
into the F2
Try the Punnett
Square for the
cross of F1 hybrids
producing and F2
generation.
Results
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After Mendel
counted the
offspring of the
dihybrid cross,
he could make
the following
conclusion.
Mendel Could Now Make A
Second Generalization:
 Genes
for different
traits segregate
independently during
the formation of
gametes.
 This became known
as Mendel’s third
principle:
Independent
Assortment
A Summary of Mendel’s Principles
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Mendel’s principles form the basis of the modern
science of genetics. These principles can be
summarized as follows:
1.
2.
3.
4.
The inheritance of biological characteristics is determined
by individual units known as genes. Genes are passed
from parents to their offspring.
In cases in which two or more forms (alleles) of the gene
for a single trait exist, some form of the gene may be
dominant and the other recessive (Principles of
Dominance).
In most sexually reproductive organisms, each adult has
two copies of each gene; one from each parent. These
genes are segregated from each other when gametes are
formed (Principles of Segregation).
The alleles for different genes usually segregate
independently of one another (Principles of independent
assortment).
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