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Heredity: Inheritance,
and Variation
Appreciate Similarities and
Differences Among Organisms
What characteristics do you share among all human species?
What characteristics do you share among your siblings?
What characteristics do you share among your close relatives?
Which traits resemble your parent/s?
Which traits neither resemble your mom’s or your dad’s?
Have you been told that you have your mother’s nose or your
grandmother’s eyebrows?
Examine closely the picture of a family, what
characteristics do these siblings share in
People have been fascinated at how children
will resemble their parents and vice versa.
As years went by, scientists began to search
for more information on how these traits are
passed on.
The passing of traits from parents to offspring
is HEREDITY and VARIATION
demonstrates differences among individuals.
GENETICS is the study of heredity and
variation. It aims to understand how traits can
be passed on to the next generation and how
variation arises.
Every living thing undergoes reproduction.
The nutrients taken by an individual will
provide for energy for metabolic processes,
for growth and development as well as
reproduction. The cellular level of
reproduction, in the form of cell division,
provides for the backdrop for the organismal
level of reproduction.
How will the environment influence
their inherited traits?
The environment may influence an individual’s growth and
development.
Height is determined by genes. Tall parents will likely
produce tall children.
However, inadequate nutrition and lack of sleep may lead
to stunted growth.
In similar way, intelligence which includes the speed of
processing information has been studied to be genetically
controlled.
But even if a person is predisposed to
become intelligent, if the environment in
which he or she grows up in is not
stimulating, the person may not reach his full
potential.
Similarly, a person may not have the perfect
combination of genes for intelligence, but if
the environment motivates and nurtures
learning, the person may develop higher
intelligence quotient.
Traits are observable characteristics determined by
specific segments of DNA called GENES.
The DNA
(deoxyribonucleic acid) is
a double helix molecule,
the thin strands of which
are twisted around each
other like a twisted ladder.
The sides of the ladder are
made up of sugar and
phosphate molecules and
the steps of the ladder are
made of nitrogen base
pairs.
A GENE is a length of DNA that codes for a
particular protein.
For example, a gene may code for the protein of
your hair or a protein like insulin which controls
the level of glucose or sugar in the blood.
The gene is responsible why you are different from
everyone else alive today and even among those
who lived in the past.
But it is also the same gene responsible why you
shares some features of your family members.
Genes store the information needed to make the
necessary proteins that will eventually lead to
specific traits.
It determines how you will look outside and how
your body will work inside.
The DNA are tightly coiled around special proteins
called HISTONES and make up the chromosomes.
Chromosomes are found inside the nucleus of the
cell.
In eukaryotic cells
(cells with
organelles), the
DNA are bound with
proteins and are
organized as beads
on strings to form
chromosomes.
Chromosomes Number of Some
Organisms
The chromosomes of a cell change form as the cell
transitions from one stage to another in a typical
cell cycle. The cell cycle may be divided into two
stages: the interphase where the chromosomes are
long and extended and are also referred to as
chromatin, and the cell division phase where the
chromosomes become condensed or thickened
The interphase refers to the period that follows one
cell division and precedes another. During this
stage, the cell does not divide; it merely grows.
The chromosome doubles or replicates itself
because the DNA molecule contained in the
chromosome produces an exact copy of itself.
The interphase is divided into three substages. The
stage from the formation of a new cell until it
begins to replicate its DNA is called the first gap
period or G1, during which time the cell grows
initially.
G1 stage is characterized by protein and
ribonucleic acid (RNA) synthesis. RNA, which is
synthesized based on the DNA, is then used to
synthesize proteins.
The middle stage of interphase, called the
synthesis stage or S, is the period of DNA
synthesis or replication. The chromosomes are
duplicated in preparation for the next cell division.
The second gap period or G2, falls between the S
period and the next cell division or M (mitosis or
meiosis, see discussion below) phase.
G2 represents a period of rapid cell growth to
prepare for cell division.
During interphase, the nucleus is clearly visible as
a distinct membrane-bound organelle. In stained
cells, this membrane can be clearly seen under the
light microscope. One or more nucleoli are visible
inside the nucleus. On the other hand, the
chromosomes cannot be clearly seen. They appear
as an irregular mass that is grainy in appearance
because the DNA they contain are stretched out
thinly in the nucleus. This facilitates the replication
of DNA during the S phase.
CELL DIVISION
Alternating with the interphase is the cell division
phase. In eukaryotic cells,
there are two types of cell division: mitosis and
meiosis.
1. Mitosis
This type of cell division produces two identical
cells with the same number of chromosomes.
Mitosis is divided into four stages.
STAGE A: Prophase. The nuclear membrane and
nucleoli may still be present. The chromosomes
are thicker and shorter because of repeated coiling.
At this stage, each chromosome is made up of two
identical sister chromatids as a consequence of
replication of DNA during the S phase. The two
chromatids produced from one chromosome are
still attached at one point, called the centromere.
The centromere may divide the chromosome into
the shorter arms, also called the p arms (‘p’ stands
for petite in French) and the longer q arms. If the
chromosomes are stained using Giemsa,
alternating dark and light regions will appear.
These are the heterochromatin and
euchromatin, respectively. The heterochromatin
are more coiled and dense than the euchromatin.
STAGE B: Metaphase. The nuclear membrane has
disappeared while the highly coiled chromosomes
align at the metaphase plate, an imaginary plane
equidistant between the cell’s two poles. Spindle
fibers are also formed. Each fiber binds to a
protein called the kinetochore at the centromere of
each sister chromatid of the chromosome.
STAGE C: Anaphase. The paired centromeres of
each chromosome separate towards the opposite
poles of the cells as they are pulled by the spindle
fibers through their kinetochores. This liberates the
sister chromatids. Each chromatid is now regarded
as a full-fledged chromosome and is only made up
of one sister chromatid.
STAGE D: Telophase. The chromosomes are now
at the opposite poles of the spindle. They start to
uncoil and become indistinct under the light
microscope. A new nuclear membrane forms
around them while the spindle fibers disappear.
There is also cytokinesis or the division of the
cytoplasm to form two separate daughter cells
immediately after mitosis.
MITOSIS PHASES
2. Meiosis
The number of chromosomes normally remains the
same within the species. It does not double or
triple for every generation. This suggests that a
different kind of cell division must take place in an
individual. This kind of cell division is called
meiosis, from a Greek word which means “to
make smaller.” Meiosis reduces the chromosome
number in half. It takes place in plants and animals
whenever gametes, or sex cells, are formed
through the process called gametogenesis.
2. Meiosis
Undergoes two rounds of cell division to produce
four daughter cells, each with half the
chromosome number as the original parent cell and
with a unique set of genetic material as a result of
exchange of chromosome segments during the
process of crossing over.
The first round of meiotic division, also known as
meiosis I, consists of four stages: prophase I,
metaphase I, anaphase I, and telophase I. Prophase
I of meiosis I, unlike its counterpart in mitosis, is
more elaborate and should be understood well in
order to grasp the mechanisms of heredity.
STAGE A: Prophase I. Meiosis starts with this
stage and includes the following substages:
Leptotene. Each chromosome is made up of two
long threads of sister chromatids as a result of
replication during the S phase of the cell cycle.
Zygotene. The chromosomes begin to pair off.
Pairs of chromosomes are called homologous
chromosomes, and this pairing process is exact.
Pachytene. The chromosomes contract due to
repeated coiling. Crossing over takes place during
this stage where a segment of a sister chromatid of
one chromosome is exchanged with the same
segment of the sister chromatid of the homologous
chromosome through the formation of a cross-
linkage of the segments called a chiasma (Figure
4). After crossing over, the sister chromatids of
each chromosome may no longer be identical with
each other based on the genetic material they
contain.
Diplotene. The chromosomes begin to uncoil.
Diakinesis. The paired chromosomes disperse in
the nucleus.
STAGE B: Metaphase I. The paired chromosomes
arrange themselves along the equatorial plate.
STAGE C: Anaphase I. Spindle fibers form and
attach to the centromeres of the chromosomes. The
homologous chromosomes separate from each
other completely and start their movement toward
the poles of the cells as they are pulled by the
spindle fibers.
As the centromere of each chromosome does not
divide, the sister chromatids remain together.
STAGE D: Telophase I. This is the stage when the
chromosomes reach their respective poles.
Cytokinesis follows and two daughter cells are
formed. Each cell now has only half the
chromosome number because only one
chromosome from each pair goes to the daughter
cell. This is called the haploid condition, in
contrast to the diploid condition at the beginning
of meiosis I where each chromosome pair is intact.
Telophase I is followed by interphase II
Note that each chromosome still has two sister
chromatids; it is therefore necessary for the cells to
undergo another round of division.
The second meiotic division, also known as
meiosis II, is mitotic in nature and consists of the
following stages: prophase II, metaphase II,
anaphase II and telophase II; these stages are
identical with the mitotic stages. The results are
four cells, two from each daughter cell from
meiosis I, with one half the diploid chromosome
number and with only one sister chromatid for
each chromosome.
MENDELIAN
GENETICS
Gregor Mendel was an
Augustinian monk in a
monastery in Brünn, Austria-
Hungarian Empire (now Brno,
Czech Republic). He was
interested in investigating how
individual traits were inherited.
He wanted to find out whether
both parents contributed
equally to the traits of the
offspring. He also wanted to
know if the traits present in the
offspring were produced by the
blending of the traits of the
parents.
When he has pure-breeding
plants, Mendel began cross-
pollinating peas with
contrasting traits. The pure-
breeding peas constituted the
parental or P1 generation. All
offspring of these crosses
resembled one another. For
example, when he crossed pea
plants that produced round
seeds with pea plants that
produced wrinkled seeds, all
the offspring had round seeds.
The offspring of the parental
cross are called the first filial
(F1) generation. In Mendel’s
experiments, the F1 generation
are also called hybrids because
they resulted from a cross
between two pure-breeding
plants with contrasting traits
(for example, pea plants with
round seeds crossed with pea
plants with wrinkled seeds).
Genes control the appearance of a trait.
Alleles – traits are controlled by a pair of genes.
Based on the results for the F1 generation, the
trait for round seeds is the dominant trait. The
trait of wrinkled seeds, which did not appear in
the F1 generation, is called the recessive trait.
Its appearance was either prevented or hidden by
the dominant trait. This is now known as the
principle of dominance: The dominant trait
dominates or prevents the expression of the
recessive trait.
Genes and Gametes
Following Mendel’s reasoning, a pure-breed,
round-seeded parent plant has an allelic
combination or genotype of RR while a pure-
breed, wrinkled-seeded parent plant has a
genotype of rr. Individuals that are pure-breeding
for a particular character therefore have identical
alleles. These individuals have a homozygous
genotype.
Genes and Gametes
An individual with contrasting alleles (a dominant
and a recessive allele) for a particular character is
said to have a heterozygous genotype. However,
Rr individuals will still produce round seeds
because of the presence of the dominant allele R.
These will be just as round as all the seeds
produced by the RR parents. The expression of the
genotype of an individual for a particular character
is referred to as its phenotype.

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Heredity: Inheritance and Variation of Traits

  • 2. Appreciate Similarities and Differences Among Organisms What characteristics do you share among all human species? What characteristics do you share among your siblings? What characteristics do you share among your close relatives? Which traits resemble your parent/s? Which traits neither resemble your mom’s or your dad’s? Have you been told that you have your mother’s nose or your grandmother’s eyebrows?
  • 3. Examine closely the picture of a family, what characteristics do these siblings share in
  • 4. People have been fascinated at how children will resemble their parents and vice versa. As years went by, scientists began to search for more information on how these traits are passed on. The passing of traits from parents to offspring is HEREDITY and VARIATION demonstrates differences among individuals.
  • 5. GENETICS is the study of heredity and variation. It aims to understand how traits can be passed on to the next generation and how variation arises. Every living thing undergoes reproduction. The nutrients taken by an individual will provide for energy for metabolic processes, for growth and development as well as reproduction. The cellular level of reproduction, in the form of cell division, provides for the backdrop for the organismal level of reproduction.
  • 6. How will the environment influence their inherited traits? The environment may influence an individual’s growth and development. Height is determined by genes. Tall parents will likely produce tall children. However, inadequate nutrition and lack of sleep may lead to stunted growth. In similar way, intelligence which includes the speed of processing information has been studied to be genetically controlled.
  • 7. But even if a person is predisposed to become intelligent, if the environment in which he or she grows up in is not stimulating, the person may not reach his full potential. Similarly, a person may not have the perfect combination of genes for intelligence, but if the environment motivates and nurtures learning, the person may develop higher intelligence quotient.
  • 8. Traits are observable characteristics determined by specific segments of DNA called GENES. The DNA (deoxyribonucleic acid) is a double helix molecule, the thin strands of which are twisted around each other like a twisted ladder. The sides of the ladder are made up of sugar and phosphate molecules and the steps of the ladder are made of nitrogen base pairs.
  • 9. A GENE is a length of DNA that codes for a particular protein. For example, a gene may code for the protein of your hair or a protein like insulin which controls the level of glucose or sugar in the blood. The gene is responsible why you are different from everyone else alive today and even among those who lived in the past. But it is also the same gene responsible why you shares some features of your family members.
  • 10. Genes store the information needed to make the necessary proteins that will eventually lead to specific traits. It determines how you will look outside and how your body will work inside. The DNA are tightly coiled around special proteins called HISTONES and make up the chromosomes. Chromosomes are found inside the nucleus of the cell.
  • 11. In eukaryotic cells (cells with organelles), the DNA are bound with proteins and are organized as beads on strings to form chromosomes.
  • 12. Chromosomes Number of Some Organisms
  • 13. The chromosomes of a cell change form as the cell transitions from one stage to another in a typical cell cycle. The cell cycle may be divided into two stages: the interphase where the chromosomes are long and extended and are also referred to as chromatin, and the cell division phase where the chromosomes become condensed or thickened
  • 14.
  • 15. The interphase refers to the period that follows one cell division and precedes another. During this stage, the cell does not divide; it merely grows. The chromosome doubles or replicates itself because the DNA molecule contained in the chromosome produces an exact copy of itself. The interphase is divided into three substages. The stage from the formation of a new cell until it begins to replicate its DNA is called the first gap period or G1, during which time the cell grows initially.
  • 16. G1 stage is characterized by protein and ribonucleic acid (RNA) synthesis. RNA, which is synthesized based on the DNA, is then used to synthesize proteins. The middle stage of interphase, called the synthesis stage or S, is the period of DNA synthesis or replication. The chromosomes are duplicated in preparation for the next cell division. The second gap period or G2, falls between the S period and the next cell division or M (mitosis or meiosis, see discussion below) phase.
  • 17. G2 represents a period of rapid cell growth to prepare for cell division. During interphase, the nucleus is clearly visible as a distinct membrane-bound organelle. In stained cells, this membrane can be clearly seen under the light microscope. One or more nucleoli are visible inside the nucleus. On the other hand, the chromosomes cannot be clearly seen. They appear as an irregular mass that is grainy in appearance because the DNA they contain are stretched out thinly in the nucleus. This facilitates the replication of DNA during the S phase.
  • 18. CELL DIVISION Alternating with the interphase is the cell division phase. In eukaryotic cells, there are two types of cell division: mitosis and meiosis. 1. Mitosis This type of cell division produces two identical cells with the same number of chromosomes. Mitosis is divided into four stages.
  • 19. STAGE A: Prophase. The nuclear membrane and nucleoli may still be present. The chromosomes are thicker and shorter because of repeated coiling. At this stage, each chromosome is made up of two identical sister chromatids as a consequence of replication of DNA during the S phase. The two chromatids produced from one chromosome are still attached at one point, called the centromere. The centromere may divide the chromosome into the shorter arms, also called the p arms (‘p’ stands for petite in French) and the longer q arms. If the chromosomes are stained using Giemsa, alternating dark and light regions will appear.
  • 20. These are the heterochromatin and euchromatin, respectively. The heterochromatin are more coiled and dense than the euchromatin.
  • 21. STAGE B: Metaphase. The nuclear membrane has disappeared while the highly coiled chromosomes align at the metaphase plate, an imaginary plane equidistant between the cell’s two poles. Spindle fibers are also formed. Each fiber binds to a protein called the kinetochore at the centromere of each sister chromatid of the chromosome.
  • 22. STAGE C: Anaphase. The paired centromeres of each chromosome separate towards the opposite poles of the cells as they are pulled by the spindle fibers through their kinetochores. This liberates the sister chromatids. Each chromatid is now regarded as a full-fledged chromosome and is only made up of one sister chromatid.
  • 23. STAGE D: Telophase. The chromosomes are now at the opposite poles of the spindle. They start to uncoil and become indistinct under the light microscope. A new nuclear membrane forms around them while the spindle fibers disappear. There is also cytokinesis or the division of the cytoplasm to form two separate daughter cells immediately after mitosis.
  • 25. 2. Meiosis The number of chromosomes normally remains the same within the species. It does not double or triple for every generation. This suggests that a different kind of cell division must take place in an individual. This kind of cell division is called meiosis, from a Greek word which means “to make smaller.” Meiosis reduces the chromosome number in half. It takes place in plants and animals whenever gametes, or sex cells, are formed through the process called gametogenesis.
  • 26. 2. Meiosis Undergoes two rounds of cell division to produce four daughter cells, each with half the chromosome number as the original parent cell and with a unique set of genetic material as a result of exchange of chromosome segments during the process of crossing over. The first round of meiotic division, also known as meiosis I, consists of four stages: prophase I, metaphase I, anaphase I, and telophase I. Prophase I of meiosis I, unlike its counterpart in mitosis, is more elaborate and should be understood well in order to grasp the mechanisms of heredity.
  • 27. STAGE A: Prophase I. Meiosis starts with this stage and includes the following substages: Leptotene. Each chromosome is made up of two long threads of sister chromatids as a result of replication during the S phase of the cell cycle. Zygotene. The chromosomes begin to pair off. Pairs of chromosomes are called homologous chromosomes, and this pairing process is exact.
  • 28. Pachytene. The chromosomes contract due to repeated coiling. Crossing over takes place during this stage where a segment of a sister chromatid of one chromosome is exchanged with the same segment of the sister chromatid of the homologous chromosome through the formation of a cross- linkage of the segments called a chiasma (Figure 4). After crossing over, the sister chromatids of each chromosome may no longer be identical with each other based on the genetic material they contain.
  • 29.
  • 30. Diplotene. The chromosomes begin to uncoil. Diakinesis. The paired chromosomes disperse in the nucleus. STAGE B: Metaphase I. The paired chromosomes arrange themselves along the equatorial plate. STAGE C: Anaphase I. Spindle fibers form and attach to the centromeres of the chromosomes. The homologous chromosomes separate from each other completely and start their movement toward the poles of the cells as they are pulled by the spindle fibers.
  • 31. As the centromere of each chromosome does not divide, the sister chromatids remain together. STAGE D: Telophase I. This is the stage when the chromosomes reach their respective poles. Cytokinesis follows and two daughter cells are formed. Each cell now has only half the chromosome number because only one chromosome from each pair goes to the daughter cell. This is called the haploid condition, in contrast to the diploid condition at the beginning of meiosis I where each chromosome pair is intact. Telophase I is followed by interphase II
  • 32. Note that each chromosome still has two sister chromatids; it is therefore necessary for the cells to undergo another round of division. The second meiotic division, also known as meiosis II, is mitotic in nature and consists of the following stages: prophase II, metaphase II, anaphase II and telophase II; these stages are identical with the mitotic stages. The results are four cells, two from each daughter cell from meiosis I, with one half the diploid chromosome number and with only one sister chromatid for each chromosome.
  • 33.
  • 35. Gregor Mendel was an Augustinian monk in a monastery in Brünn, Austria- Hungarian Empire (now Brno, Czech Republic). He was interested in investigating how individual traits were inherited. He wanted to find out whether both parents contributed equally to the traits of the offspring. He also wanted to know if the traits present in the offspring were produced by the blending of the traits of the parents.
  • 36. When he has pure-breeding plants, Mendel began cross- pollinating peas with contrasting traits. The pure- breeding peas constituted the parental or P1 generation. All offspring of these crosses resembled one another. For example, when he crossed pea plants that produced round seeds with pea plants that produced wrinkled seeds, all the offspring had round seeds.
  • 37. The offspring of the parental cross are called the first filial (F1) generation. In Mendel’s experiments, the F1 generation are also called hybrids because they resulted from a cross between two pure-breeding plants with contrasting traits (for example, pea plants with round seeds crossed with pea plants with wrinkled seeds).
  • 38.
  • 39.
  • 40.
  • 41. Genes control the appearance of a trait. Alleles – traits are controlled by a pair of genes. Based on the results for the F1 generation, the trait for round seeds is the dominant trait. The trait of wrinkled seeds, which did not appear in the F1 generation, is called the recessive trait. Its appearance was either prevented or hidden by the dominant trait. This is now known as the principle of dominance: The dominant trait dominates or prevents the expression of the recessive trait.
  • 42. Genes and Gametes Following Mendel’s reasoning, a pure-breed, round-seeded parent plant has an allelic combination or genotype of RR while a pure- breed, wrinkled-seeded parent plant has a genotype of rr. Individuals that are pure-breeding for a particular character therefore have identical alleles. These individuals have a homozygous genotype.
  • 43. Genes and Gametes An individual with contrasting alleles (a dominant and a recessive allele) for a particular character is said to have a heterozygous genotype. However, Rr individuals will still produce round seeds because of the presence of the dominant allele R. These will be just as round as all the seeds produced by the RR parents. The expression of the genotype of an individual for a particular character is referred to as its phenotype.