BIOLOGY CHAPTER 11

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BIOLOGY CHAPTER 11
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INTRODUCTION TO GENETICSPG 262
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11-1 THE WORK OF GREGOR MENDEL
Gregor Johann Mendel- (born 22nd July 1822, died
6th January 1884)
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Introduction"EARLY IDEAS ABOUT HEREDITY"
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Until the 19th century people believed that
heredity was a BLENDING inheritance and the
nature of these factors was unknown
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The resemblance of children was explained by the
THEORY OF BLENDING INHERITANCE.
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Indeed you do see a little of both parents in a
child...
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GENETICS- the branch of biology that studies
heredity.the scientific study of heredity.
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HEREDITY- the passing of traits from parents to
their young...biological inheritance.
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Heredity is the reason we have different
species....cats have kittens, dogs have puppies,
oak trees produce acorns...
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NO THIS CANNOT HAPPEN!
NO! This CANNOT Happen!
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"GREGOR MENDEL" PG 263
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Gregor Mendel , an Austrian monk born in 1822, is
known as the "father of genetics".
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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.
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He spent 2 years at the University and then
returned to the monastery and spent the next 14
years teaching at the monastery.
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Mendel was a teacher was also responsible for
tending the garden at the monastery. From this
responsibility came the foundation of genetics.
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Mendel studied the garden pea plants and
conducted experiments that unlocked some of the
secrets of heredity.
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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.
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TRUE BREEDING- when allowed to self pollinate,
the parents produce offspring identical to the
parent plant.
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SELF POLLINATION- the process in which pollen
falls from the male part of a flower to the
female part of a flower of the same plant.
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PUREBRED- belonging to a group of organisms that
can produce offspring having only one form of a
trait in each generation.
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Mendel used pea plants that had been allowed to
self pollinate for several generations because
they were purebred.
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The purebred pea plants would produce offspring
that were identical to themselves.
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Purebred pea plants were the basis of Mendel's
experiments.
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Tall plants produced only tall offspring. Short
plants produced only short offspring. Plants that
had green seeds produced offspring with green
seeds.
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Mendels pea plants produced seeds by self
pollination. These true breeding plants were the
basis for Mendel's experiments.
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The seeds that were produced inherited all of
their characteristics from the single plant that
"created" them.
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Mendel prevented self-pollination in some plants
and fertilized the eggs of a flower with the
pollen from a different plant.
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CROSS POLLINATION- the 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.
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Cross pollination produces seeds that are the
offspring of 2 different plants.
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Through cross pollination Mendel was able to
cross plants with different characteristics.
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Mendel studied a few isolated traits...ones that
were easily observed.He studied 7 traits...See
figure 11-3 page 264
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TRAIT- a specific characteristic that a living
thing can pass on to its young
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Traits Mendel Studied Seed Shape, Seed Color,
Seed Coat Color, Pod Shape, Pod Color, Flower
Position, Plant Height.
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Studying only a few traits made measuring the
effects of heredity much easier.
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"GENES AND DOMINANCE" PG 264
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Mendel crossed pea plants with different
characteristics for the same trait....tall with
short.
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Flower positions axial and terminal...axial
along the sides...terminal at the end.
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HYBRID- an organism that results from crossing
parents with differing traits or characteristics
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From Mendels crosses he got HYBRIDS.
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Mendel thought that he would get half short and
half tall plants when he crossed a short and tall
plant
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to his surprise all of the offspring were
tall....the short characteristic had apparently
disappeared.
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Conclusions from Mendels 1st Set of Experiments
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1. The individual factors that do not blend with
one another, control the trait of a living thing.
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Mendel used the word merkmal to refer to these
factors..In German merkmal means character
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Merkmal - factors that control traits.Today the
word merkmal is replaced with the word GENE.
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Bill Nye Gene video Clip
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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
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ALLELES- different forms of a gene.
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Allele
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(conclusions from 1st set of
experiments...) 2. PRINCIPLE OF DOMINANCE- some
of the alleles or factors are dominant and some
are recessive.
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The effects of a dominant allele is seen even
when the contrasting recessive allele is present.
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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
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Dominance is seen in many traits but does not
apply to all genes.
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"SEGREGATION" PG 265
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Mendel wanted to know what happened to the
recessive characters.So, he allowed several of
the hybrid plants to reproduce by
self-pollination.
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To keep track of the different groups of seeds he
gave them names.
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P GENERATION- PUREBRED PARENTAL PLANTS
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F1 FIRST FILIAL GENERATION- the first generation
of plants produced by cross pollination.
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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.
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"THE F1 CROSS" PG 266When the 1st filial
generation was crossed the plants produced (F2)
showed the recessive traits. WHY?
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"EXPLAINING THE F 1 CROSS" PG 266
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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.
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This segregation or separation puzzled Mendel
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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.
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SEGREGATION- the separation of alleles during
gamete formation.
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11-2
11-2
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PUNNETT SQUARE- a diagram that shows the possible
gene combinations in the offspring that result
from a cross.
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Alleles are represented by letters that serve as
symbols.The DOMINANT allele is represented by a
capital letter.
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The RECESSIVE allele is represented by a lower
case letter that corresponds to the dominant
allele symbol.
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11-2 page 267 "GENETICS AND PROBABILITY
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Mendel applied the math concept of "probability"
to biology. PROBABILITY- the likelihood that a
particular event will occur.
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PROBABILITY times a particular event occurs/
of trials EX flipping a coin
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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.
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Previous events do not affect future outcomes.Ex
each flip of the coin is a separate independent
event
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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.
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The offspring are represented by each square.The
probable results are often expressed as ratios.
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PHENOTYPE- physical characteristics GENOTYPE-
genetic make-up
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HOMOZYGOUS- organisms that have 2 identical
alleles for a particular trait. (TT or
tt)Homozygous organisms are PUREBRED.
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HETEROZYGOUS- organisms that have two different
alleles for a particular trait. (Tt)
Heterozygous organisms are hybrids.
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11-3
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"INDEPENDENT ASSORTMENT" (page 271)
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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?
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To find out the answer to this Mendel crossed
purebred plants that produce round yellow seeds
with purebred plants that produced wrinkled green
seeds.
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TWO FACTOR CROSS- a cross that involves 2 traits.
See the cross in figure 11-9 11-10 pages 270
271
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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.
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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.
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"THE 2 FACTOR CROSS F2
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Mendel concluded from his results of the F1
cross that genes could segregate independently
during the formation of gametes...genes can
undergo independent assortment.
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The only exception to independent assortment is
for genes that are located on the same
chromosome...they cannot undergo independent
assortment.
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A SUMMARY OF MENDEL'S WORK
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-Genes control heredity. In sexually reproductive
organisms genes are inherited from each parent.
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-When 2 or more forms of one gene exist, some
forms of the gene may be dominant and some forms
may be recessive.
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-Genes of different traits may assort
independently of one another.
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APPLYING MENDEL'S PRINCIPLES
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Mendel's ideas about heredity and his
applications of mathematics and statistics to
Biology were ahead of their time.
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Mendel's pioneering work in genetics remained
unappreciated in his lifetime.
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More than 20 years after his death, Mendel's
experiments and conclusions were recognized as
important breakthroughs in Biology.
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"USING THE PUNNETT SQUARE"
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ONE FACTOR CROSS...crossing one trait
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TEST CROSS- the cross of an organism of unknown
genotype with a homozygous recessive individual.
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"TWO FACTOR CROSS
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SEE FIGURE 11-9 page 270
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11-4 MEIOSIS PG 275
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The sex cells or gametes that carry the genetic
information are formed by a special kind of cell
division known as MEIOSIS.
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The number of chromosomes, the structures that
contain the genetic information, is reduced by
half during meiotic division.
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By fertilization the full number of chromosomes
is restored.
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These 2 processes Meiosis and fertilization-
allow for infinite variety in the selection and
recombination of genetic traits.
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From Mendel's genetics we know that an organism
inherits a single copy of each gene from each of
their parents.
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These 2 copies are segregated from one another
during the formation of gametes.
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EX fruit fly...each body or somatic cell of a
fruit fly contains 8 chromosomes.
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If chromosomes were not separated the offspring
would have 16 chromosomestheir offspring would
have 32 and so on.
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Drosophila melanogaster
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CHROMOSOME NUMBER PG 275
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The chromosomes of the fruit fly can be divided
into 2 sets4 chromosomes from the male parent
and 4 chromosomes from the female parent.
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Each chromosome in the male set has a
corresponding chromosome in the female set.
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HOMOLOGOUS- a description of chromosomes that
occur in pairs having a corresponding
structurethe chromosomes are called HOMOLOGS.
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EX Each of the chromosomes from the male parent
have a corresponding chromosome from the female
parent.
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A cell that contains both sets of homologous
chromosomes are said to be DIPLOID.
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A diploid cell contains 2 complete sets of
chromosomes and 2 complete sets of genes.The
diploid number is sometimes represented by 2Nso
for the Drosophila 2N 8.
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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.
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Gametes or sex cells contain only a single set of
genes because alleles are separated during the
process of gamete formation.
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The GAMETES of Sexually reproducing organisms
contain a single set of chromosomes and genes.
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HAPLOID- a cell that contains a single set of
chromosomes..represented by the symbol
N.Drosophila, N 4
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THE PHASES OF MEIOSIS
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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.
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In most organisms meiosis takes place in 2
stages..1st 2nd meiotic divisions.
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The phases of meiosis are very different from the
phases of MITOSIS.
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SEE FIG 11-15 page 276
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MEIOSIS I.
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Special cells in reproductive organs undergo a
round of DNA replicationthis resembles mitosis
but it is not the same!
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In Prophase 1 of meiosis, each chromosome seeks
out its corresponding homologous chromosome to
form a special structure called a tetrad.
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There are 4 chromatids in a tetrad
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In a process called CROSSING OVER homologous
chromosomes may exchange portions of their
chromatids, an exchange of genes that produces
new combinations of genessee figure 11-16 page
276
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METAPHASE 1
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Metaphase ITetrads (rather than individual
chromosomes) line up in the center of the
cell.ANAPHASE IThe homologous chromosomes
separate and (telophase 1) two new cells are
formed.
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The 2 new cells have sets of chromosomes that are
different from the parent cells and different
from each other.
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MEIOSIS II.
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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 cells
chromosomes contains 2 chromatids.
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In METAPHASE II of meiosis, chromosomes line up
in the middle of the cell. In ANAPHASE II they
separateeach of the 4 daughter cells receives 2
chromatids .
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The daughter cells contain the HAPLOID number 2
chromosomes each.
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The amount of genetic material has been reduced
and the combinations of chromosomes in each
gamete have been made at random.
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MEIOSIS GENETICS
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Meiosis I results in segregation and independent
assortment.
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GAMETE FORMATION
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In males the haploid gametes produced by meiosis
are called SPERM.Pollen grains contain haploid
sperm cells.
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The female gamete is called an egg in animals and
an OVULE in higher plants.
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In females the cell divisions are uneven, and
the egg or ovule gets most of the cytoplasmsee
figure 11-17 pg 278
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3 other cells called polar bodies are produced in
the female during meiosis..they are called polar
bodies.
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COMPARING MITOSIS AND MEIOSIS
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Mitosis results in the production of 2
genetically identical cells.
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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.
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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?
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Homologous chromosomes are separated during the
1st meiotic division and crossing over
occursgiving new gene combinations on the
chromosomes.
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MITOSIS results in the production of 2
genetically identical diploid cells.
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MEIOSIS results in the production of 4
genetically DIFFERENT haploid cells.
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TEST CH 11