Transcript PPT File

The Work of Gregor Mendel
Gregor Mendel’s Peas
Genetics is the scientific study of
heredity.
Gregor Mendel was an Austrian monk. His
work was important to the understanding
of heredity.
Mendel carried out his work with ordinary
garden peas.
Mendel knew that
• the male part
of each flower
produces
pollen,
(containing
sperm).
• the female part
of the flower
produces egg
cells.
During sexual reproduction, sperm and egg
cells join in a process called fertilization.
Fertilization produces a new cell.
Pea flowers are self-pollinating.
Mendel had true-breeding pea plants that, if allowed
to self-pollinate, would produce offspring identical to
themselves.
Cross-pollination
Mendel was able to
produce seeds that
had two different
parents.
Genes and Dominance
A trait is a specific characteristic that
varies from one individual to another.
Mendel studied seven pea plant traits,
each with two contrasting characters.
He crossed plants with each of the seven
contrasting characters and studied their
offspring.
Mendelian Genetics Video
Each original pair of plants is
the P (parental) generation.
The offspring are called the F1,
or “first filial,” generation.
The offspring of crosses
between parents with different
traits are called hybrids.
Mendel’s F1 Crosses on Pea Plants
Mendel’s Seven F1 Crosses on Pea Plants
Mendel’s F1 Crosses on Pea Plants
Mendel's first conclusion
•biological inheritance is determined
by factors that are passed from one
generation to the next.
Today, scientists call the factors that
determine traits genes.
Each trait is controlled by one gene with
two contrasting forms that produced
different characters for each trait.
The different forms of a gene are called alleles.
Mendel’s second conclusion is called the
principle of dominance.
The principle of dominance states that
some alleles are dominant and others are
recessive.
Mendel’s F1 Crosses on Pea Plants
Segregation
Mendel crossed the F1 generation
with itself to produce the F2 (second
filial) generation.
The traits controlled by recessive
alleles reappeared in one fourth of the
F2 plants.
Mendel's F2 Generation
P Generation
Tall
Short
F2 Generation
F1 Generation
Tall
Tall
Tall
Tall
Tall
Short
The reappearance of the trait
controlled by the recessive allele
indicated that at some point the
allele for shortness had been
separated, or segregated, from the
allele for tallness.
Mendel suggested that the alleles for tallness and
shortness in the F1 plants segregated from each
other during the formation of the sex cells, or
gametes.
Alleles separate during gamete formation.
Quiz 11-1
Gametes are also known as
a. genes.
b. hybrids.
c. alleles.
d. sex cells.
The offspring of crosses between parents with
different traits are called
a. alleles.
b. hybrids.
c. gametes.
d. dominant.
In Mendel’s pea experiments, the male gametes
are the
a. pollen.
b. seeds.
c. eggs.
d. sperm.
In a cross of a true-breeding tall pea plant with a
true-breeding short pea plant, the F1 generation
consists of
a. all short plants.
b. all tall plants.
c. half tall plants and half short plants.
d. all plants of intermediate height.
If a particular form of a trait is always present
when the allele controlling it is present, then the
allele must be
a. mixed.
b. recessive.
c. dominant.
d. hybrid.
Probability and Punnett Squares
Genetics and Probability
The likelihood that a particular event will
occur is called probability.
The principles of probability can be used to
predict the outcomes of genetic crosses.
Punnett Squares
The gene combinations that might
result from a genetic cross can be
determined by drawing a diagram
known as a Punnett square.
Punnett squares can be used to
predict and compare the genetic
variations that will result from a
cross.
A capital letter
represents the
dominant allele for
tall.
A lowercase letter
represents the
recessive allele
for short.
In this example,
T = tall
t = short
Gametes
produced by each
F1 parent are
shown along the
top and left side.
Organisms that have two identical
alleles for a particular trait are said to
be homozygous. ex. TT or tt
Organisms that have two different
alleles for the same trait are
heterozygous. ex. Tt
Homozygous organisms are truebreeding for a particular trait.
Heterozygous organisms are hybrid for
a particular trait.
Physical characteristics are called
the phenotype.
The alleles are called the genotype.
The plants have
different
genotypes (TT
and Tt), but they
have the same
phenotype (tall).
TT
Homozygous
Tt
Heterozygous
Probability & Segregation
One fourth (1/4) of the
F2 plants have two
alleles for tallness (TT).
1/2 have one allele
for tall (T), and one for
short (t).
One fourth (1/4) of the
F2 have two alleles for
short (tt).
Phenotype Ratio
3:1
Genotype Ratio
1:2:1
Probabilities Predict Averages
Probabilities predict the average outcome of a
large number of events.
Probability cannot predict the precise outcome
of an individual event.
In genetics, the larger the number of offspring, the
closer the resulting numbers will get to expected
values.
Quiz 11-2
Probability can be used to predict
a. precise outcome of any event.
b. average outcome of many events.
c. how many offspring a cross will produce.
d. which organisms will mate with each other.
Compared to 4 flips of a coin, 400 flips of the
coin is
a. more likely to produce about 50% heads and
50% tails.
b. less likely to produce about 50% heads and
50% tails.
c. guaranteed to produce exactly 50% heads
and 50% tails.
d. equally likely to produce about 50% heads
and 50% tails.
Organisms that have two different alleles for a
particular trait are said to be
a. hybrid.
b. homozygous.
c. heterozygous.
d. recessive.
Two F1 plants that are homozygous for
shortness are crossed. What percentage of the
offspring will be tall?
a. 0%
b. 25%
c. 50%
d. 100%
The Punnett square allows you to predict
a. only the phenotypes of the offspring from a
cross.
b. only the genotypes of the offspring from a
cross.
c. both the genotypes and the phenotypes
from a cross.
d. neither the genotypes nor the phenotypes
from a cross.
Exploring Mendelian Genetics
Independent Assortment
To determine if the segregation of
one pair of alleles affects the
segregation of another pair of
alleles, Mendel performed a twofactor cross.
The Two-Factor Cross: F1
Mendel crossed true-breeding plants
that produced round yellow peas
(genotype RRYY) with true-breeding
plants that produced wrinkled green
peas (genotype rryy).
RRYY
x
rryy
All of the F1 offspring produced round
yellow peas (RrYy).
The alleles for round (R) and yellow (Y)
are dominant over the alleles for
wrinkled (r) and green (y).
The Two-Factor Cross: F2
Mendel crossed the heterozygous F1
plants (RrYy) with each other to
determine if the alleles would
segregate from each other in the F2
generation.
RrYy × RrYy
The Punnett square predicts a 9 : 3 : 3 :1
ratio in the F2 generation.
Represents:
Independent Assortment
The alleles for seed shape segregated
independently of those for seed color. This
principle is known as independent
assortment.
Genes that segregate independently do not
influence each other's inheritance.
The principle of independent
assortment states that genes for
different traits can segregate
independently during the
formation of gametes.
Independent assortment helps
account for the many genetic
variations observed in plants,
animals, and other organisms.
A Summary of Mendel's Principles
• Genes are passed from parents
to their offspring.
• If two or more forms (alleles) of
the gene for a single trait exist,
some forms of the gene may be
dominant and others may be
recessive.
a.In most sexually reproducing
organisms, each adult has two
copies of each gene. These genes
are segregated from each other
when gametes are formed.
b.The alleles for different genes
usually segregate independently
of one another.
Some alleles are neither
dominant nor recessive,
and many traits are
controlled by multiple
alleles or multiple genes.
Incomplete Dominance
When one allele is not completely
dominant over another it is called
incomplete dominance.
In incomplete dominance, the
heterozygous phenotype is between the
two homozygous phenotypes.
A cross
between red
(RR) and white
(WW) four
o’clock plants
produces pinkcolored flowers
(RW).
RR
WW
Codominance
In codominance, both alleles contribute to the
phenotype.
In certain varieties of chicken, the allele for black
feathers is codominant with the allele for white
feathers.
Heterozygous chickens are speckled with both
black and white feathers. The black and white
colors do not blend to form a new color, but
appear separately.
Multiple Alleles
Genes that are controlled by more than two
alleles are said to have multiple alleles.
An individual can’t have more than two
alleles. However, more than two possible
alleles can exist in a population.
A rabbit's coat color is determined by a
single gene that has at least four different
alleles.
Different combinations of alleles result in
KEY
the colors shown here.
C=
full color; dominant
to all other alleles
cch = chinchilla; partial
defect in pigmentation;
dominant to
ch and c alleles
ch = Himalayan; color in
certain parts of the
body; dominant to
c allele
chhc
ch,cCc
h
ch
AIbino:
Chinchilla:
Himalayan:
cc CC,
cc
c,hCc
, or
cch
c,hhor
cch
c
Full color:
, or
Cc
c = albino; no color;
recessive to all other
alleles
Polygenic Traits
Traits controlled by two or more
genes are said to be polygenic traits.
Skin color in humans is a polygenic
trait controlled by more than four
different genes.
AaBbCc
aabbcc
20/64
Fraction of progeny
15/64
6/64
1/64
Aabbcc
AaBbCc
AaBbcc AaBbCc AABbCc AABBCc AABBCC
Quiz 11-3
In a cross involving two pea plant traits,
observation of a 9 : 3 : 3 : 1 ratio in the F2
generation is evidence for
a. the two traits being inherited independently
of each other.
b. an outcome that depends on the sex of the
parent plants.
c. the two traits being inherited together.
d. multiple genes being responsible for each
trait.
Traits controlled by two or more genes are
called
a. polygenic traits.
b. multiple-allele traits.
c. codominant traits.
d. hybrid traits.
In four o'clock flowers, the alleles for red flowers
and white flowers show incomplete dominance.
Heterozygous four o'clock plants have
a. pink flowers.
b. white flowers.
c. half white flowers and half red flowers.
d. red flowers.
A white male horse and a tan female horse
produce an offspring that has large areas of
white coat and large areas of tan coat. This is
an example of
a. codominance.
b. multiple alleles.
c. incomplete dominance.
d. a polygenic trait.
Mendel's principles apply to
a. all organisms.
b. fruit flies only.
c. pea plants only.
d. only plants and animals.
Meiosis
Each organism must inherit a
single copy of every gene from
each of its “parents.”
Gametes are formed by a process
that separates the two sets of
genes so that each gamete ends up
with just one set.
Chromosome
Number
All organisms have
different numbers of
chromosomes.
A body cell in an adult
fruit fly has 8
chromosomes: 4 from
the fruit fly's male parent,
and 4 from its female
parent.
These sets of chromosomes are
homologous (Single Structure).
Each of the 4 chromosomes that came
from the male parent has a corresponding
chromosome from the female parent.
A cell that contains both sets of
homologous chromosomes is said to be
diploid.
The number of chromosomes in a diploid
cell is sometimes represented by the symbol
2N.
For Drosophila, the diploid number is 8,
which can be written as 2N=8.
The gametes of sexually reproducing
organisms contain only a single set of
chromosomes, and therefore only a single
set of genes.
These cells are haploid. Haploid cells are
represented by the symbol N.
For Drosophila, the haploid number is 4,
which can be written as N=4.
Phases of Meiosis
Meiosis is a process of reduction
division in which the number of
chromosomes per cell is cut in
half through the separation of
homologous chromosomes in a
diploid cell.
Meiosis Video
Meiosis involves two divisions,
meiosis I and meiosis II.
By the end of meiosis II, the
diploid cell that entered meiosis
has become 4 haploid cells.
Meiosis I
Interphase I
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
Cells undergo a
round of DNA
replication, forming
duplicate
chromosomes.
Interphase
Each chromosome
pairs with its
corresponding
homologous
chromosome to
form a tetrad.
There are 4
chromatids in a
tetrad.
Prophase I
When homologous chromosomes form tetrads in
meiosis I, they exchange portions of their
chromatids in a process called crossing over.
Crossing-over produces new combinations of
alleles.
Spindle fibers attach
to the
chromosomes.
Metaphase I
The fibers pull the
homologous
chromosomes
toward opposite
ends of the cell.
Anaphase I
Nuclear membranes
form.
The cell separates into
two cells.
The two cells
produced by meiosis I
have chromosomes
and alleles that are
different from each
other and from the
diploid cell that
entered meiosis I.
Telophase I and
Cytokinesis
Meiosis II
The two cells produced by meiosis I
now enter a second meiotic division.
Unlike meiosis I, neither cell goes
through chromosome replication.
Each of the cell’s chromosomes has
2 chromatids.
Meiosis II
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
Anaphase II Telophase II
and
Cytokinesis
Meiosis I results in
two haploid (N)
daughter cells, each
with half the number
of chromosomes as
the original cell.
Prophase II
The chromosomes
line up in the center
of cell.
Metaphase II
The sister
chromatids separate
and move toward
opposite ends of the
cell.
Anaphase II
Meiosis II
results in four
haploid (N)
daughter cells.
Telophase II and Cytokinesis
Gamete Formation
In male animals, meiosis results in four equal-sized
gametes called sperm.
In many female animals, only one egg
results from meiosis. The other three cells,
called polar bodies, are usually not involved
in reproduction.
Comparing Mitosis and Meiosis
Mitosis results in the
production of two
genetically identical diploid
cells.
Meiosis produces four
genetically different haploid
cells.
Mitosis
Meiosis
a.Cells produced by
a.Cells produced by
meiosis have half the
mitosis have the same
number of
number of chromosomes
chromosomes as the
and alleles as the original
parent cell.
cell.
b.These cells are
b.Mitosis allows an
genetically different
organism to grow and
from the diploid cell
replace cells.
and from each other.
c.Some organisms
reproduce asexually by
mitosis.
c.Meiosis is how
sexually-reproducing
organisms produce
gametes.
Quiz 11-4
If the body cells of humans contain 46
chromosomes, a single sperm cell should have
a. 46 chromosomes.
b. 23 chromosomes.
c. 92 chromosomes.
d. between 23 and 46 chromosomes.
During meiosis, the number of chromosomes
per cell is cut in half through the separation of
a. daughter cells.
b. homologous chromosomes.
c. gametes.
d. chromatids.
The formation of a tetrad occurs during
a. anaphase I.
b. metaphase II.
c. prophase I.
d. prophase II.
In many female animals, meiosis results in the
production of
a. only 1 egg.
b. 1 egg and 3 polar bodies.
c. 4 eggs.
d. 1 egg and 2 polar bodies.
Compared to egg cells formed during meiosis,
daughter cells formed during mitosis are
a. genetically different, while eggs are
genetically identical.
b. genetically different, just as egg cells are.
c. genetically identical, just as egg cells are.
d. genetically identical, while egg cells are
genetically different.