This document provides an overview of genetics concepts including: - Genetics is the study of heredity and variation in living organisms. It explains similarities and differences between related organisms. - Variation exists between individuals of the same species, which can be continuous (gradual differences) or discontinuous (distinct categories). Variation arises from processes like independent assortment and crossing over during meiosis. - DNA contains the genetic information passed from parents to offspring. Genes located on chromosomes determine inherited traits. Genetic information is expressed through processes like DNA replication, transcription, and protein synthesis.

1. GENETICS
By Mr. Kanyoro Peter

2. What’s genetics?
 Scientific study of inheritance and variations among living
organisms.
 Explains the similarities and differences between organisms of
the same species.
 Resemblance of offspring to parents in the reproduction process
is a result of transmission of certain characteristics known as
heredity.
 In most cases, there are slight differences between parents and
offspring - variation

3. Concepts of Genetics
 Variation – observable differences among living organisms.
 Two types of variation;
 Continuous variation – with wide range of differences for the same
characteristic (a lot of intermediates) e.g. skin color, eye color, height,
weight etc.
 Discontinuous variation – with distinct groups of individuals without
intermediate forms e.g. tongue rolling, blood groups etc.

4. Variation – examples
Continuous variation- eye color Discontinuous variation – tongue rolling

5. Causes of Variation
1. Gamete formation – in the process of meiosis. Two processes;
a. Independent Assortment
 homologous chromosomes come together and separate into daughter cells
independently during metaphase I bringing varied combinations.
 Combination depends on number of pairs of chromosomes in parent cell (2n).
n = haploid number of chromosomes. In human, 223 = 8,388,608; siblings are
never alike!!
b. Crossing over
 Homologous chromosomes come in contact in prophase I at chiasmata.
 Genetic exchange occur resulting into variation.

6. Causes of variation contd.
Independent Assortment Crossing-over

7. Causes of variation contd.
2. Fertilization
 Parental genes are brought together in different combinations resulting in
different qualities of the offspring.
3. Mutations
 Genetic composition of living organisms may change spontaneously.
 Mutations bring about variation when the changes are inherited by the offspring.
 Apart from the inherited characteristics, there are acquired characteristics.
 The latter caused by environmental influences like accidents or deliberate efforts;
accidental scars or knowledge and skills.
 They are not passed to offspring.

8. Chromosomes
 Long, thread-like structures
 Found in the nucleus and become visible with microscope only during cell
division.
 Contain genes, which are hereditary materials, Deoxyribonucleic acid (DNA).
 Genes contain information that dictates how an organism grows and
develops.
 Each chromosome is made of two/pair of parallel strands, known as
chromatids.
 Chromatids are connected at one point by a centromere.
 Occurs in pairs in somatic cells and referred to as homologous chromosomes.

9. Structure of chromosomes

10. Genes and DNA
 Genes are the hereditary materials or factors contained in chromosomes
which are passed form the parents to the offspring.
 Genes occupy definite positions on chromosome called gene loci.
 Oswald Avery (1944) established that gene is in form of nucleic acid known as
Deoxyribonucleic acid (DNA).
 Francis Crick and James Watson (1953) established that DNA is complex
molecule made of 3 components;
 A 5-carbon sugar
 A phosphate
 Nitrogenous bases’
 The three forms a nucleotide.

11. Gene loci

12. Four types of nitrogenous bases;
 Adenine (A)
 Guanine (G)
 Cytosine (C)
 Thymine (T)
 The nucleotides join to form long chains (DNA strands).
 Two DNA strands twist around each other to form a double helix.
 Quantities of nitrogenous bases; A=T and C=G.
 Always combine as above when forming double helix of DNA structure.

13. Double helix structure of DNA molecule

14. Untwisted DNA strands

15. Gene
 A section of DNA on the chromosome made of chains of bases along
the DNA strand.
 Role of DNA
i. Store genetic information in a coded form as nucleotide bases.
ii. Enables the transfer of genetic information unchanged to the
daughter cell through replication.
iii. Translates the genetic information into the characteristics of an
organism through protein synthesis.

16. DNA Replication
 Making an exact copy of each of DNA strands by base pairing with
complementary nucleotides – without losing any genetic information.
 Replication of chromosome DNA takes place during interphase of cell
division. It gives rise to two sets of identical DNA molecules.
 Products of replication are visible during prophase as sister chromatids
attached at centromere.
 During replication, the twisted double helix unwinds and the strands
unzip from one end through breakage of hydrogen bonds that link the
strands.

17. DNA replication contd.
 A=T – Two weak hydrogen bonds between Adenine and Thymine. Are three
bonds between C and G.
 Unzipping exposes the base sequence.
 New DNA strands develop and the information on the parent DNA template is
copied unto them.
 Replication is important as it avails sufficient DNA for each of daughter cells.

18. DNA replication diagram

19. DNA Transcription
 The process by which the info. in DNA is copied by base pairing to
form a strand of ribonucleic acid called messenger RNA (mRNA).
 It leads to synthesis of proteins required for proper body functioning,
growth and development.
 mRNA molecule mediates between DNA in the nucleus and the
cytoplasm where proteins are synthesized.
 The DNA portion with bases needed for protein synthesis opens up.
 mRNA strand is formed from the DNA section that serves as template
and information on the template copied on it.
 mRNA leaves nucleus when transfer of DNA base sequence onto mRNA
is complete.
 It takes the instruction in form of triplets/codons, used to make
amino acids on protein polypeptide chains.

20. DNA Transcription contd.
 RNA contains base Uracil (U) in place of Thymine (T).
 Information on m-RNA is translated by the ribosomes and used to
assemble amino acids into specific protein molecules.
 Transfer RNA molecule (t-RNA) is the molecule that translates that
information.
 t-RNA transport amino acids from their pool in cytoplasm to protein
assembly point depending on base sequence on m-RNA.
 Amino acids are condensed to form a protein (long polypeptide chain)
– protein synthesis, which determine the inherited features.

21. DNA Transcription – RNA synthesis

22. The First Law of Heredity
 Aka the Law of Segregation.
 It states;
 The characteristics of an organism are determined by the hereditary factors which
occur in pairs and only one member of a pair can be represented in a single
gamete.
 Or;
 The paired factors that determine an organism’s characteristics segregate
(separate) from each other during sex cell formation and only one of the factors is
represented in a single gamete.

23. Mendel’s experiments
 John Gregor Mendel (father of Genetics) studied inheritance using garden pea
Pisum sativa.
 He studied contrasting characteristics; height (tall-dwarf), pod color (green-
yellow), seed texture (smooth-wrinkled), flower color (red-white) etc.
 Expt 1; dwarf X dwarf = dwarf (selfed dwarf severally and results were same)
 Expt 2; tall X tall = tall – as above
 Above are called PURE BREEDS – characteristics that when self pollinated
(selfed) produce characteristics of their own.

24. Reasons why Mendel’s experiments were
successful
 Used favorable experiment materials – the pea plant
 Focused on particular traits/ a particular characteristic at
a time.
 He kept accurate data in every experiment.

25. Why use pea plant (Pisum sativa)?
 Peas have sharply contrasting characteristics/traits.
 They naturally rely on self-pollination and are easily cross-pollinated.
 They mature fast.
 They produce many seeds.

26. Concepts of Genetics (more…)
 Diploid – a cell condition with two complete sets of chromosomes one from each
parent.
 Haploid – a cell condition in which only one set of chromosomes is present.
 Chromosome – long thread-like structure composed of long DNA strand. It contains
genes.
 Gene – basic unit of heredity consisting of a sequence of nucleotide bases of DNA
that encodes for a particular characteristics.
 Alleles/allelomorphs – alternative forms of the same gene responsible for
determining contrasting characteristics.
 Homologous chromosomes – a pair of chromosomes that have the same length and
the same number of genes at the same loci in an individual.

27. Concepts contd.

28. Concepts….
 Locus – position of an allele/gene on a chromosome.
 Homozygous – a condition where both alleles are identical.
 Heterozygous – a condition where the alleles are different.
 Genotype – the allele combination in an individual that determines a
particular characteristic.
 Phenotype – the observable trait in an individual resulting from influence of
genotype.
 Dominant gene – the allele that is expressed in the phenotype when present
either in homozygous or heterozygous state.
 Recessive gene – the allele that is only expressed when present in homozygous
state.

29. Concepts….
 F1 = first filial generation which refers to the first
generation/offspring produced when two varieties are crossed in
genetic experiment.
 F2 = second filial generation which refers to the second
generation/offspring produced when is F1 selfed/ individual of F1 is
crossed with another genetic experiment.

30. Monohybrid inheritance
 Aka the first law of inheritance.
 It’s the inheritance of one characteristic controlled by a single pair of
hereditary factors (genes) contributed by both parents.
 Involves transmission of just a pair of contrasting characteristics/ traits such
as tall and dwarf for height or smooth and wrinkled for seed coat texture.
 Allelic pair – a pair of genes on chromosomes in diploid organisms

31. Genetic Crossing
 Shows the diagrammatic illustration on inheritance.
 Genotype of an organism is represented using paired letter symbols.
 Capital letter represent the dominant gene while the small letter represents the
recessive gene e.g. T for tallness (dominant) and t for shortness/dwarf (recessive)
 Conventional symbols
 = male = female
 During gametogenesis, each gene in allelic pair segregates/separates into
different gametes.
 Upon fusion of male and female gametes during fertilization, offspring contain
same number of genes as in each parent.
v v

32. Genetic Crossing…..
 Inheritance of dwarf trait (pure breed dwarf plants selfed)
Parental phenotype Dwarf Dwarf
Parental genotype tt X tt
Gametes
Fusion
F1 offspring genotype tt tt tt tt (all tt)
F1 offspring phenotype: all short.
t t t t

33. Genetic Crossing…..
 Inheritance of tallness trait (pure breed tall plants selfed)
Parental phenotype Tall Tall
Parental genotype TT X TT
Gametes
Fusion
F1 offspring genotype TT TT TT TT (all TT)
F1 offspring phenotype: all tall.
T T T T

34. Genetic Crossing…..
 Cross between pure breeds
Parental phenotype Tall dwarf
Parental genotype TT X tt
Gametes
Fusion
F1 offspring genotype Tt Tt Tt Tt (all Tt)
F1 offspring phenotype: all tall.
T T t t

35. Genetic Crossing….
 F2 (second filial generation) = Cross between F1 (first filial generation) above
Parental phenotype Tall Tall
Parental genotype Tt X Tt
Gametes
Fusion
F2 offspring genotype TT Tt Tt tt (1TT, 2Tt, 1tt)
F2 offspring phenotype: 3 tall, 1 dwarf/short.
T t T t

36. Types of crossing
 Reciprocal cross – a cross of reversed genotype such that the formerly male’s
genotype becomes female’s and female’s become male’s.
 Repeat example above in the order tt X TT
 Back cross – a cross between a hybrid offspring with one of its parents.
 Test cross – a cross between an organism of unknown genotype with a
homozygous recessive organism.
 Test cross is used to determine/ is important in determining the genotype of
organism under test.

37. Test cross
 Test cross is used to determine the genotype of organism under test.
 Are two possible outcomes;
 If the unknown genotype was a homozygous dominant, all the offspring will have
dominant trait phenotype.
Parental phenotype Tall dwarf
Parental genotype TT X tt
Gametes
Fusion
F1 offspring genotype Tt Tt Tt Tt (all Tt)
F1 offspring phenotype: all tall.
 If heterozygous, the ration of offspring will be 1dominant:1recessive.
T T t t

38. Components of Genetic Cross
 Parental phenotypes
 Parental genotypes – crossing (X) MUST be shown here.
 Gametes (conventionally circled) e.g.
 Fusion lines to show fertilization.
 Filial generation genotypes.
 Filial generation phenotypes.
T

39. Genetic Crossing….
 Using Punnet square.
 Replaces use fusion lines fertilization of the genetic cross.
Filial generation
Gametes
Gametes
T t
T TT Tt
t Tt tt

40. Complete dominance
 The ratio 3:1 of a certain trait in F2 generation in a characteristic of
monohybrid inheritance.
 Complete dominance is a condition where the dominant gene completely
masks the expression of recessive gene in heterozygous condition e.g. Tt for
tallness, the individual is tall.
 For the above ratio, one individual is homozygous tall (TT), two are
heterozygous tall (Tt) and one is homozygous dwarf (tt).

41. Ratios and probabilities
 Ratios such as 3:1 can be expressed as probability.
 Probability is an expression of a chance of occurrence.
 Probability for a tall individual for the above ration is ¾ and for short/dwarf
is ¼.
 Can also be percentage. ¾ = 75% and ¼ = 25%.
 In heterozygous condition, the probability of producing either T or t is ½ for
each case.
 Probability of T fusing with t is ½ X ½ = ¼.

42. Incomplete dominance/ Codominance
 It is a condition where fusion of two genes results in a blended
trait/characteristic such that no allele completely dominates the other.
 It is represented with two different capital letters to represent two genes in a
genetic cross.
 In Mirabilis jalapa (4 O’clock plant), a cross between red and white flowered
plants results in pink flowered F1 generation.

43. Incomplete dominance; genetic cross
Parental phenotype Red White
Parental genotype RR X WW
Gametes
Fusion
F1 offspring genotype RW RW RW RW (all RW)
F1 offspring phenotype: All Pink.
R R W W

44. ABO blood groups inheritance
 ABO blood group system is an inheritable trait in humans.
 It’s transmitted from parents to offspring in the usual Mendelian fashion.
 It is determined by three genes, a multiple allele case.
 Alleles are responsible for the presence of respective antigen types on RBC i.e.
gene A, antigen A and gene B, antigen B.
 O isn’t responsible for any antigen. It is recessive to A and B. It is only expressed in
homozygous condition as blood group O.
 A and B are co-dominant and both express themselves when they occur together as
blood group AB.
 Genotypes for the four blood types are formed by allelic pairs

45. Human blood groups and their genotypes
Blood group (phenotype) Genotype Antigens
A AA, AO A
B BB, BO B
AB AB A and B
O OO O (none)

46. Genetic cross; blood groups
Parental phenotype (Blood group); A B
Parental genotype AO X BO
Gametes
Fusion
F1 offspring genotype AB AO BO OO
F1 offspring phenotype: (Blood group); AB, A, B and O.
A O B O

47. Rhesus Factor
 The same way that blood types are determined by the types of antigens on the
blood cells, Rhesus factor is.
 Antigens are proteins on the surface of blood cells that can cause a response from
the immune system.
 The Rh factor is a type of protein on the surface of red blood cells.
 Possession of rhesus antigen type D in human is controlled by the presence of
rhesus gene Rh+ while its absence is due to lack of rhesus gene denoted by Rh-

48. Rhesus Factor…
 People with the Rhesus antigen are Rhesus-positive (Rh+) while those without
are Rhesus-negative (Rh-).
 Rh+ is while Rh- is recessive; if one parent is homozygous Rhesus positive (Rh+)
F
1 offspring will be Rh+
 Rh- is expressed in homozygous recessive condition only i.e. in offspring, it
only happens when both parents are Rhesus negative.

49. Rhesus Factor Inheritance
 Follows the normal Mendelian fashion – monohybrid inheritance.
Parental phenotype Rhesus positive Rhesus negative
Parental genotype Rh+ Rh+ X Rh- Rh-
Gametes
Fusion
F1 offspring genotype: Rh+ Rh- Rh+ Rh- Rh+ Rh- Rh+ Rh- (all Rh+ Rh-)
F1 offspring phenotype: all Rhesus positive.
Rh-Rh-Rh+ Rh+

50. Effects on Blood transfusion
 A Rhesus negative person cannot receive blood from a Rhesus positive person.
 The Rh- recipient produce antibodies against Rh+
 Production of antibodies lead to agglutination of RBC of Rh- person thus
death.

51. Effects on Marriage
 If female Rh- marries a male Rh+, the fetus is Rh+
 The antigen passes from fetus to mothers blood through placenta. Also during
labor and birth.
 Mother produces antibodies that find their way back to fetus’ blood.
 There’s antigen-antibody reaction and fetus’ RBC are haemolysed.

52. Effects on Marriage…
 The antibodies build-up in second and consecutive births developing a
disease/disorder known as Erythroblastosis foetalis.
 It’s a hemolytic disease (aka hemolytic anemia).
 Injection with Rh immunoglobulin (RhIg); from donated blood is given to a
non-sensitized Rh negative person targeting any Rh positive cells in the
bloodstream and prevents the production of Rh antibodies.

53. Effects on Marriage…
 Given to an Rh negative woman who hasn’t produced antibodies against the
Rh factor, it can prevent fetal hemolytic anemia in a later pregnancy.
 RhIg is given:
 At around the 28th week of pregnancy to prevent Rh sensitization for the
rest of the pregnancy
 Within 72 hours after the delivery of an Rh positive infant
 After a miscarriage, abortion, or ectopic pregnancy
 After amniocentesis or chorionic villus sampling

54. Sex Determination
 Determined by a specific pair of chromosomes known as sex chromosomes.
 In human, there are 23 pairs of homologous chromosomes.
 Among them are sex chromosomes X and Y chromosomes which determine
whether a child becomes male of female.
 The rest 22 chromosomes are autosomes responsible for other inheritable
traits.
 A genetic male carries XY chromosomes hence heterogametic while a genetic
female carries XX hence homogametic.

55. Sex determination- gamete formation
 After meiosis in male, spermatozoa can carry either X or Y chromosomes.
 In female, ova carry X chromosomes.
 The spermatozoon fertilizing the ovum determine the sex of the child.
 If it carries X chromosome, the child is a female and if it has Y chromosome,
the child is a male.
 Are four possible genotypic combination during fertilization; 2XX and 2XY, 50%
chance for each sex.

56. Sex determination in human
Parental phenotype Male Female
Parental genotype XY X XX
Gametes
Fusion
F1 offspring genotype XX XX XY XY (2XX and 2XY)
F1 offspring phenotype: 2 Males and 2 females.
X X XY

57. Other animals…
 Birds; male XX (homogametic) and female XY (heterogametic).
 Insects e.g. Drosophila melanogaster; resembles that of human beings
exactly. XY for male and XX for female.
 Some other insects; males are XO and female XX.

58. Gene Linkage
 Refers to location of different genes closely on the same chromosome which
do not segregate during meiosis and are transmitted/inherited together.
 Such genes are said to be linked genes.
 All linked genes constitute linkage group.
 In Drosophila melanogaster genes for wing length, abdomen size and body
color are linked.

59. Sex-linked Genes
 These are all the genes located on the sex chromosomes.
 They are transmitted together with the genes that determine sex.
 In Drosophila melanogaster the gene that determine eye color is located on X
chromosome. Its allele is however lacking on Y chromosome.
 Most sex-linked genes are located on X chromosome whereas Y chromosome
carries very few genes in Drosophila melanogaster.

60. Sex-linked Genes…….
 Human beings have similar linkage as Drosophila melanogaster.
 Genes located on Y chromosome control traits that are exclusively male;
premature baldness and hair tuft on ear pinna and in nose.
 Genes located on X chromosomes control traits that can arise in either male
or female; color blindness and hemophilia.

61. Color Blindness
 Red-green color blindness – inability to distinguish between red and green
color by some people.
 The gene for color blindness is located on X chromosomes and its allele is
absent/lacking on Y chromosome.
 Gene for normal color is dominant (N) while that of blindness is recessive (n).
 Heterozygous girls/females are carriers; carry the recessive gene for color
blindness but it is suppressed by the dominant gene for normal color.

62. Inheritance of color blindness (a)
Parental phenotype Blind Male Normal Female
Parental genotype XnY X XNXN
Gametes
fusion
F1 offspring genotype XnXN XnXN XNY XNY
F1 offspring phenotype: 2 carrier females and 2 normal males.
Xn Y XN XN

63. Inheritance of color blindness (b)
Parental phenotype Normal Male Carrier Female
Parental genotype XNY X XnXN
Gametes
fusion
F1 offspring genotype XNXn XNXN XnY XNY
F1 offspring phenotype: 1 carrier female, 1 normal female, 1 blind male and 1
normal male.
XN Y XNXn

64. Hemophilia
 A condition where the blood of the person with the disorder takes abnormally
long to clot.
 Aka bleeder’s disease; in event of cut, the person experience prolonged
bleeding.
 Caused by a recessive gene (h) on X chromosome. Its allele is absent on Y
chromosome.
 It is inherited the same way as colorblindness.
 Recessive is denoted by Xh while normal by XH
 Work out a cross between a hemophiliac male and a carrier female.

65. Sex linked disorders are common in
males
 This is because males inherit only one X chromosome from the mother.
 If it carries the trait for the disorder, it is expressed as its allele is
lacking/absent in the Y chromosome.
 In females, it is expressed only in homozygous recessive on X chromosomes.

66. Other functions of sex chromosomes
 Carry genes that control feminine and masculine characteristics.
 These are the secondary traits expressed at onset of puberty among girls and boys
respectively.
 For girls, the genes are on X chromosomes while are found on Y chromosomes for
boys.
 List the feminine and masculine characteristics for girls and boys at adolescence.

67. Mutations
 Refers to spontaneous change in genetic make-up of an organism resulting in
variations.
 It is a result of crossing-over during meiosis.
 Crossing-over result in new combinations (recombinants) by separating certain
linked genes.
 An organism with different combinations is called a mutant.
 Normally caused by recessive genes transmitted in the Mendelian fashion.

68. Specific examples of mutations
 Hemophilia in humans.
 Vestigial wings in Drosophila melanogaster.
 Resistance to DDT among insects such as mosquitoes.

69. Occurrence of mutations
 Occurs naturally.
 Caused by environmental factors at times. Factors are known as mutagens.
 Includes;
 High energy electromagnetic radiations; gamma rays X-rays and U.V rays.
 Certain chemicals; colchicine and mustard gas

70. Occurrence of mutations…
 Heavy metals; mercury and lead
 High temperatures
 Some viruses such especially retroviruses such human papilloma virus
 Important mutations occur in gametes than somatic cells.
 Are the basis of discontinuous variations in populations.

71. Types of mutation
 Are of two types;
 Chromosomal mutations aka chromosome aberrations – change in structure or
number of chromosomes.
 Gene mutations aka point mutations – change in gene structure.

72. Chromosomal mutations
 Takes place during meiosis.
 Homologous chromosomes intertwine at chiasmata during crossing over.
 They then break and create plenty of chances for changes on chromatids thus
chromosome mutations.
 Are five chromosomal mutations;
 Deletion
 Duplication
 Inversion
 Translocation
 Non-disjunction

73. Chromosomal Deletion
 Aka chromosome aberrations.
 Sections of chromatids break off and do not reconnect to any of chromatids.
 Sections are completely lost and genetic material deleted.
 Involves loss of genes.
 It is lethal as an organism may lose gene necessary for vital protein molecule
synthesis

74. Chromosomal Deletion…
A B C D E F
A B C D
Cut and drop off

75. Chromosomal Duplication
 A section of chromatid replicates and adds an extra length to itself.
 It involves addition of genes by repeating a set of genes.
 It may result in overemphasizing a certain trait in the organism.

76. Chromosomal Duplication
A B C D E F
A B C D E F D E F
Extra piece added to chromatid

77. Chromosomal Inversion
 A chromatid breaks at two points and when rejoining, the middle part rotates
at 1800 and rejoins at an inverted position.
 Its effects are that it reverses gene sequence along the chromatid.

78. Chromosomal Inversion
A B C D E F
C D E
A B E D C F
Inverted section of chromatid

79. Chromosomal Translocation
 Occurs when a section of one chromatid breaks and attaches to another
chromatid of a non-homologous pair.
 It involves movement of genes form one non-homologous to another.

80. Chromosomal Translocation
A B C D E F
Q R S T U V
Q R S T U V D E F
Section that broke off and is attached to non-homologous pair
Breaks off and it is attached to non-homologous pair

81. Chromosomal Non-disjunction
 A chromosomal abnormality that lead to addition or loss of one or more whole
chromosome(s)
 If it occurs during anaphase I, two homologous chromosomes doesn’t
segregate hence move to the same gamete (sex cell) and others have no
chromosomes at all.
 If during anaphase II, sister chromatids doesn’t separate hence half of the
resulting gametes contains two of the same chromosomes while the other half
have none.

82. Chromosomal Non-disjunction
Non-disjunction at the first meiotic division

83. Chromosomal Non-disjunction
Comparing chromosomal non-disjunction at first and second meiotic divisions

84. Effects of Non-disjunction
 Fusion between a gamete containing homologous chromosomes with a normal
gamete results in an individual with three such chromosomes.
 Non-disjunction leads to disorders such as Down’s syndrome, Klinefelter’s
syndrome and Turner’s syndrome.
 Characteristics of each disorder; Secondary Biology Bk4 KLB 5th Ed. Pg. 35-26

85. Polyploidy
 A condition that arise when a normal haploid gamete fuse with a diploid
gamete or two diploid gametes fuse.
 The resulting zygotes are triploid and tetraploid zygotes respectively
 It can also occur as a result of doubling of the whole set of the chromosomes
after fertilization.
 It is common in plants. It is associated with increased yields, early maturity
and crop resistance to drought, pests and diseases.
 Artificially induced by use of chemicals such as colchicine - prevents spindle
formation hence lead to non-disjunction of chromosomes.
 The resultant cell is 4n – has double number of chromosomes.

86. Gene Mutations
 Aka point mutations
 Refers to spontaneous change in gene structure due to change in the chemical
nature of the gene.
 May involve alteration in DNA molecule e.g. sequence of nucleotides in some
section.
 Change in DNA nucleotide sequence lead to change in amino acid sequence
used for protein synthesis, thus a different protein is formed.

87. Types of gene mutation
 Are four main types;
 Insertion
 Substitution
 Inversion
 Deletion

88. Insertion
 Addition of an extra base onto an existing DNA strand.
 For example, addition/insertion of a base Guanine (G) between two Adenine
(A) bases on DNA chain.
 The resulting m-RNA base triplet and subsequent amino acid alignment will be
altered thus different protein or none.

89. Insertion…
A A A C T A C Original DNA
A G A A C T A C DNA after insertion
(G) inserted at this point

90. Deletion
 A portion of a gene is removed.
 E.g. removal of a base Thymine (T) from its position in a section of DNA
strand.
 The base sequence is altered at this point.
 Sequence of amino acids on polypeptide chain is therefore altered.
 This in turn lead to synthesis of wrong (unintended) protein.

91. Deletion...
A A A C A C
A A A C T A C
Lost/ Deleted
Original DNA
DNA after deletion

92. Substitution
 Replacement of a portion of a gene with another portion.
 E.g. Adenine (A) is exchanged/substituted with Guanine (G) on a DNA strand
thereby altering the base sequence at a particular portion.
 Also leads to formation of unintended protein molecule.

93. Substitution….
A A A C T A C
(A) is substituted with (G)
A A G C T A C
Original DNA
DNA after substitution

94. Inversion
 A portion of DNA strand breaks off at two points.
 The middle portion rotates at 1800 and rejoins the DNA strand at an inverted
position.
 Bases are inverted resulting in alteration of base sequence at that point.

95. Inversion...
A A A C T A C
Portion (A)(C) is inverted
A A C A T A C
Original DNA
DNA after inversion

96. Disorders due to gene mutations
 Include the following in human beings;
 Albinism
 Sickle cell anemia
 Hemophilia
 Color blindness

97. Albinism
 Condition where synthesis of melanin (the skin pigment), fails. It is a
derivative of phenylalanine and tyrosine.
 The person has light skin, white hair and pink eyes.
 The person is described albino.
 It is a result of substitution. A gene designated A is substituted with a
(a recessive gene).

98. Albinism…
 In homozygous condition aa blocks synthesis of melanin by preventing
formation of enzyme tyrosinase, resulting to albinism.
 A carrier of albinism, male or female is heterozygous (Aa) for the condition.
 Skin of albino is susceptible to sunburn and eyes sensitive to bright light.
 Use of sunglasses and sunscreen lotions remedies the problems.
 Work out the genotype and phenotype of F1 generation and state the
probability of having a carrier and an albino in a cross between carriers.

99. Sickle-cell anemia
 Caused by substitution.
 Normal hemoglobin type A has two polypeptide chains.
 In sickle cell condition, the amino acid glutamic acid is replaced
by/substituted with amino acid valine in the chains of hemoglobin.
 A defective hemoglobin type S results.
 In homozygous recessive condition, sickle cell anemia is expressed. The
individual synthesizes Hb type S where most of RBC’s are Sickle-shaped (S-
shaped).

100. Problems associated with sickle cell anemia
 A person with sickle cell anemia experiences oxygen shortage hence can’t
carry out strenuous activities.
 Sickle-shaped cells are unable to squeeze through blood capillaries hence clog
and block them.
 Blockage of vessels causes severe pain in joints, arms, legs and stomach.

101. Image of sickle cell anemia

102. Sickle cell trait
 It is a heterozygous condition where less than half of the number RBC’s are
sickle-shaped while the rest are normal and oxygen loading efficient.
 A person with trait experience mild anemia.
 Persons with sickle cell trait has an adaptive advantage in surviving malaria
attacks compared to persons with normal Hb.

103. Inheritance...
 Inheritance of sickle cell condition is an example of incomplete dominance.
 Work out the genotype and phenotype of F1 generation and state the
probability of having a carrier in a cross between;
i. A carrier man and a normal woman.
ii. Two carries
 N.B: Normal hemoglobin is denoted by HbA while recessive (trait) is denoted
by HbS

104. Normal and Defective Hemoglobin comparison
Normal HbA
 A position in each polypeptide is
occupied by glutamic acid.
 Doesn’t easily crystallize in low
oxygen concentration.
 It is efficient in oxygen loading
and transportation.
 Red blood cells carrying it have
the normal biconcave shape.
Defective HbS
 The same position in each
polypeptide is occupied by valine.
 Easily crystallizes in low oxygen
concentration.
 It is inefficient in oxygen loading and
transportation.
 Red blood cells carrying it are
crescent or sickle- shaped.