Zoology –
Cell
Division and Inheritance
I. A Code for All Life
A.
Before Genetics - __________________________________________
1. If a very tall man married a short woman, you
would expect their children to
be
intermediate, with average height.
2. The history of blending inheritance, as an
idea, remains in some animal
scientific
names. For example…
a. Giraffe = Giraffa
camelopardalis (described as having characteristics
like
both a camel and a leopard)
b. Mountain Zebra = Equus Hippotigris zebra (having characteristics
of a
hippo and a
tiger)
B. History of Central Tenets of Genetics
1. Genetics
accounts for resemblance and fidelity of reproduction. But, it also accounts for variation.
2. Genetics is a major unifying concept of
biology.
3. _______________________________________________
described
particulate inheritance.
4. ____________________&_____________________described
nature of the
coded
instructions (the structure of DNA).
C. Some vocabulary
1. _______________________ – a unit of
heredity. A discreet part of the DNA
of
a chromosome that encodes for one trait, or protein, or enzyme, etc.
2. _______________________ – (deoxyribonucleic
acid) a molecule that carries
the
genetic instructions for what the cell will do, how it will do it, and when it
will
do
it.
3. _______________________ – a linear sequence
of genes, composed of DNA
and
protein.
a.
Think of the chromosome as a bus that the genes are riding in. Where
the
bus goes, the genes must go.
4. _______________________- The location of any
one gene on a
chromosome.
5. _______________________ - Alternative forms
of a gene; one or both may
have
an effect and either may be passed on to progeny.
6. _______________________pairs of chromosomes – Chromosomes that
have
the same banding pattern, same centromere position, and that encode for
the
same traits.
a. One member of the pair you inherit from your
mother & one member of
the
pair you inherit from your father.
II. Sexual Reproduction
A. Humans have 46 chromosomes in
_______________________
(normal
body cells)
1. This represents _______________________
homologous pairs
of
chromosomes (you inherit one member of the pair from your
mother
& one member of the pair from your father)
2. Normal body cells are _______________________ or
____________________.
In diploid
cells, each organism has two genes for each trait, one on
each
homologous chromosome. (In other words,
you inherit one
gene for a
trait from your mother & one gene for a trait from your
father.)
a.
Remember that normal body cells reproduce through
mitosis and cytokinesis.
B. _______________________ are sex cells. Females produce eggs,
and
males produce sperm.
1. Gametes are ____________________ or ________________.
Gametes
do NOT have homologous chromosome
pairs. When
you
make gametes, the gamete receives EITHER the gene you
inherited
from your mother OR the gene you
inherited from your
father,
but not both.
2. In humans, eggs and sperm have 23
chromosomes.
3. Sexual reproduction involves
_______________________,
when
the sperm joins the egg.
a. The sperm carries 23 paternal chromosomes
& the egg
carries
23 maternal chromosomes.
b. 23 maternal + 23 paternal chromosomes = 23
homologous
pairs or 46 chromosomes in the newly formed
_______________________
(a fertilized egg).
c. This restores the diploid condition in the
offspring.
C. Gametes are produced through a special type of
cell division,
_______________________. In meiosis, one duplication and two
divisions
result in gametes with one member of each homologous pair.
Animal
Cell Meiosis
Sex
Cell (Gamete) Production
Sexual Reproduction:
Sexual reproduction
involves the union of male and female gametes (sex cells; eggs and sperm) through fertilization. Each gamete
contributes half of the genetic makeup of the resulting zygote (fertilized egg).
This insures that the offspring are not genetic clones of their mother or
father. It increases genetic variation
in the population and helps the species survive environmental challenges.
Watch this
video covering the basics of meiosis: http://www.youtube.com/watch?v=D1_-mQS_FZ0
What does haploid and
diploid mean?
Ploidy is a term referring to the number of
sets of chromosomes. Humans have 23
pairs of chromosomes. The chromosome
pairs are numbered 1-23. These pairs are
homologous chromosomes that have the
same banding pattern, centromere position, and carry genes for the same
traits. People inherit one member of
each homologous pair from their mother (the maternal chromosome) and one member of each homologous pair from
their father (the paternal chromosome). Haploid and diploid are terms referring to
the number of sets of chromosomes in a cell.
Most animal cells are said to be diploid
because they have pairs of every chromosome.
Di refers to “two.” We abbreviate
diploid as 2n.
Meiosis is a special type of nuclear division
which segregates the homologous chromosomes into separate gametes, reducing the
chromosome number by half. A sperm will
receive one member of each homologous chromosome pair from the man. Likewise, an egg will receive one member of
each homologous chromosome pair from the woman.
Because these gametes have only one member of each chromosome pair, they
are said to be haploid, abbreviated
as n.
Meiosis is
necessary because when the sperm fertilizes the egg, the resulting zygote will
receive one of each type of chromosome from the sperm and one of each type from
the egg. This will produce a baby that
is again diploid, with the normal chromosome number.
Gametogenesis (Gamete
Production):
Spermatogenesis refers to production of sperm, the
male gamete. Oogenesis refers to the production of ovum (eggs), the female
gamete. Notice that meiosis produces four sperm, but only one egg. Sperm primarily contribute only nuclear DNA
to the zygote. All the organelles,
cytoplasm, etc. come from the egg. Thus,
the egg is much larger than the sperm.
(The diagram below illustrates a species in which 2n = 4 chromosomes.)
Events
of Meiosis:
Meiosis
is actually a series of two cellular divisions (termed meiosis I and meiosis
II) that produces four haploid cells.
·
Meiosis
I (also known as reductional division) reduces the
ploidy level from 2n to n.
·
Meiosis
II (also
known as equational division)
divides the remaining sets of chromosomes in a mitosis-like process. Most of the differences between mitosis
(which produces 2 diploid cells for growth and repair) and meiosis (which
produces sex cells) occur during prophase I.
Meiosis I (or
reductional division):
Below
is a diagram illustrating the events that take place during Meiosis I. Meiosis I begins with a diploid primary sex cell. Note that in this example, 2n = 6. The primary sex cell has 3 pairs of
homologous chromosomes. By the
conclusion of Meiosis II, two haploid secondary
sex cells will have been formed. The
secondary sex cells are n. They have no
homologous pairs of chromosomes.
Prophase I:
Prophase
I begins with a primary sex cell that
is diploid. Most of what occurs in Prophase I is similar
to prophase of mitosis. Just as in mitosis,
the spindle forms and spindle fibers attach to the chromosomes via their
centromeres. Also, the nuclear envelope
breaks down as the nuclear chromatin condenses into visible chromosomes. However, two unique events occur in Prophase
I:
- Homologous chromosomes pair through a process termed
synapsis.
- The synapsed chromosomes are called tetrads because they consist of 4
(tetra = 4) chromatids.
- Synapsed chromosomes are also known as bivalents because they consist of
2 chromosomes (bi = 2).
- During synapsis, crossing-over may occur.
Crossing-over is the reciprocal exchange of chromosome arms between
homologous chromosomes. It occurs
when the synapsed chromosomes become tightly wound together and
break. Sometimes the broken arms
then “heal” (are reattached) to the wrong chromosome.
- This process provides for greater genetic variation among a
species.
|
|
Electron
Microscope View of Chiasmata
|
Metaphase
I:
The
homologous pairs of chromosomes line up across from each other at the cell’s
equator. This event explains Mendel’s Law of Independent Assortment,
which will be explored further during our genetics lecture.
Anaphase
I:
The spindle fibers
drag the homologous chromosome pairs away from each other, towards opposite
poles of the cell. *Note that no centromeres are broken in this process.
Telophase
I:
The
chromosomes reach opposite poles as the spindle begins to break down. Cytokinesis,
the division of the cell’s cytoplasm between what will be two daughter cells
begins with a cleavage furrow. The daughter cells are known as secondary sex cells. They are haploid. This event explains Mendel’s Law of Segregation, which will be explored further during
our genetics lecture.
Meiosis
II (or equational division):
Below
is a diagram illustrating the events that take place during Meiosis II. Meiosis II begins with two haploid secondary sex cells. By the conclusion of Meiosis II, 4 sperm or 1
egg (the gametes) will be
produced. The events of Meiosis II are
very similar to those of mitosis, except that the cells are haploid.
Prophase II:
Prophase
I begins with a two secondary sex cells that
are haploid.
1. The spindle forms and spindle fibers
attach to the chromosomes via their centromeres.
2. The nuclear envelope breaks down as
the nuclear chromatin condenses into visible chromosomes.
Metaphase
II:
The spindle
fibers drag the chromosomes into a line at the cell’s equator.
Anaphase
II:
The centromeres break, allowing the sister chromatids to be dragged toward
opposite poles by the spindle fibers.
Once separated, the sister chromatids are termed daughter chromosomes.
Telophase
II:
The daughter
chromosomes reach opposite poles as the spindle begins to break down. Cytokinesis,
the division of the cell’s cytoplasm begins with a cleavage furrow. The
resulting haploid cells are known as
sex cells or gametes.
Complete this
study guide covering the events of meiosis:
E. Sex Determination
1.
McClung in 1902 studied bugs.
a. Half the sperm lacked one chromosome found in
the
other
half and in all eggs.
b. When the sperm with the full number
fertilized an egg, a
female
resulted; when a sperm lacking one chromosome
fertilized
an egg, it produced a male.
c. Sex chromosomes were those that
determined sex;
_______________________
were the remainder.
d. The bug’s sex determination system is called
XX-XO
indicating
the missing chromosome as “O.”
2. Humans
and many others use an XX-XY system; the male has
the
different sex chromosomes.
a. Half the sperm carry X and half carry Y; they
fertilize an X
egg to
produce 50% of each sex in offspring.
b. The Y chromosome is smaller than the X and
contains
fewer
genes.
3. Birds, moths, butterflies and some fish use
an XX-XY system
but
the female is the XY.
4. Some animals across many taxa, use
environmental and
behavioral
conditions rather than sex chromosomes.
a. In crocodiles, turtles and lizards,
temperature of the nest
determines
sex ratio; lower temperatures may produce
females,
higher produce males.
b. Some fish are _______________________ and
sensory
stimuli
trigger male or female development.
III.
Gregor Mendel – ______________________________________________
A. Mendel’s Investigations
1.
Gregor Mendel conducted his plant breeding experiments from
_______________________.
2. His
discoveries were published in 1866 but not appreciated until
1900,
16 years after his death.
3.
Genes and chromosomes were as yet unknown; his experiments
were
based on crossbreeding.
a.
Mendel carefully controlled pollination of pea plant
stigmas
by stamens.
b.
Mendel carefully documented offspring of different parents
(hybrids)
and then crossed the hybrids.
B. Some Vocabulary
1. Pure
line – strains that consistently yield offspring with the
same
traits generation after generation
2. F1
generation – 1st filial generation (the children)
3. F2
generation – 2nd filial generation (the grandchildren)
4. _______________________ – the combination of
alleles
producing
a trait in an individual
5. _______________________ – an individual’s
visible, physical
trait
6. _______________________ – an individual in
which the two
alleles
of a pair are the same
7. _______________________ – an individual in
which the two
alleles
of a pair differ
C. Mendelian Laws of Inheritance
1. ______________________________________________–
when the gametes are
formed, only one member of each allele pair
is included in a gamete.
a. For any given trait, the gamete will have
either the
maternal
allele OR the paternal allele, but not both
b. When fertilization occurs, the union of the
egg & the
sperm will
restore the allele pair
c. **Refuted blended inheritance.**
1) Tall and dwarf plants produce tall F1
progeny;
hence
there is no blending.
2) Self-pollinating the F1 progeny produce tall
and
short
in a 3:1 ratio; again there was no blending and
this
ratio held for crosses of six other traits.
2. ______________________________________________ –
whenever the 2 alleles
of a pair in an individual differ, only one, the
dominant, will be
expressed.
a. Dominant allele – the allele that
indicates the
appearance
of _______________________.
1) One allele is said to be dominant over
another if a
heterozygote
has the same appearance as an
individual
homozygous for the trait
2) Mendel
called the tall factor dominant; when it
was present the recessive factor
is not expressed.
b. Recessive allele – an allele whose
phenotype effects are
masked in heterozygotes
by the presence of a dominant
allele
1) Recessive traits appear only in homozygotes
who
inherit
both recessive alleles for a that trait.
3. ______________________________________________ -
during formation of gametes, paired alleles
on different
chromosomes segregate
independently. (Genes located on
different pairs of homologous
chromosomes assort independently
during meiosis.)
a.
This deals with two different characters on two different
chromosomes.
b. When tall plants with yellow seeds (both
dominant traits)
were
crossed with dwarf plants with green seeds, the F1
plants
were all tall and yellow as expected.
c.
When the F1 hybrids were crossed, it yielded a 9:3:3:1
ratio, which is a combination of the two
3:1 ratios for each
set or
a dihybrid cross.
d.
Segregation of alleles for heights was independent of
segregation
of alleles for seed color.
IV.
NeoMendelian Genetics – remember that when Mendel did his work,
chromosomes and genes were still
unknown. Since Mendel, further
discoveries
in genetics have been made.
A. Intermediate Inheritance
1. Sometimes,
neither allele is completely dominant, resulting in
intermediate
inheritance or incomplete dominance.
2. Red
and white homozygous four-o-clock flowers cross to form
heterozygous
pink flowers.
B. Multiple Alleles
1. While
only two alleles can exist at one locus, more than two
types of
alleles may exist in a population.
(While you have only 2
alleles for
a trait, more alleles may exist in the population.)
2. For
instance, rabbits may possess two alleles from among four
for
coat color: C (normal), cch (chinchilla), ch (Himalayan) and c
(albino).
C. Gene Interaction
1.
Many different genotypes may affect a single phenotype.
2.
Many genes have more than one effect (i.e. eye color,
intelligence,
etc.).
3. An
allele at one location that masks expression of an allele at
another
locus acting on the same trait is called epistasis.
4.
Several sets of alleles may produce a cumulative effect on the
same
character.
5.
Polygenic characters show continuous variation between
extremes
(blending or quantitative inheritance); skin pigmentation in
humans
probably involves 3 or 4 genes.
D. Sex-Linked Inheritance
1. Some
traits depend on the sex of the parent carrying the gene.
a.
Hemophilia is a recessive trait on the X chromosome.
b.
Red-green color blindness is also a recessive trait and on
the X
chromosome.
c.
Carriers are heterozygous for these genes and are
phenotypically
normal.
d.
Males are hemizygous for traits carried on the X
chromosome.
E. Autosomal
Linkage and Crossing Over
1.
Linkage
a. Not
all factors segregate as stated in Mendel’s Law of
Independent
Assortment
b. Genes
on the same chromosome are linked, and the traits
are
inherited together.
2.
Crossing Over
a.
Linkage is not absolute; some separation of alleles on the
same
chromosome occurs due to crossing over.
F.
Chromosomal Aberrations
1.
Structural and numerical deviations from the norm that affect
many genes
are chromosomal aberrations.
2. It
is estimated that five of every 1,000 humans are born with a
serious
genetic defect from chromosomal anomalies.