biology chapter 11. introduction to geneticspg 262
TRANSCRIPT
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
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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.
TRUE BREEDING- when allowed to self pollinate,
the parents produce offspring identical to the
parent plant.
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.
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 self-pollination in some
plants and fertilized the eggs of a flower with the pollen from a different
plant.
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.
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 self-
pollination.
To keep track of the different groups of
seeds he gave them names.
P GENERATION- PUREBRED 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 1 CROSS" PG 266
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.
11-211-2
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. PROBABILITY- the 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
HOMOZYGOUS- organisms that have 2 identical alleles for a particular trait. (TT or
tt);Homozygous organisms are
PUREBRED.
HETEROZYGOUS- organisms 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 takes place in 2 stages..1st & 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;
In a process called
“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 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.
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 1st 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