Transcript Mendelian Genetics: Heredity

• Compare your personal traits with your
parents. Make a column for each.
• What controls the inheritance of traits in
organisms?
• Students know plant and animal cells
contain many thousands of different genes
and typically have two copies of every
gene. The two copies (or alleles) of the
gene may or may not be identical, and one
may be dominant in determining the
phenotype while the other is recessive.
Mendelian
Genetics
• Genetics is the
science of heredity.
• Heredity is the
passing on of
genetic material and
thus traits from
parent to offspring.
Gregor Mendel
• Austrian Monk and
science teacher 18221884
• Considered the
“Father of Heredity”
• He conducted plant
breeding experiments
in a monastery
garden.
• In 1865 he made his
work public about
heredity.
Gregor Mendel
• It took him 2 years to select the pea
plant as his subject.
• He collected data for 8 years.
• His sample sizes were large; he
tabulated results from 28,000 pea
plants.
• He repeated his experiments.
• He analyzed his data with statistics
(probability theory).
• His work was not well known until
early 1900s—34+ years later!
Mendel observed seven pea traits that are easily
recognized and apparently only occur in one of two
forms: The trait on the left is dominant and the trait on the right
is recessive.
Some of his
experimental
procedure.
Mendel’s Pea Plant Experiment
3.He
Mendel
Hecrossed
recorded
chose
trueon
2.1.
a data
true
5.4.He
recorded
data
on
the
He
self
pollinated
breeding
lines
of
the offspring
thisa
breeding
plantofwith
each
plant/trait
he F1)
offspring
of the
second
the
F1
offspring
cross
(First
Filial,
plant
of the
opposite trait
studied
(true
generation,
calling
it
the
or
firstxgeneration.
(purple
white).
breeding
lines
Second
Filial
always
He
calledproduced
this the
generation
(F2)
offspring
of
the
Parental (P) generation.
same type).
Mendel’s Experiment
In
cross-pollinating
plants
thatgenerations
either produce
This
3:1 ratio occurs
in later
as
However,
the
following
generation
(f2)
yellow
or green
peasthat
exclusively,
well.
Mendel
realized
this wasMendel
the key to
consistently
has
abasic
3:1
ratio
ofgeneration
yellow to
found that the
first
offspring
(f1)
understanding
the
mechanisms
of green.
always has yellow peas.
inheritance.
He came to three important conclusions from
these experimental results:
1.3. The
inheritance
of each
trait
isindividual
determined
A
trait
may
not
show
up
in
an
2. An individual inherits one such unit from
by
"units"
or
"factors"
that
are
passed
on
to
but
canparent
still befor
passed
on to the next
each
each
trait.
descendents
generation. unchanged (these units are
now called genes).
Mendel established three principles (or
Laws of Genetics) from his research:
3. Principle of Independent Assortment 2. Principle of Segregation - the two factors
1.The
Principle
of Dominance
and
factors of
a trait separate
independently
of one
Chromosomes,
genes,
or DNA
wasgamete
not
(alleles) for a trait
separate
during
another
during gamete
(sex trait
cell) division
called
Recessiveness
one
is
masked
known of during
Mendel’s
lifetime
in the
mid
formation;
the
pair
of
alleles
of
each
parent
meiosis
or covered
up one
by another
trait. from each
1800s.
separate
and
only
allele
passes
Another way to look at this is, whether a
We
now
know
that there
aresex
parent
on
to
an
offspring.
Neither
did
he
know
the
process
of
how
flower is purple has nothing to do with the length
alleles
(genes)
that
are
more
dominant
Which
allele
in
a
parent's
pair
of
alleles is
cells
(gametes)
are
made.
of the plants stems - each trait is independently
inherited
is a matter of chance.
than others.
inherited.
We now know that this segregation of alleles
occurs during the process of sex cell (gamete)
division (meiosis).
Meiosis
What are genes made of?
• Chromosomes are
made of DNA
• Genes are located
on Chromosomes
• Genes are a
section of DNA that
codes information
for making
proteins.
What are Genes?
• A gene is a segment of
DNA on a chromosome.
• A gene codes for a
specific trait.
• Gene is inherited from the
biological parents.
Therefore, the traits of
any organism is from the
biological parents.
• Genes come in pairs.
What is a dominant gene?
• The gene that
expresses itself; the
most powerful and
dominant gene used
by the cell.
• It has the power to
overshadow the
recessive gene when
there is complete
dominance.
• Some examples are:
brown hair, right
handed, etc.
What is a Recessive Gene?
• The gene that is
overshadowed by a
dominant gene
• Recessive genes can
only express
themselves in the
absence of the
dominant gene.
• A recessive trait can
only show in a person
with both genes in a
pair are recessive
genes.
Genetic Terms to know
• Gene- A sequence of base-pairs (ATCG) of
various lengths that code for proteins.
• Loci- Location of a gene on the chromosome.
• Phenotype- Outward appearance of a trait, for
example: hair color, size, horn shape.
• Genotype- Genetic classification of a gene,
AA, Aa, aa. Always come in pairs.
• Homozygous- contains 2 identical genes for
the same trait, AA, BB, cc – “Purebred”
• Heterozygous- contains 2 different genes for
the same trait, Aa, bB, Cc-- “Hybrid”
Genotype
• Genotypes of your body
cells contain two copies
of the gene.
Aa or AA or aa
However, Gamete (sex
cells) genotypes contains
only one copy (allele) of
the gene.
A or a
Steps for Solving a Genetics Problem:
• Trait – dominant = A (AA or Aa)
• Trait – recessive = a (aa)
Aa x ________
Aa
• _______
=genotype
A
____
____
a
• Punnett Square
Parent
A
____
AA
Aa
a
____
aa
aA
Parent
• The Punnett Square diagram is used to see the
probable outcome of a cross.
Fill in the punnet square of this
cross (fertilization).
If a heteroyzous
round seed is
crossed with itself
(Rr x Rr) a Punnett
square can help you
figure out the ratios
of the offspring.
Practice!
Showing a genetic cross!
Heterozygote cross:
Rr x Rr
R
r
R
RR
Rr
r
Rr
rr
What
Do
youpercentage
see why of
the offspring will
1 out of 4 dominant
traits are
probably have white
or 25% more common?
flowers?
Offspring from a Collie Cross!
Who
has on
thethe
The dog
dominant
allele?
lower left side
What
percentage of
the offspring will
probably have
fur that is black
and white?
50%
Each parent has just one
version of the gene in
each of its sex cells. Not
paired. It is random what
each sex cell gets as the
gene.
The End!
Using a PUNNETT SQUARE
Finally, take each letter in each column and
combine it with each letter from each
row in the corresponding square. You
should now have a picture close to this:
Incomplete
Dominance
Pink
A blend of dominant and recessive traits
Dihybrid Cross: Punnett Square
AB
Ab
aB
ab
AB
ASBB ASBb AaBB AaBb
Ab
AABb AAbb AaBb Aabb
aB
AaBB AaBb aaBB aaBb
ab
AaBb Aabb
aaBb
aabb
Example 2: Answer the
Question
• What is the probable offspring phenotype
ratio for a cross involving two parents that
are heterozygotes for both traits?
– 9/16 Pink, tall
– 3/16 Pink, short
– 3/16 Pink, tall
– 1/16 White, short
Incomplete Dominance:
• Incomplete Dominance: is a condition in which
the dominant allele cannot completely mask the
expression of another allele.
• For example, red-flowered snapdragons
crossed with white ones yield pink in the
first generation.
• The heterozygote phenotype is
somewhere between that of two
homozygotes.
Codominance
• Codominance: is a condition in which both alleles
are expressed in heterozygotes.
• It is NOT a middle combination of two alleles like
the red and white snapdragons make pink.
• Instead it is where both alleles and their traits are
both equally expressed.
• Best example is blood type. IA and IB are each
dominant to i (type O), but are codominant to each
other. Therefore, some persons can express both
genes and have AB blood.
Mendel’s Experiment
With all of the seven pea plant traits that Mendel
Note
that each
of
theappeared
f1 generation
plants over
(shown
examined,
one
form
dominant
the
However,
theadominant
yellow
allele
does
above)
inherited
Y allele from
one
parent
and a
other.
Which
is
to
say,
it
masked
the
presence
of
not alter
green
in any
G allele
from the recessive
other. When
the one
f1 plants
the
other
allele.
For
example,
when
the
genotype
breed,
eachBoth
hasalleles
an equal
of passing
way.
canchance
be passed
on to on
for
pea
color
is
YG
(heterozygous),
the
either
Y
or
G
alleles
to
each
offspring.
the next generation unchanged.
phenotype is yellow.