Chapter 5 Form 5

1. CHAPTER 5
INHERITANCE

2. 5.1 MENDEL EXPERIMENT
• The passing on genetic instructions from
generation to generation is called inheritance.
• The scientific study of inheritance is called
genetics.
• It was in the 1850’s that the study of inheritance
began.
• Discoveries made by Gregor Mendel, an
Austrian monk, contributed to the basis of
modern genetics.

3. MENDEL EXPERIMENT
• Mendel chose pure-breeding pea plants to study the
inheritance of several characteristics.
• A pure-breeding plant is obtained after many
generations of self-pollination.
• They produce identical offspring and the offspring show
the same traits as their parents.
• Table 5.1 shows the difference in meaning between
characters and traits.
Character Trait
Plant height Tall or short (dwarf)
Seed colour Green or yellow
Seed shape Round or wrinkled

4. MENDEL’S RESULT
• In one of his experiment, Mendel chose two
parent plants, one a pure-breeding tall plant and
other a pure breeding short plant.
• He called this generation the parental
generation or P generation.
• He carried out cross-pollination on the two
plants by transferring the pollen grains from the
tall plant onto the stigma of the short plant.
• He collected the seed, planted them and found
that all grew to become tall plants.
• The results of the parental cross appeared in the
first generation called the first filial
generation or F1 generation.

5. Mendel’s experiment with tall and short pea
plants

6. 5.2 Schematic diagram of Mendel’s Experiment

7. • Mendel then allowed the plants of the F1 generation
to self pollinate to produce the second filial
generation or F2 generation.
• He discovered that when he planted the seeds from
the F2 generation, about three quarters of the
offspring were tall and one quarter was short. The
ratio tall to short is 3:1.
• Mendel found similar results when he crossed pure-
breeding pea plants for each of the other
characteristics.

8. Character and traits of pea plants studied by
Mendel

9. A pair of homologous chromosomes

10. From the results of his experiments, Mendel concluded
that:
a) Inheritance depends on the transfer of heredity factors from
parents to offspring. There is a heredity factor that
determine a particular characteristics.
b) Each characteristic is controlled by a pair of factors.
c) The factors are passed from generation to generation
unaltered.
d) The factors may be dominant or recessive. Some factors are
not expressed in every generation.
e) The factors segregate or separate during gamete formation
so that each gamete contains only one pair of factors for a
given characteristic.

11. Genes and alleles
• Genes are the basic units of inheritance which occupy specific
positions on chromosomes.
• The position of the gene is called its locus.
• Gene determine specific characters in an organism.
• For example, in Mendel’s experiments there is a gene which determine
height of the pea plants.
• The height of the pea plant is expressed as tall or short.
• It is determined by factors called alleles.
• Alleles are different forms of the same gene for a trait and occupy the
same relative position on a pair of homologous chromosomes.
• For example, there is an allele for the tall and an allele for the short
trait.
• Alleles also appear in pairs.

12. Dominant & recessive alleles
• Dominant allele will mask or cover the effect of the
recessive allele.
• A recessive allele only express itself when the dominant
allele is absent.
• A dominant allele is represented by a capital letter, for
example, T for tall.
• A recessive allele is represented by a small letter, for
example, t for short.
• If the organism is pure-breeding tall, we represent it by
TT.
• If it is pure-breeding short, we represent it by tt.
• Tt stands for hybrids of the F1 generation.
• Hybrid are offspring o parents differing in at least one
cahracteristic.

13. Phenotype and genotype
• Phenotype is the observable characteristics of
an organism like colour, size, form and
structure.
• Example of phenotype are the various traits like
tall and short plants, round and wrinkled, and
yellow and green seeds.
• Genotype refers to the genetic composition of
an organism and cannot be seen.
• The genotype TT and Tt indicate tall pea plants
• The genotype tt shows the phenotype for short.

14. Homozygote and heterozygote
• Homozygote  two same allele such as TT(tall)
and tt(short)
• Heterozygote  two different allele such as Tt
(tall)

15. Importance of meiosis I in the
segregation of allelles
• In meiosis I, during metaphase I, the homologous
chromosomes are arranged on the equator.
• During anaphase I, the homologous pairs separate
and each pair moves to the opposite poles.
• On each pair of homologous chromosomes, there are
many pairs of alleles controlling different traits.
• When the homologous chromosomes separate, the
pairs of alleles also segregate.
• At the end of the meiosis process, only one allele for
each trait is found in each gamete.

16. Mendel’s First law of Inheritance
THE LAW OF SEGREGATION
The members of each pair of alleles separate or
segregate during the formation of gametes. Only
one allele can be carried in a single gamete
• Monohybrid cross : involve only one character
• Monohybrid inheritance : A cross which involves a
single characteristic determined by one gene
• Dihybrid inheritance : A cross which involves two
pairs two pair of allele determining two
characteristics.

17. Dihybrid inheritance

18. Mendel’s Second Law of Inheritance
THE LAW OF INDEPENDENT ASSORTMENT
Two or more pairs of allele segregate
independently of one another during the
formation of gametes.

19. 5.2 INHERITANCE OF TRAITS IN HUMAN
• The ABO system
• Autosome and sex chromosome
• Different type human karyotypes
• Sex determination in offspring
• Sex-linked inheritance
• Haemophilia
• Colour blindness
• Other heredity diseases

20. The ABO System
• Some traits in human are controlled by more than two alleles.
• The ABO system, is an example of a trait that is controlled by
multiple allele.
• Multiple allele means there are more than two possible allele
for a particular gene.
• There are four blood groups within the ABO system namely
O, A, B and AB.
• These blood groups are determine by three alleles.
• They are allele A, allele B and allele O.
• Each individual carries only two of the three alleles.
• The three alleles can be written as IA,IB and IO.
• Alleles IA and IB are dominant to allele IO.
• Allelle IA and IB show codominance. This means when both
alleles are present, they both have an effect.

21. Human blood groups and genotype
Blood Group Possible genotype
(phenotype)
Blood group A IAIA or IAIO
Blood group B IBIBor IBIO
Blood group AB IAIB
Blood group O IOIO

22. Use your knowledge on Mendel’s First Law to
draw a schematic diagrams and show the
inheritance of ABO blood group and the
genotypes of offspring from the following:
• A father with blood group AB and a mother with
blood group A (heterozygous)
• A father blood group is O and a mother with
blood group AB.

23. Do you know that ?
• If two different blood types are mixed together, the
blood cells may begin to clump together in the blood
vessels, causing a potentially fatal situation.
Therefore, it is important that blood types be
matched before blood transfusions take place. In an
emergency, type O blood can be given because it is
most likely to be accepted by all blood types.
However, there is still a risk involved.

24. • A person with type A blood can donate blood to a person with
type A or type AB. A person with type B blood can donate
blood to a person with type B or type AB. A person with type
AB blood can donate blood to a person with type AB only. A
person with type O blood can donate to anyone.
• A person with type A blood can receive blood from a person
with type A or type O. A person with type B blood can receive
blood from a person with type B or type O. A person with
type AB blood can receive blood from anyone. A person with
type O blood can receive blood from a person with type O.
• Because of these patterns, a person with type O blood is said
to be a universal donor. A person with type AB blood is said
to be a universal receiver.

25. The Rhesus factor
• The Rhesus factor is a group of antigens in red blood cells.
• This antigens will cause agglutination when it react with
antibodies from individuals without this antigen.
• The Rhesus factor is controlled by a pair of alleles.
• If an individual has the Rhesus factor, he is known as Rh-
positive.
• If he does not have the Rhesus factor, he is known as Rh-
negative.
• The Rh allele is dominant to the rh allele.
• The inheritance of the Rhesus factor follows Mendel’s Law.
• An individual who is Rh-positive has the genotype Rh-Rh or
Rh-rh.
• An individual who is Rh-negative has the genotype rh-rh.

26. Inheritance of Rhesus factor

27. • The Rhesus factor can become a problem when a
Rh-negative person receives Rh-positive blood
during blood transfusion.
• For the first transfusion, there is no reaction.
• In subsequent transfusions, the recipient’s blood
in the body of recipient reacts by producing Rh
antibodies.
• This results in agglutination with the receiver’s
blood.

28. Autosomes and sex chromosome
• Each human somatic cell has 46 chromosomes.
• In the human male:
 The chromosome are arranges into 22 homologous pairs
which are identical in size and shape.
In addition, there is one odd pair made up one larger
chromosomes and one smaller chrmosome.
The larger chromosome is called the X chromosome, while the
smaller chromosome is called the Y chromosome.
• In the human female:
There are 22 homologous pairs of chromosomes.
In addition, there is a pair of X chromosome
• The 22 homologous pairs are called the autosomes
• The X and Y chromosome in the male, and the X and X
chromosomes in the female are called the sex
chromosomes which determine the sex of an organism.

29. • When chromosomes are arranged and
numbered by size, starting from the largest pair
to the smallest pair their form what is called a
karyotype for an individual.
• The autosome are numbered 1 – 22 and the sex
chromosome are numbered 23.

30. The karyotype of a normal male

31. The karyotype of a normal human female

32. Observe the following human karyotype

33. Different human karyotypes
• The cells of individuals with a genetic disease or
genetic disorder show a karyotype that is
different from that of a normal human being.
• For example, an individual with Down’s
syndrome shows a karyotype different from that
a normal individual.

34. Sex determination in offspring
• In somatic cells of a human male there are 44+XY
chromosome.
• In the female, there are 44+XX chromosome.
• A male produces two types of sperms. One carries a
Y chromosome and the other an X chromosomes.
• The female only produces ova with X chromosomes.
• During fertilisation, when a sperm carrying a Y
chromosome fuses with an ovum carrying an X
chromosome, a male offspring is produced.
• However, when a sperm carrying an X chromosome
fuses with an ovum carrying an X chromosome, a
female offspring is produced.

35. • Draw a schematic diagram to show the sex
determination in offspring.

36. Sex-linked inheritance
• Genes that control various traits which are found
on the X and Y chromosome are called sex-
linked gene.
• The Y chromosome is shorter than the X
chromosome.
• It does not have many alleles that are found on
the comparable portion of X chromosome.
• Hence, in the male, any trait caused by a
dominant or recessive allele that is found on the
X chromosome, will be expressed.

37. Haemophilia
• Haemophilia is a condition which the blood does not
clot normally.
• This condition is due to the lack of a protein needed
for normal blood clotting.
• If often results in excessive bleeding.
• The ability to produce a protein for the blood to clot
is due to a dominant allele.
• An individual’s inability to produce the protein is
caused by a recessive allele found on the X
chromosome.
• Individual who suffer from hemophilia are called
hemophiliacs.
• Hemophiliacs can bleed to death from minor
wounds.

38. • We can show the inheritance of haemophilia by
using letters.
• To represent the normal dominant allele on the
X chromosome, we use XH.
• The genotype for normal female is XH XH . The
genotype of a heterozygous normal female is XH
Xh . Heterozygous females are known as carrier
of disease.
• A normal male XH Y and haemophiliac male is
Xh Y.

39. • A sex-linked gene shows a criss-cross method of
inheritance.
• This means the inheritance is from father to
daughter and mother to son.
• This is due to the fact that the Y chromosome
carries no such homologous allele on the same
locus.

40. Schematic diagram of the sex-linked
inheritance of haemophilia

41. • From the example, it can be seen that the
haemophilia gene is passed from the mother to son.
• The probability of getting a normal son or a
haemophiliac son is 0.5.
• The daughter are all normal but it is 50% likely that
one of the daughters is carrier.
• The probability of the son to be a haemophiliac is
higher compared to the daughter.
• This is because the recessive haemophilia gene is
able to express itself in a male individual.

42. Colour blindness
• Colour blindness is a condition in which colours
cannot be distinguished.
• In most cases, a person who suffer from this
condition is unable to distinguish between red and
green colours.
• Colour blindness is caused by a recessive allele
carried on the X chromosome.
• The pattern of inheritance of this condition is
similar to haemophilia.
• We can represent a normal vision gene as XC and a
colour blind gene as Xc.

43. Thalassaemia
• Thalassaemia is a childhood disease.
• This disease is caused by a recessive gene and
affects the haemoglobin in the red blood cells.
• The red blood cells of a person with
Thalassaemia are smaller.
• They may be deformed and pale in colour.
• This is because the haemoglobin content is less.
• Thalassaemia is common among people in
Mediterranean and South-East Asian countries.

44. Huntington’s disease
• Huntington disease is caused by a single gene
mutation which causes nerves to degenerate.
• This causes brain cells to die leading to
behavioral changes and loss of mental powers.

46. 5.3 GENES & CHROMOSME
• A gene is the basic unit of inheritance and has a
specific location or locus on a chromosome.
• A chromosome is a thread-like structure found
in the nucleus. Each chromosome is made up of
a long DNA molecule coiled around protein
molecules called histon.
• A DNA molecule contains thousands of genetic
codes while the protein molecules do not carry
any genetic information.

47. • The DNA molecule consists of two polynucleotide
chains twisted about each other to form a double
helix structure.
• Each polynucleotide chain is made up of many
nucleotides through condensation.
• Each nucleotide is made up of a deoxyribose sugar, a
nitrogenous base and a phosphate group.
• There are four different bases, thymine (T), adenine
(A), cytosine (C) and guanine (G).
• Adenine is linked to thymine while cytosine to
guanine by hydrogen bonds.
• The sequence of the bases forms the genetic codes.
• The genetic codes determine the characteristics of
an organism through the synthesis of proteins.

48. A nucleotide

50. Structure
of DNA

51. Application of knowledge in genetics
• Genetic knowledge and research are widely used
in selective breeding and genetic engineering.
• DNA fingerprinting and the human genome
project are two major fields involving genetic
knowledge.

52. Selective breeding
• Desireable characteristics in plants and animals
can be produced through selective breeding.

54. Genetic engineering
• Genetic engineering or recombinant DNA
technology is the modification of the
characteristics of an organism by manipulating
its DNA.
• Genetic modified organisms (GMO), genetically
modified food (GMF), gene therapy and
production of medicine are applications of
genetic engineering.

56. Genetic modified organisms (GMO)
• Genetic codes of an organism can be
altered, added or taken out to produce new
desirable characteristics.
• For example, the transfer of a gene from a firefly
to a tobacco plant enables the plant to glow in
the dark.

58. Genetically modified food (GMF)
• Genetic codes from bacteria can be transferred
to crops such as maize plants to protect plants
from pests.
• GMF gives higher yield, pest resistance, enhance
quality, last longer and increases nutritional
values.

59. Gene therapy
• Gene therapy is the insertion of genes into an
individual’s cells to treat heredity disease such as
sickle-cell anemia.

60. Production of medicine or drugs
• A genetically modified form of insulin can be
produced by inserting human insulin gene into
the E.coli bacteria.

61. DNA FINGERPRINTING
• DNA fingerprinting is a technique used to
analyse a person’s DNA fragments.
• Certain nucleotide segments of the DNA do not
code for proteins and exist as repeated short
sequences of bases.
• DNA of a person is obtained from sample of
semen, hair or blood and is cut into fragments
using specific enzyme.
• The DNA fragments are separated and arranged
to form specific band pattern.
• Each person has his own set of DNA except for
identical twins.

64. Uses of DNA fingerprinting:
• To identify criminals from samples of blood,
saliva, hair or semen collected at the scene of
crime.
• To settle paternity disputes
• To detect genetic diseases
• To test compatibility of potential organ donor
with recipient
• To confirm genotypes of animals and plants

65. HUMAN GENOME PROJECT
• A genome is an organism’s complete set of genes.
• The project aims to map the position of the genes, to
read and decode every code in the 46 human
chromosome.
• Advantages of the human genome project:
▫ To understand the mechanism of a genetic disease and
find ways to prevent the disease
▫ To obtain information for diagnosis, treatment and
possible prevention of disorders or diseases such as
cancer, diabetes and heart diseases.