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BIOLOGY CHAPTER 11
INTRODUCTION TO
GENETICSPG 262
11-1 THE WORK OF
GREGOR MENDEL
Gregor Johann Mendel(born 22nd July 1822, died 6th January 1884)
Introduction:"EARLY
IDEAS ABOUT
HEREDITY"
Until the 19th century
people believed that
heredity was a
BLENDING inheritance
and the nature of these
factors was unknown…
*The resemblance of
children was explained by
the THEORY OF
BLENDING
INHERITANCE.
Indeed you do see a little
of both parents in a
child...
GENETICS- the branch
of biology that studies
heredity….the scientific
study of heredity.
HEREDITY- the passing
of traits from parents to
their young...biological
inheritance.
Heredity is the reason we
have different
species....cats have
kittens, dogs have
puppies, oak trees produce
acorns...
NO!
This
CANNOT
Happen!
NO THIS CANNOT HAPPEN!
"GREGOR MENDEL" PG
263
Gregor Mendel , an
Austrian monk born in
1822, is known as the
"father of genetics".
At the age of 21 Mendel
entered a monastery in
"Brno"and in 1851 was
sent to the Univeristy of
Vienna to study science
and math.
He spent 2 years at the
University and then
returned to the
monastery and spent the
next 14 years teaching at
the monastery.
Mendel was a teacher &
was also responsible for
tending the garden at the
monastery. From this
responsibility came the
foundation of genetics.
Mendel studied the
garden pea plants and
conducted experiments
that unlocked some of the
secrets of heredity.
Teacher Tube Video Clip
• http://www.teachertube.com/v.php?viewkey
=2cbe4813cae2f7715336
Mendel found that pea
plants have both male and
female parts and that
normally, pollen from the
male part of a pea flower
fertilizes the female egg
cells of the same pea
flower.
TRUE BREEDING- when
allowed to self pollinate,
the parents produce
offspring identical to the
parent plant.
SELF POLLINATIONthe process in which
pollen falls from the male
part of a flower to the
female part of a flower of
the same plant.
PUREBRED- belonging to a
group of organisms that
can produce offspring
having only one form of a
trait in each generation.
*Mendel used pea plants
that had been allowed to
self pollinate for several
generations because they
were purebred.
The purebred pea plants
would produce offspring
that were identical to
themselves.
Purebred pea plants were
the basis of Mendel's
experiments.
Tall plants produced only
tall offspring. Short
plants produced only short
offspring. Plants that had
green seeds produced
offspring with green
seeds.
Mendel’s pea plants
produced seeds by self
pollination. These true
breeding plants were the
basis for Mendel's
experiments.
The seeds that were
produced inherited all of
their characteristics from
the single plant that
"created" them.
Mendel prevented selfpollination in some plants
and fertilized the eggs of
a flower with the pollen
from a different plant.
CROSS POLLINATIONthe transfer of pollen
from the flower of one
plant to the flower of
another
plant...fertilization of a
plant's eggs by the pollen
of another plant.
Cross pollination produces
seeds that are the
offspring of 2 different
plants.
Through cross pollination
Mendel was able to cross
plants with different
characteristics.
Mendel studied a few
isolated traits...ones that
were easily observed.He
studied 7 traits...See
figure 11-3 page 264
TRAIT- a specific
characteristic that a
living thing can pass on to
its young
Traits Medel Studied:
Seed Shape, Seed Color,
Seed Coat Color, Pod
Shape, Pod Color, Flower
Position, Plant Height.
Studying only a few traits
made measuring the
effects of heredity much
easier.
"GENES AND
DOMINANCE" PG 264
Mendel crossed pea plants
with different
characteristics for the
same trait....tall with
short.
Flower positions: axial and
terminal...axial along the
sides...terminal at the end.
HYBRID- an organism
that results from crossing
parents with differing
traits or characteristics…
From Mendels crosses he
got HYBRIDS.
Mendel thought that he
would get half short and
half tall plants when he
crossed a short and tall
plant…
to his surprise all of the
offspring were tall....the
short characteristic had
apparently disappeared.
Conclusions from Mendels
1st Set of Experiments:
1. The individual factors
that do not blend with one
another, control the trait
of a living thing.
Mendel used the word
merkmal to refer to these
factors..In German
merkmal means
character…
Merkmal - factors that
control traits.Today the
word merkmal is replaced
with the word GENE.
Bill Nye Gene video Clip
GENE-chemical factors
that control traits. Each
of the traits that Mendel
studied was controlled by
one gene that had 2
contrasting forms..tall and
short;
ALLELES- different
forms of a gene.
Allele
(conclusions from 1st set of
experiments...) 2. PRINCIPLE OF
DOMINANCE- some of the alleles
or factors are dominant and some
are recessive.
The effects of a dominant
allele is seen even when
the contrasting recessive
allele is present.
The effects of a recessive
allele are not seen when the
dominant allele is present.In
his 1st experiments tall and
yellow alleles were dominant
while short and green alleles
were recessive…
Dominance is seen in many
traits but does not apply
to all genes.
"SEGREGATION" PG 265
Mendel wanted to know
what happened to the
recessive characters.So,
he allowed several of the
hybrid plants to
reproduce by selfpollination.
To keep track of the
different groups of seeds
he gave them names.
P GENERATIONPUREBRED PARENTAL
PLANTS
F1 FIRST FILIAL
GENERATION- the first
generation of plants
produced by cross
pollination.
The word filius is a Latin
word that means son.The
next generation produced
from crossing the F one
plants would be referred
to as the F2 generation
and so on.
"THE F1 CROSS" PG 266;
When the 1st filial
generation was crossed
the plants produced (F2)
showed the recessive
traits. WHY?
"EXPLAINING THE F
CROSS" PG 266
1
Mendel assumed that the
presence of the dominant
tall allele had masked the
recessive short allele.In
some of the offspring of
the F2 generation the
allele was not masked.…
This segregation or
separation puzzled
Mendel…
He suggested that during
the formation of the egg
and pollen cells, the tall
and short alleles in the F1
plants were separated
from each other.
SEGREGATION- the
separation of alleles
during gamete formation.
11-2
11-2
PUNNETT SQUARE- a
diagram that shows the
possible gene
combinations in the
offspring that result from
a cross.
Alleles are represented
by letters that serve as
symbols.The DOMINANT
allele is represented by a
capital letter.
The RECESSIVE allele is
represented by a lower
case letter that
corresponds to the
dominant allele symbol.
11-2 page 267
"GENETICS AND
PROBABILITY”
Mendel applied the math
concept of "probability"
to biology. PROBABILITYthe likelihood that a
particular event will occur.
PROBABILITY = # times
a particular event occurs/
# of trials EX: flipping a
coin
In probability you only get
the expected ratio for
large numbers of
trials...the larger the
number of trials the
closer you get to
expected values.
Previous events do not
affect future
outcomes.Ex: each flip of
the coin is a separate
independent event;
GAMETES- reproductive
cells (sex cells);The
gametes produced by each
parent are placed along
the left hand side and
the top of the punnett
square.
The offspring are
represented by each
square.The probable
results are often
expressed as ratios.
PHENOTYPE- physical
characteristics;
GENOTYPE- genetic
make-up
HOMOZYGOUSorganisms that have 2
identical alleles for a
particular trait. (TT or
tt);Homozygous organisms
are PUREBRED.
HETEROZYGOUSorganisms that have two
different alleles for a
particular trait. (Tt);
Heterozygous organisms
are hybrids.
11-3
"INDEPENDENT
ASSORTMENT" (page
271)
Mendel also questioned if
the segregation of one
pair of alleles affect the
segregation of another
pair.For example: does
the gene for seed shape
have anything to do with
seed color?
To find out the answer to
this Mendel crossed
purebred plants that
produce round yellow
seeds with purebred
plants that produced
wrinkled green seeds.
TWO FACTOR CROSS- a
cross that involves 2
traits. See the cross in
figure 11-9 & 11-10 pages
270 & 271
The F1 plants from the
above mentioned cross will
produce seeds that are
round and yellow...the
dominant traits show up in
a hybrid and the recessive
ones seem to disappear.
Segregation is still not
proved to be
independent...another
cross is needed.The F1
generation plants need to
be crossed to produce an
F2 generation.
"THE 2 FACTOR CROSS
F2”
Mendel concluded from
his results of the F1 cross
that genes could
segregate independently
during the formation of
gametes...genes can
undergo independent
assortment.
The only exception to
independent assortment is
for genes that are located
on the same
chromosome...they cannot
undergo independent
assortment.
A SUMMARY OF
MENDEL'S WORK:
-Genes control heredity.
In sexually reproductive
organisms genes are
inherited from each
parent.
-When 2 or more forms of
one gene exist, some
forms of the gene may be
dominant and some forms
may be recessive.
-Genes of different traits
may assort independently
of one another.
APPLYING MENDEL'S
PRINCIPLES
Mendel's ideas about
heredity and his
applications of
mathematics and
statistics to Biology were
ahead of their time.
Mendel's pioneering work
in genetics remained
unappreciated in his
lifetime.
More than 20 years after
his death, Mendel's
experiments and
conclusions were
recognized as important
breakthroughs in Biology.
"USING THE PUNNETT
SQUARE"
ONE FACTOR
CROSS...crossing one trait
TEST CROSS- the cross
of an organism of unknown
genotype with a
homozygous recessive
individual.
"TWO FACTOR CROSS”
SEE FIGURE 11-9 page
270
11-4 MEIOSIS PG 275
The sex cells or gametes
that carry the genetic
information are formed by
a special kind of cell
division known as
MEIOSIS.
The number of
chromosomes, the
structures that contain
the genetic information, is
reduced by half during
meiotic division.
By fertilization the full
number of chromosomes is
restored.
These 2 processes Meiosis
and fertilization- allow for
infinite variety in the
selection and
recombination of genetic
traits.
From Mendel's genetics
we know that an organism
inherits a single copy of
each gene from each of
their parents.
These 2 copies are
segregated from one
another during the
formation of gametes.
EX: fruit fly...each body
or somatic cell of a fruit
fly contains 8
chromosomes.
If chromosomes were not
separated the offspring
would have 16
chromosomes…their
offspring would have 32
and so on.
Drosophila melanogaster
CHROMOSOME NUMBER
PG 275
The chromosomes of the
fruit fly can be divided
into 2 sets…4
chromosomes from the
male parent and 4
chromosomes from the
female parent.
Each chromosome in the
male set has a
corresponding
chromosome in the female
set.
HOMOLOGOUS- a
description of
chromosomes that occur
in pairs; having a
corresponding
structure…the
chromosomes are called
HOMOLOGS.
EX: Each of the
chromosomes from the
male parent have a
corresponding
chromosome from the
female parent.
A cell that contains both
sets of homologous
chromosomes are said to
be DIPLOID.
A diploid cell contains 2
complete sets of
chromosomes and 2
complete sets of
genes.The diploid number
is sometimes represented
by 2N…so for the
Drosophila 2N= 8.
DIPLOID- "two sets"…a
cell that contains both
sets of homologous
chromosomes.All of an
organisms cells (except
for sex cells) contain 2
alleles for a given trait.
Gametes or sex cells
contain only a single set
of genes because alleles
are separated during the
process of gamete
formation.
The GAMETES of
Sexually reproducing
organisms contain a single
set of chromosomes and
genes.
HAPLOID- a cell that
contains a single set of
chromosomes..represented
by the symbol N.Drosophila,
N =4…
“THE PHASES OF
MEIOSIS”
MEIOSIS- a process of
reduction division in which
the number of
chromosomes per cell is
cut in half and homologous
chromosomes that exist in
a diploid cell are
separated.
In most organisms meiosis
st
takes place in 2 stages..1
& 2nd meiotic divisions.
The phases of meiosis are
very different from the
phases of MITOSIS.
SEE FIG 11-15 page 276…
MEIOSIS I….
Special cells in
reproductive organs
undergo a round of DNA
replication…this
resembles mitosis but it is
not the same!
In Prophase 1 of meiosis,
each chromosome seeks
out its corresponding
homologous chromosome
to form a special
structure called a tetrad.
There are 4 chromatids
in a tetrad;
“CROSSING OVER”
homologous chromosomes
may exchange portions of
their chromatids, an
exchange of genes that
produces new
combinations of
genes…see figure 11-16
page 276
METAPHASE 1
Metaphase I
Tetrads (rather than individual
chromosomes) line up in the
center of the cell.
ANAPHASE I
The homologous chromosomes
separate and (telophase 1) two
new cells are formed.
The 2 new cells have sets
of chromosomes that are
different from the parent
cells and different from
each other.
MEIOSIS II….
The 2 cells produced by
meiosis I enter meiosis
II.In the second meiotic
division the cells do not
under go DNA replication
so each cell’s
chromosomes contains 2
chromatids.
In METAPHASE II of
meiosis, : chromosomes
line up in the middle of
the cell. In ANAPHASE II
they separate…each of
the 4 daughter cells
receives 2 chromatids .
The daughter cells contain
the HAPLOID number = 2
chromosomes each.
*The amount of genetic
material has been
reduced and the
combinations of
chromosomes in each
gamete have been made
at random.
“MEIOSIS & GENETICS”
Meiosis I results in
segregation and
independent assortment.
GAMETE FORMATION…
In males the haploid
gametes produced by
meiosis are called
SPERM.Pollen grains
contain haploid sperm
cells.
The female gamete is
called an egg in animals
and an OVULE in higher
plants.
In females the cell
divisions are uneven, and
the egg or ovule gets most
of the cytoplasm…see
figure 11-17 pg 278;
3 other cells called polar
bodies are produced in
the female during
meiosis..they are called
polar bodies.
“COMPARING MITOSIS
AND MEIOSIS”
Mitosis results in the
production of 2
genetically identical cells.
A diploid cell divides and
gives rise to 2 diploid
daughter cells that are
genetically identical to
each other and identical
to the original parent cell.
Meiosis begins with a
diploid cell but produces
4 haploid cells that are
different from the
original diploid cell and
different from one
another.WHY?
Homologous chromosomes
are separated during the
st
1 meiotic division and
crossing over
occurs…giving new gene
combinations on the
chromosomes.
*MITOSIS results in the
production of 2
genetically identical
diploid cells.
*MEIOSIS results in the
production of 4
genetically DIFFERENT
haploid cells.
TEST CH 11