lesson of grade 9 science. the topics includes the following (a) heredity, inheritance and variation

1. Heredity, Inheritance,
and Variation
By: Victor Rea Oribe

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 the science that deals
with the study of heredity is GENETICS.

5. The family picture shows how some traits of the parents
blend into the traits of the offspring.
Can you name some of the traits?

6. Nature Versus Nurture
Human grow and develop according to the instructions in our
GENES.
The Human Genome Project (HGP -1987-2003) was aimed to map
out the sequence of the genes that will benefit many fields including
medicine and human evolution.
The findings concluded that there are about 20,000 genes that shape
how humans develop.
But despite this big number, it is very difficult to separate the relative
influence of heredity and environment on an individual’s
characteristics.
Human development is not just genetic.

7. The girl and boy in the picture may have had their father’s eye but
got their mother’s lips.
The uniqueness is brought about by the combination of genes of
their parents.

8. 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.

9. 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.

10. Check Your Understanding
1.Why does father or mother who paints well
will likely have children who are excellent
painters as well?
2.Two identical twins (twins who look exactly
alike and come from the same fertilized egg;
same genetic make-up; same sex) were
separated at birth. One was adopted by a rich
family and the other by a poor sidewalk vendor.
How will this scenario affect the twins?

11. 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.

12. 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.

13. 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.

14. Chromosomes replicate before a cell divides to
make sure that each daughter cell will contain the
complete set of chromosomes.

15. From Haploid to Diploid
All living things have the ability to reproduce
in order to perpetuate their own kind.
But multi cellular organisms like human
being go through a complex series of events
in some cells of their body, specially in their
sex cells or gametes, to maintain the
chromosomes number of a particular species.

16. Chromosomes Number of Some
Organisms
Organisms Diploid
Number (2N)
Haploid (N)
Human 46 23
Mosquito 6 3
Housefly 12 6
Cat 38 19
Chimpanzee 48 24
Dog 78 39
Onion 16 8
Rice 24 12
Potato 48 24
Cotton 52 26

17. The process by which sex cells divide and shuffle
their genetic materials is called MEIOSIS.
Meiosis takes place in the sperm and egg cell of
animal and in the anther and ovaries of plants.
It results in daughter cells which posses half the
number of chromosomes of the parent cell.
This is necessary so that when sex cells unite
during the fertilization, the original number of
chromosomes is maintained.

18. Human being have 23 chromosomes in their
GAMETES, and therefore contain only half of the
complete set of chromosomes.
These sex cells are said to be HAPLOID. It is
represented by letter n

19. During fertilization, the sperm carries only half the
number of chromosomes (haploid) and unites with
the egg cell which is also haploid.
Both father and mother contribute a copy of each
gene to the resulting offspring.

20. Two sets of
chromosomes from
both parents now
results in 46
chromosomes in the
developing embryo.
The resulting zygote
from the union of the
egg and sperm now
have a complete set of
chromosomes and is
said to be diploid,
represented by 2n.

21. All cells of our body with the exception of the sperm cells contain
the complete set of chromosomes.
The first 22 pairs are body chromosomes or autosomes and the 23
pair make up the sex chromosomes.

22. The sex chromosomes determine the sex of an
individual.
Egg always carry an X
sex chromosomes
while the sperm may
carry either X or a Y
chromosomes.
The egg and the sperm
carry one member of
the sex chromosome
pair.

23. If an X bearing sperm
fertilizes an egg, the
resulting offspring is a
female (XX)
But if a Y-bearing sperm
fertilizes and egg, the
resulting offspring is a
male (XY)

24. In short, the 44 chromosomes are autosomes and the 45th
and 46th chromosomes are sex chromosomes.
In human,
chromosomes
number 1 is
the biggest
containing
8,000 genes
and
chromosomes
21 is the
smallest with
300 genes.

25. Variation and Inherited Human
Traits
The combination of the sex cells during
fertilization results in VARIATION in the
offspring.
Several factors influence this variation which
actually happens during the meiotic process.
CROSSING OVER during the prophase stage
of meiosis.

26. Crossing over allows
the exchange of a part
of a chromosomes
with another part
from its homologous
or identical
chromosomes.
It occurs at a number
of different points on
the same
chromosomes, this
leads to greater
variation.

27. MUTATION or the permanent change of DNA
sequence can also result in variation.
These changes may be brought about by
environmental factors such as:
a) exposure to ultraviolet or nuclear radiation
b) exposure to chemicals
It may also happen during the time when a cell
copies its DNA before it divides.
If the mutation takes place in the gametes, they can
readily passed on to the offspring.

28. Most genes have two or more variations called
ALLELES.
For instance, the two alleles for hairline shape are
straight and heart-shape hairline.
A child may inherit the same allele or two different
alleles from his/her parents.
Two different alleles will interact in specific ways
to cause a trait to be expressed.

29. Other inherited traits include:
a) Free and attached airlobe
b) Straight or the bent thumb
c) Tongue rolling
d) Color of eyes
e) Shape of lips
f) Color of hair
g) Blood type
h) Diabetes
i) Texture of the hair

30. The actual set of genes carried by an organism is its
genotype and expression of manifestation of an
organism, the gene type is called PHENOTYPE.
An individual is said to be HOMOZYGOUS if the
alleles for a traits are identical or the same.
A person with two alleles for freckles is
homozygous for that particular trait.
Similarly, if a person also has two alleles for no
freckles, then he or she is homozygous as well.

31. An individual is said to be
HETEROZYGOUS when the two inherited
alleles are different for a particular gene.
In this case, an individual who possesses one
allele for freckles and one allele for no
freckles is heterozygous.
Alleles interact in a dominant or recessive
manner.

32. A DOMINANT ALLELE expressed itself and
masks the effect of the allele for the same traits.
For example, a person has an allele for free or
hanging earlobes and one allele for attached
earlobe, yet what was expressed in his physical
appearance is free earlobes.

33. A RECESSIVE ALLELE on one hand is one
expressed in the presence of the other allele.
Recessive allele can only be expressed when the
organism is homozygous for the recessive alleles.
A trait is symbolized by one letter.
for example: earlobe may be represented by
the letter E
Its alleles may be written in the following
manner: EE, Ee, ee.

34. Its alleles may be written in the following manner:
EE, Ee, ee.
Since free earlobes is dominant it is represented by
a capital letter E, attached earlobe is recessive thus
it is written as a small letter e.
a) homozygous dominant genotype EE = free
b) heterozygous Ee = free
c) homozygous recessive ee = attached

36. Traits and Expression in Human
Traits Expression
1. Shape of face
Oval : dominant,
Square : recessive
2. Cleft in chin
No cleft: dominant
Cleft : recessive

37. Traits Expression
3. Hair curl
(probably
polygenic)
Assume
incomplete
dominance
Curly: homozygous
Wavy: heterozygous
Straight: homozygous
4. Hairline
Widow’s peak: dominant
Straight hairline: recessive

38. Traits Expression
5. Eyebrow size
Broad: dominant
Slender: recessive
6. Eyebrow
shape
Separated: dominant
Joined :recessive

39. Traits Expression
7. Eyelash length
Long: dominant
short: recessive
8. Dimples
Dimples: dominant
No dimples : recessive

40. Traits Expression
9. Earlobes
Free lobe: dominant
Attached : recessive
10. Eye shape
Almond: dominant
Round :recessive

41. Traits Expression
11. Freckles Freckles: dominant
No freckles: recessive
12. Tongue
rolling
Roller: dominant
Non roller : recessive

42. Traits Expression
13. Tongue
folding
Inability: dominant
ability: recessive
14. Finger mid-
digital hair
Hair : dominant
No hair :recessive

43. Traits Expression
15. Hitch hiker
thumb
Straight thumb : dominant
Hitch-hiker thumb: recessive
16. Hair on Back
hands
No Hair: dominant
Hair :recessive

44. Gene Mutation
Sometimes, an organism may appear with a
genetic trait totally unlike anything that is
seen in otjer members of the species.
This total unlikeness is a physical
manifestation of changes at the biochemical
level which are called MUTATIONS.

45. Mutations may either SOMATIC or GERM
SOMATIC MUTATION occurs in any body cells
except the reproductive cells.
It is not passed on to the offspring and will cease
to exists when the parent or organism dies.
GERM MUTATION occurs in reproductive cells
and is transmitted to offspring.
It may be passed on from one generation to
another.

46. Causes of Mutation
1. High energy radiation
Exposure to different kinds of rays : cosmic
rays, radiation from radioactive elements, X-
ray, gamma rays, beta particles and ultraviolet
rays.
High-energy radiation is one of the most
frequent causes of mutations.

47. 2. Chemical as mutagens (agent of mutation)
among these mutagens are formaldehyde, nitrous
acid, peroxide, mustard gas, marijuana plants
61 cannabinoids with the principal component
delta-9-THC. This component has been traced
as radioactive, and it takes 5 to 8 days for just half
the THC in a single marijuana cigarette to clear
from the body.
constant exposure to THC diminishes the capacity of
individual cells to begin life according to genetic
plan built into cellular molecules.

48. 3. Induced mutations
One form of induced mutation comes from
recombinant DNA experiments.
Here, DNA from one kind of organism is treated with
enzymes to isolate a specific sequence of genes.
These genes are then added to another kind of
organism, and this added DNA recombines (with the
help of some enzymes) with DNA already present in
the recipients organisms.
Consequently, the organism can be considered a
mutant

49. MUTATIONS
Changes which can be
inherited
Changes in
Chromosomes
Cause by
Nature
Changes in
Chromosomes
Changes in
Chromosomes
Genetic Changes

50. NN Non-Mendelian Inheritance
Our modern understanding of how traits may be inherited
through generations comes from the principles proposed by
Gregor Mendel in 1865.
His experiment on Pisum sativum, or pea plants led to the
following principles:
a) Independent assortment – traits are inherited
independent of each other.
b) Dominance- when pure parents with opposite traits are
mated, the first generation shows only one traits
(dominant). The other trait (recessive) is hidden
c) Segregation – when hybrid are crossed, the opposite
traits are separated into different offspring in a ratio of
3:4 (dominant : recessive)

51. However, not all pattern of inheritance obey
the principle of Mendelian genetics. In fact,
many traits we observe are due to a combined
expression of alleles
Non-Mendelian inheritance is a term that refer
to any pattern of inheritance in which traits do
not segregate in accordance with Mendel’s
principle (that is, each parent contributes one
of two possible alleles for a traits)

52. A. INCOMPLETE DOMINANCE and
CODOMINANCE
Incomplete dominance occurs when one allele is
unable to express its full phenotype in a
homozygous individual.
This results to an individual with a phenotype that
is intermediate to homozygous individual.

53. Red Carnation is
dominant over white
carnation.
Following Mendel’s
principle of dominance,
when these traits are
crossed, the dominant
red carnation should
hide the recessive white.

54. Scientists after Mendel
found out differently.
In 1760, Josef Kolreuter
crossed pure red
carnation (RR) and pure
white carnation (WW)
and produced pink
carnation – an
intermediate phenotype
of red and white.
The phenotype of the offspring is a “blend” of the parent’s
phenotype. This is an example of incomplete dominance.

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56. CRCR
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all CRCW
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F2 generation
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57. Codominance is a situation in which both alleles are equally
strong and both alleles are visible in the hybrid genotype.
An example of codominance is found in chickens.
When a white chicken is cross with black chicken, the result
is not a grey chicken, but a chicken with both black and
white feathers.

60. B. Multiple Alleles
A gene with more than two alleles is said to have multiple
alleles.
Many genes exist in multiple alleles.
A rabbit’s coat, for example has at least four different
alleles
Hair color is determined by a single gene with a series of
alleles, each resulting in different colors like alleles for
black, brown, blond, etc.

62. C. Epistasis
The interaction between two or more genes to
control a single phenotype result in an
inheritance pattern called EPISTASIS.
It occurs when the action of one gene is
modified by one or several other genes, which
are sometimes called MODIFIER genes.
A gene can either mask or modify the
phenotype controlled by the other gene.

63. Masking epistasis occurs when a gene at one
locus masks the expression of a gene at the
second locus so its phenotype is not
expressed.
Modifying epistasis occurs when a gene at one
locus modifies or change the expression of the
phenotype of a gene at the second locus
The gene that does masking / modifying is
referred to as epistatic, while the gene that is
masked / modified is referred to as hypostatic.

64. Labrador’s coat
color vary from
yellow to black
and is controlled by
two dominant
alleles, one for the
presence of dark
pigment (allele E)
and another for the
degree of
pigmentation
(allele B)
The coat of Labrador retrievers show masking epistasis.

65. Chaff Colour in Oat. In Oat, the chaff can have three colours - black, grey and white.
Black colour (B-) is dominant over all others. In its absence grey colour (bbG-) is
dominant over white (bbgg). The dominant gene of black colour (B) is epistatic over the
alleles for grey and white chaff colour (G-and gg). When a pure black chaff producing
plant (BBGG) is crossed with a pure white chaff producing plant (bbgg) the hybrids of
F1 generation plants have black chaff (BbGg). On self breeding the resultant plants of
F2 generation have three types of chaff in the ratio of 12 black : 3 grey : 1 white.

66. Fruit Color in Cucurbita pepo.
In Summer Squash or Cucurbita pepo, there are three types of fruit color-
yellow, green and white.
White color is dominant over other colors
while yellow is dominant over green.
Yellow color is formed only when the dominant epistatic gene is represented by its
recessive allele (w). When the hypostatic gene is also recessive (y), the color of the fruit
is green.

71. D. Sex-linked Traits
After Mendel’s garden peas, an American geneticist
Thomas Hunt Morgan studied genetic variations in
drosophila melanogaster (fruit fly).
In 1910, while examining a large number of drosophila,
Morgan found one that had white eyes instead of the normal
red ones; the fly was a male.

72. Morgan had the white-eyed male mate with a re-eyed
female.
All of the offspring in the first generation had red eye.

73. He then mated flies from this generation. As a result, three-
fourths of the second generation offspring had red eyes and
one-fourth had white eyes.
The results seemingly
conformed with the
results Mendel
observed in garden
peas.
However, there was
one striking
difference, that is, all
of the white-eyed flies
were males.

75. Legend
W : Dominant gene for red eyes
XW Xw Female, pure red eyed
XW Xo Male, red eyed (W cover O)
White-eyed male X Red-eyed female (pure)
XW Xo XW Xw
FEMALE
MALE
XW
Xw
XW
Xo
XW XW
XW Xw
Xw X0
Xw X0
Female, red Female, red
Male, red Male, red
Resulting F1
Genotype:
all female are carriers of
white-eyes recessive gene
Phenotype:
All males are normal; red-
eyed
All females appear red-
eyed because W is
dominant.

76. Legend
w : Recessive gene for white eyes
XW Xw Female, hybrid, red eyed (W dominant
Xw Xo Male, white eyed (O cannot cover w)
Red-eyed male X Red-eyed female (hybrid)
Xw Xo XW Xw
FEMALE
MALE
XW
Xw
Xw
Xo
XW Xw
Xw Xw
XW X0
Xw X0
Female, red Female, red
Male, red Male, red
Resulting F1
Genotype:
One-half of the females
are normal, pure red-eyed
One-half of the females
appear red but are carriers
Phenotype:
One-half of the males are
normal; red eyed