Download Cell Reproduction

MEIOSIS
Why is Meiosis Important ?
• Meiosis is important because it
prevents the increase, or decrease, in
the number of chromosomes (amount
of DNA) in an offspring …. Abnormal
chromosome numbers, extra
chromosomes or fewer chromosomes
than normal, can seriously effect the
traits and survival chances of the
offspring.
Karyotype
Photograph of Chromosomes
Non-disjunction
Non-disjunction is one of the Two major occurrences of Meiosis
(The other is Crossing Over)
• Non-disjunction is the failure of homologous
chromosomes, or sister chromatids, to
separate during meiosis.
• Non-disjunction results with the production
of zygotes with abnormal chromosome
numbers…… remember…. An abnormal
chromosome number (abnormal amount of
DNA) is damaging to the offspring.
Non-disjunctions usually occur
in one of two fashions.
The first is called Monosomy, the second
is called Trisomy. If an organism has
Trisomy 18 it has three chromosomes
in the 18th set, Trisomy 21…. Three
chromosomes in the 21st set. If an
organism has Monosomy 23 it has only
one chromosome in the 23rd set.
Common Non-disjunction Disorders
Down’s Syndrome – Trisomy 21
Turner’s Syndrome – Monosomy 23 (X)
Kleinfelter’s Syndrome – Trisomy 23 (XXY)
Edward’s Syndrome – Trisomy 18
Patau’s Syndrome – Trisomy 13
Down’s Syndrome – Trisomy 21
Down syndrome causes
mental retardation, a
characteristic face, and
multiple malformations.
Down syndrome is a
relatively common birth
defect. The chromosome
abnormality affects both
the physical and
intellectual development
of the individual.
Turner’s Syndrome – Monosomy 23 (X)
A congenital
condition of females
associated with a
defect or absence of
an X-chromosome,
characterized by
short stature, sexual
underdevelopment,
and other physical
abnormalities.
Kleinfelter’s Syndrome – Trisomy 23 (XXY)
Affects males
Edward’s Syndrome – Trisomy 18
Results in physical
abnormalities such as low
birth weight; a small,
abnormally shaped head;
small jaw; small mouth;
low-set ears; and clenched
fists with overlapping
fingers. Those with
Edward's syndrome also
have heart defects, and
other organ malformations
such that most systems of
the body are affected. The
survival rate for Edward's
Syndrome is still incredibly
low. Only 5 - 10% of liveborn
infants will survive their
first year of life.
Patau’s Syndrome – Trisomy 13
Patau's
Syndrome is
usually fatal,
with most of the
babies dying
before birth and
those who do
make it to birth
typically living
only a few
days. However,
a small number
of babies
(<10%) live at
least one year.
3 Ways of assessing the risk of
passing on possible Genetic Disorders
1. Genetic Counseling
* Helps identify parents at risk of passing on
genetic disorders.
* Helps parents with deciding whether of not they
should have children.
* Use family pedigrees to trace family histories.
2. Pedigree
a diagram that shows the occurrence of genetic traits
over several generations of a family
3. Amniocentesis
An Amniocentesis
is a procedure a
pregnant woman
can have in order
to detect some
genetics
disorders…..such
as nondisjunction.
Amniocentesis
Amniotic fluid withdrawn
Chromosome Types
Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
In Humans the “Sex
Chromosomes” are
the 23rd set
XX chromosome - female
XY chromosome - male
Sex Chromosomes
“Sex Chromosomes”
…….the 23rd set
This person has 2
“X” chromosomes…
and is a female.
23
Sex Chromosomes
“Sex Chromosomes”
…….the 23rd set
23
This person has 1
“X” chromosome and
1 “Y” chromosome…
this is a male.
Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans the
“Autosomes”
are sets 1 - 22
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
• Homologous pairs (tetrads) carry genes controlling
the same inherited traits.
• Each locus (position of a gene) is in the same
position on homologues.
• Humans have 23 pairs of homologous chromosomes.
22 pairs of autosomes
1 pair of sex chromosomes
Homologous Chromosomes
(because a homologous pair consists of 4 chromatids it is called a “Tetrad”)
Paternal
Chromosome
Maternal
Chromosome
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologous set #1
Homologue
Organisms that reproduce Sexually are
made up of two different types of cells.
1. Somatic Cells are “body” cells and contain the
normal number of chromosomes ….called the
“Diploid” number (the symbol is 2n). Examples
would be … skin cells, brain cells, etc.
2. Gametes are the “sex” cells and contain only ½
the normal number of chromosomes…. called
the “Haploid” number (the symbol is n)…..
Sperm cells and ova are gametes.
Diploid Cells
(symbolized as 2n)
• Diploid cells (2n) have 2 copies of every
chromosome, forming what is called a
“Homologous” chromosome pair.
In Diploid cells, one pair of chromosomes
comes from the mother and the other pair
comes from the father.
The diploid number for humans is 46, or
2n = 46,…. Or you could say…..Humans have
23 homologous pairs, or…. Humans have 92
chromatids.
n = number of chromosomes
in a set… so….2n means 2
chromosomes in each set ….
Polyploid cells have more
than two chromosomes in
each set … example: 3n (3
chromosomes in each set )
Development of Gametes
The Male Gamete is the Sperm and is
produced in the male gonad the Testes.
The Female Gamete is the Ovum (ova =
pl.) and is produced in the female gonad
the Ovaries.
During Ovulation the ovum is
released from the ovary and
transported to an area where
fertilization, the joining of the
sperm and ovum, can occur……
fertilization, in Humans, occurs in
the Fallopian tube. Fertilization
results in the formation of the
Zygote. (fertilized egg)
Sperm + Ovum (egg)
(n)
+
(n)
fertilization
Zygote
(2n)
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Meiosis
is the process by which ”gametes” (sex cells) , with half
the number of chromosomes, are produced.
During Meiosis diploid cells are reduced to haploid cells
Diploid (2n)

Haploid (n)
If Meiosis did not occur the chromosome
number in each new generation would
double…. The offspring would die.
How Meiosis Works
• In organisms that reproduce Sexually,
meiosis produces gametes (sex cells). The
male gamete (the sperm) and the female
gamete (the ovum, egg) produced contain
only ½ the number of chromosomes as the
normal somatic cells.
• Later, when the gametes unite (through
“fertilization”) the normal number of
chromosomes is restored. This produces an
offspring with the normal number of
chromosomes (DNA).
Meiosis
Meiosis is Two cell divisions
(called meiosis I and meiosis II)
with only one duplication of chromosomes.
During Prophase I
“Crossing Over” occurs.
Crossing Over is one of the Two major occurrences of Meiosis
(The other is Non-disjunction)
• During Crossing over chromosomes
exchange segments of their DNA.
Crossing Over
creates variation (diversity) in the offspring’s traits.
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Question:
• A cell containing 20 chromosomes (diploid)
at the beginning of meiosis would, at its
completion, produce cells containing how
many chromosomes?
Answer:
• 10 chromosomes (haploid)
Meiosis in males is called
Spermatogenesis and produces
sperm.
Meiosis in females is called
Oogenesis and produces ova.
Spermatogenesis
Secondary Spermatocyte
n=23
human
sex cell
2n=46
sperm
n=23
Primary Spermatocyte
n=23
Secondary Spermatocyte
haploid (n)
n=23
diploid (2n)
n=23
4 sperm cells are
produced from each
primary spermatocyte.
meiosis I
n=23
meiosis II
Oogenesis
*** The polar bodies die… only one ovum
(egg) is produced from each primary oocyte.
Sexual Reproduction
Sexual Reproduction involves
the union of gametes (sperm
and ovum (egg) from two
separate individuals.
Advantages of Sexual Reproduction
There is greater genetic variation of the
offspring and therefore, greater chance
of survival in changing environments.
Disadvantages of Sexual Reproduction
Energy is expended finding a mate in
many organisms. However some
organisms have both male and female
reproductive organs that are able to
produce gametes simultaneously.
Classical Mendelian Genetics
(along with some other variations)
• Genetics is the study of
Heredity…………. Heredity is
the passing of traits from
one generation to the next.
In February 1865, Mendel presented his
findings to the Natural History Society of
Brünn and published them under the title
"Experiments in Plant Hybridization" in a
scientific journal. No one seemed
interested in Mendel's findings., his
discoveries went unappreciated.
Gregor Mendel died on January 6, 1884 at the
age of 62. Sixteen years later, Mendel's
findings were rediscovered.
Gregor Mendel
In 1900, three botanists in different parts of
Europe came across his papers. They
praised Mendel's research and achieved
similar results in their own studies.
Mendel's principles of heredity began to be
referred to as the Mendelian Laws, and
these laws are considered to be the
foundation of the modern study of
genetics.
Your “Traits” are
your characteristics!
Examples would be: The color of
your hair, your eyes, whether
or not you are a taster, tall
stem, short stem, stripes, no
stripes…..etc.
Why did Mendel use Sweet peas?
1. The traits of the
sweet pea seem to
“Breed True”……….
Tall plants were
“Tall”… short plants
were “short”….. Etc…
2. Pollination was easy to control.
(transfer of pollen from the male part of the plant to the female part of the plant)
Flowers
are the reproductive structures of the plant.
- Attract pollinators, and protects the reproductive
structures
Pistil
- Protects young flower
Site where the eggs are produced base of the flower
-
The Male Reproductive Structures
are called the Stamens.
They consist of two parts:
• The anther -- a small
case in which the pollen
grains (sperm) form.
• The filament -- a slender
stem that supports the
anther.
The Female Reproductive
Structure is called the Pistil.
It consists of three parts:
• The stigma -- the pollen
grains stick to this
small sticky pad
• The style -- the pollen
grains grow down
through this stem-like
cylinder
• The ovary -- this is
where the young seeds
(eggs) wait for the
chromosomes in the
pollen (sperm), and
where they grow into
mature seeds…. Which
contain the embryos.
also called a Carpel
Pollination
is the transfer of pollen from the male part of the
plant to the female part of the plant.
Self- Pollination
is the transfer of pollen from the male part of the
plant to the female part of the same plant.
Pollen from plant A
pollinates plant A…
the offspring now is
composed of the
DNA from only one
parent. Selfpollination ensures
genetic
continuity….. every
generation is
genetically identical.
Cross- Pollination
is the transfer of pollen from the male part of one
plant to the female part of different plant.
Pollen from plant
B is transferred to
plant A… the
offspring now is
composed of the
DNA of both
parents. Crosspollination
creates diversity.
Mendel
prevented his
sweet peas
from selfpollinating by
removing the
anthers from
the flowers.
Some Vocabulary You’re
Going to Need
Genes
• A Gene is a
segment of
DNA which
codes for a
particular trait.
• Your traits are
determined by
your Genes.
Your Genes are located on your
chromosomes. Genes always
occur in pairs (sets of 2). You
inherit one set of genes from
your Father, and one set of
genes from your Mother.
Allele
An Allele is one of the possible options
for a given characteristic or trait.
• Human Examples:
- you can have an allele for brown eyes or an
allele for blue eyes
- you can have an allele for brown hair or an
allele for blonde hair
Pea Plant Examples:
– An allele for flower color: purple or white
– An allele for height: tall or short
– An allele for flower position: terminal or axial
Genotype
• What genes you inherited for that trait
• Example: If you have Brown hair it is
because you have inherited a gene for
Brown Hair.
Phenotype
• The actual expressed trait
• What you see when you look at an
organism…. Blue eyes, brown hair ???
Dominant Trait
• An allele that can mask or hide the
expression of another allele for the same
trait.
• Is represented by a capital (big) letter
• Example: In Humans, Brown eyes is a
Dominant trait
• The dominant trait will always be
expressed in the next generation.
Recessive Trait
• An allele that is masked or hidden when
present with another allele for the same
trait
• Is only expressed when two are inherited
• Is represented by a lower case (little) letter
• In Humans: Blue eyes is a Recessive trait
Homozygous
If an organism is homozygous it has either two dominant genes
(homozygous dominant)for a trait or two recessive genes
(homozygous recessive) for a trait
• When the two alleles inherited are the same
• Homozygous dominant
– Both are dominant alleles
Example: TT = tall + tall = a tall plant
Key
T = Tall, t = short
• Homozygous recessive
– Both are recessive alleles
Example: tt = short + short = a short plant
Heterozygous
If an organism is heterozygous it has a dominant gene for
one trait and a recessive gene for the other trait
• When the two alleles inherited are different
• Only the dominant allele is expressed or visible
Example: Tt = tall + short = a tall plant
Key
T = Tall, t = short
Individuals
• Purebreds = have only one allele for a trait
Example: TT, tt, PP,pp
(They are either Homozygous Dominant or Homozygous Recessive)
• Hybrids = have different alleles for a trait.
• Example: Tt or Pp
Probability
Is the likelihood that a specific event will occur
A probability may be expressed as a decimal percent age or a fraction
Use the following equation to determine probability
Probability =
Number of times an event is expected to happen
Number of times an event could happen
Sample: A trait occurs in 35 individuals out of a total of 105 individuals
occurs with a probability of?
Solution: 35/105 = 1/3 or 33% or 0.33
Terminology and Symbols used
by Mendel
• P1 = parent generation
• F1 = first generation of children
• F2 = second generation of children
“F” stands for filial, or sons and daughters
Mendel’s Laws
Law of Segregation
• Each pair of alleles separate during the
formation of the gametes…. traits could
disappear and then reappear.
NB: The Law of Segregation describes meiosis… which
had not been discovered yet.
Example
TT
x
tt
P1
Gametes
T
T
t
t
Alleles have
separated to
form gametes
Law of Independent Assortment
• Each pair of alleles separate independent from other
pairs of alleles during the formation of the gametes.
• One trait has no effect on the inheritance of the others!
In other words… the
big ”G” and the big
“W” don’t also have
to go together into
the same gametes.
likewise… the little
”g” and the little “w”
don’t have to go
together into the
same gametes.
Types of Crosses
(One)
Monohybrid Cross = cross one trait, TT x tt
(Two)
Dihybrid Cross = cross for two traits, TTpp x ttPP
• Yes!… there are trihybrid crosses…. etc…. But
we won’t cover them in this class
Monohybrid Cross
• A cross between two individuals
• Examines only one trait
• Parents have contrasting traits
Let P = Purple, p = white
P1
PP
Purple Flower
x
pp
White Flower
Punnett Squares
(named after RC Punnett)
An easy (nonmathematical)
way to calculate
the probability
of the traits for
the offspring of
two specific
individuals.
Website that performs genetic Punnett squares
http://www.athro.com/evo/gen/punexam.html
Problem 1
Is it possible for two brown-eyed parents to have a
blue-eyed child?
Let B = Brown eyes, b = blue eyes
“B” = is a capital letter, so…. “B” (brown
eyes) is dominant. “b” = is lower case,
so… blue eyes are recessive.
Possible Genotypes are: BB – Homozygous Dominant = Brown eyes
Bb - Heterozygous = Brown eyes
bb – Homozygous Recessive = blue eyes
***A heterozygous male marries a heterozygous female.
Give the “genotypic” and “phenotypic” ratios for the F1 generation.
Solution to Problem 1
P1
gametes
Bb
B
x
b
F1
Bb
B
b
B
b
B
BB
Bb
b
Bb
bb
Genotypic ratio – 1 BB:2 Bb: 1bb (or 1:2:1)
Phenotypic ratio - 3 Brown eyes: 1 blue eyes (3:1)
*** Is it possible for two brown-eyed parents to have a
blue-eyed child? YES
Problem 2
Let B = Brown eyes, b = blue eyes
“B” = is a capital letter, so…. “B” (brown
eyes) is dominant. “b” = is lower case,
so… blue eyes are recessive.
Possible Genotypes are: BB – Homozygous Dominant = Brown eyes
Bb - Heterozygous = Brown eyes
bb – Homozygous Recessive = blue eyes
***A homozygous recessive male marries a heterozygous female.
Give the “genotypic” and “phenotypic” ratios for the F1 generation.
Solution to Problem 2
P1
gametes
bb
b
x
b
F1
Bb
B
b
B
b
b
Bb
bb
b
Bb
bb
Genotypic ratio - 2 Bb: 2 bb (reduced to 1Bb:1bb)
Phenotypic ratio - 2 Brown eyes: 2 blue eyes (reduced to 1Brown:1Blue)
B
B
Gametes
P
?
p
Pp
?p
p
Pp
?p
Test Cross
• A cross used to determine what genes
(genotype) a parent has.
• Cross the unknown individual (PP or Pp) with
an individual that is homozygous recessive
for that trait (pp).
• The Children will show the second unknown
gene.
Dihybrid Cross
• A cross between two individuals
• Examines two traits at the same time
• Parents have contrasting traits
Let P = Purple, p = white
T = Tall, t = short
P1 = PPTT
Purple Flower, Tall
Gametes
PT
Pt
x
pptt
White Flower, short
pT
pt
Determining the Sex of the Offspring
• The sex of the Offspring is determined by the
male’s sperm cell.
Females only have
“X” chromosomes
which code for
females. Males
have “X”
chromosomes and
“Y” chromosomes
which code for
males. If the ”X”
sperm fertilizes the
egg the offspring
will be a female. If
the ”Y” sperm
fertilizes the egg
the offspring will
be a male.
P1
gametes
Male
Female
XY
XX
X
Y
X
F1
X
Y
XX
XY
X
X
X
XX
XY
Results
50% XX, 50% XY
50% female, 50% male
Some Human Sex-linked Traits
are traits that are coded for on the “X” chromosomes
… never the “Y” chromosomes
Examples include:
Hemophilia
Red-green color blindness
Baldness
Sex-linked traits are rarely found in women because
they would require an afflicted male to reproduce
with a carrier female.
Hemophilia
sometimes called the “Bleeders” disease, Hemophilia is an inherited
disease. It is caused by a defect in one of the genes that determine how
the body makes blood clotting factors.
The major signs and symptoms of hemophilia are: Bleeding and Bruising.
• Internal bleeding is common in those with severe hemophilia.
• The extent of bleeding depends on the type and severity of the disease:
• Children with mild to moderate disease may not have any symptoms at
birth.
• Boys with severe disease may bleed heavily after circumcision.
•
•
•
•
In most children, the first signs are:
1. Heavy bruising and bleeding from the gums as they cut their baby teeth
2. Bumps and bruises from frequent falls as they learn to walk
3. Swelling and bruising from bleeding in the joints and muscles.
Normal Mother + Father with Hemophilia
Case 1
Each pregnancy has a 50% chance of resulting in a female carrier and a 50% chance of
resulting in a normal male. Sons of hemophiliac fathers and normal mothers will not have
hemophilia.
Carrier Mother + Normal Father
Case 2
Each pregnancy has a 25% chance of resulting in a normal female, a 25% chance
of resulting in a female carrier, a 25% chance of resulting in a normal male, and a
25% chance of resulting in a male with hemophilia.
Red-green color blindness
(can you find the number?... If not you may be Red-Green color blind)
Multiple Alleles
• Three or more alleles
for a given trait.
• Example: blood types
you can have A, B, O
or AB type blood.
Codominance
(can produce 3 phenotypes)
• Both alleles are completely expressed
• Neither hides the other
• Example: Let R = Red Flower, W = White Flower
Possible Offsprings
RR = Red Flower
WW = White Flower
RW = Red and White Stripes
P1
RR
Gametes
R
F1
WW
R
W
W
W
R
RW
RW
R
RW
RW
Results
Genotypic ratio: 100% RW
x
Phenotypic ratio: 100% Red and White Striped
W
Incomplete Dominance
• Neither Allele is dominant
• Both alleles are expressed, or blended
Example: Let R = Red Flower, W = White Flower
Possible Offsprings
RR = Red Flower
WW = White Flower
RW = Pink
P1
Gametes
RR
R
F1
WW
R
W
W
W
R
RW
RW
R
RW
RW
Results
Genotypic ratio: 100% RW
x
Phenotypic ratio: 100% Pink
W
Fraternal Twins
• Fraternal (Dizygotic)
Twins form when
two separate eggs
are fertilized by
separate sperm.
Fraternal twins are
no more genetically
similar than siblings
born apart.
Identical Twins
• Identical twins occur
when a single egg is
fertilized by a single
sperm to form one zygote
(monozygotic) but the
zygote then divides into
two separate embryos.
The two embryos develop
into fetuses sharing the
same womb.
Pleiotropy
• One gene influences many phenotypes. (traits)
• Example in peas: One gene determines round
or wrinkled… The Same gene affects starch
metabolism and water absorption.
Polygenic Inheritance
• Many genes determine a single phenotype
(trait)
• Not just 2 or 3 varieties, but a continuous
variation or range of phenotypes
• The opposite of pleiotropy
• Example: Human Skin Color