1. title
Genetics

2. 22.2 Chromosomes, genes and DNA
Genetics
22.1 What is genetics?
22.2 Chromosomes, genes and DNA
22.3 How are genes passed on from generation to generation?
22.4 Studying the pattern of inheritance
22.5 How to solve problems involving monohybrid inheritance?
22.6 Sex determination in humans
22.7 How to study human inheritance?
22.8 Variation among individuals of the same species
22.9 Why are we all different?
Significance of variations

3. Passing on of characteristics from one generation to the next
22.1 What is genetics?
Why are you similar
to your parents in
many ways?
Passing on of characteristics from one generation to the next is
the process is
the study is
heredity
inheritance
genetics

4. 22.2 Chromosomes, genes and DNA
nucleus
chromatids
cell
DNA (helical)
gene

5. Gene
short segment of a DNA at specific location of chromosome
basic unit of heredity
controls inheritable characteristics

6. types of proteins and enzymes control
twisted together into a helix
deoxyribonucleic
acid D N A
carries genetic information that determines the sequence of amino acids in proteins
types of proteins and enzymes control
body characteristics
metabolic activities

7. e.g. human: 46 chromosomes;
cat: 38
chromosomes
DNA molecules
special proteins
Chromosome
specific
number of chromosomes in each species
same
number in
all body cells except sex cells

8. chromosomes in pairs (homologous chromosomes) Body cells diploid
Sex cells (gametes)
either paternal or maternal chromosome
haploid
Two haploid gametes combine to form one diploid cell

9. 22.3 How are genes passed on from generation to generation?
starting point of life
male gamete
female gamete
zygote
haploid gametes
each carries half of the genes which determine the characteristics of the parent
as vehicles of inheritance

10. Stages in meiotic cell division
1 diploid cell
4 haploid gametes
meiotic cell division
meiosis (nuclear division which reduces the chromosome number by half)
followed by division of cytoplasm
Stages in meiotic cell division

11. Occurrence of meiotic cell division
flowering plants
male gametes in pollen grains
egg cells in ovules
animals
sperms in testes
ova in ovaries

12. The significance of meiosis
Halving of chromosome numbers in gametes
produces haploid gametes
the diploid number of chromosomes can be restored at fertilization
Independent assortment
produces gametes with different genetic make-up
variations among offspring of the same species

13. combination 1 either Imagine the enormous number of combinations in
The two possible combinations of chromosomes in the gametes formed from only 2 pairs of chromosomes during meiosis
combination 1
either
Imagine the enormous
number of
combinations in
humans which have
23 pairs of
chromosomes!
combination 2 or

14. Carry out Practical 22.1
Observation of meiosis in a testis squash of the grasshopper or in photomicrographs

15. 22.4 Studying the pattern of inheritance
Monohybrid inheritance
inheritance of a characteristic which is controlled by only one pair of alleles for each gene
first studied by Gregor Mendel
who investigated the inheritance of two contrasting characters (tall & short stems) in garden peas

16. About the experiment tall (pure breeding) produced offspring only tall
dwarf
(pure breeding)

17. About the experiment tall (pure breeding) produced offspring only
dwarf
dwarf
(pure breeding)

18. About the experiment 3 : 1 ? ? ? tall (pure breeding) dwarf
cross-pollination & fertilization
first filial (F1) generation
all tall (1064)
self-pollination & fertilization of F1
second filial (F2) generation
787 tall
277 dwarf
3 : 1
? ? ?

19. Other characters studied
Character studied
Shape of seed coat
Colour of cotyledons
Colour of seed coat
Shape of pods
inflated
smooth
yellow
grey 
Cross   
constricted
All show similar pattern of inheritance.
wrinkled
green
white F1
all smooth
all yellow
all grey
all inflated
Ratio of characters in F2
2.96 : 1
3.01 : 1
3.15 : 1
2.95 : 1

20. Interpretation of experimental results
These characteristics are controlled by pairs of genes.
tall
dwarf 
parents TT tt

21. Interpretation of experimental results
Each gamete receives only one gene (allele) from each pair.
tall
dwarf 
parents TT tt
gametes T t

22. Interpretation of experimental results
All offspring (F1) are tall because T gene is dominant to t gene.
tall
dwarf 
parents TT tt
gametes T t F1
Tt (tall)

23. Interpretation of experimental results
F1 generation can produce 2 types of gametes (T or t).
tall
dwarf 
parents TT tt
gametes T t F1
Tt (tall)
gametes T t

24. Interpretation of experimental results
There are 4 possible combinations of gametes when random fertilization happens.
tall
dwarf 
parents TT tt
gametes T t  F1
Tt (tall)
Tt (tall)
gametes T t T t F2 TT Tt Tt tt
tall
dwarf
ratio :

25. Common terms in genetics
Allele
tall
dwarf 
parents
a length of DNA controlling a certain character TT tt
which may have 2 or more alternate forms
gametes T t  F1
Tt (tall)
Tt (tall)
each form of a gene is called an allele
gametes T t T t
e.g. T : the allele for tallness
t : the allele for dwarfness F2 TT Tt Tt tt
tall
dwarf
ratio :

26. Common terms in genetics
Phenotype
tall
dwarf 
parents
the appearance of a character of an organism TT tt
gametes T t
e.g. the phenotype of the F1 generation is tall  F1
Tt (tall)
Tt (tall)
gametes T t T t F2 TT Tt Tt tt
tall
dwarf
ratio :

27. Common terms in genetics
Genotype
tall
dwarf 
parents
the genetic make-up of an organism in relation to the gene being investigated TT tt
gametes T t  F1
Tt (tall)
Tt (tall)
e.g. the genotype of the F1 generation is Tt
gametes T t T t F2 TT Tt Tt tt
tall
dwarf
ratio :

28. Common terms in genetics
Homozygote
tall
dwarf 
parents TT tt
an organism whose genotype contains two identical alleles for a particular characteristic
gametes T t  F1
Tt (tall)
Tt (tall)
e.g. the tall parent plants (TT) or the dwarf plants (tt)
gametes T t T t F2 TT Tt Tt tt
homo = the same
tall
dwarf
ratio :

29. Common terms in genetics
Heterozygote
tall
dwarf 
parents TT tt
an organism whose genotype contains two different alleles for a particular characteristic
gametes T t  F1
Tt (tall)
Tt (tall)
e.g. the tall plants with Tt make-up in F1 generation
gametes T t T t F2 TT Tt Tt tt
hetero = different
tall
dwarf
ratio :

30. Common terms in genetics
Dominant
tall
dwarf 
parents
a dominant gene can express itself or produce its effect in both homozygous and heterozygous conditions TT tt
gametes T t  F1
Tt (tall)
Tt (tall)
e.g. T represents a dominant gene which causes the plants to be tall in either the homozygous (TT) or heterozygous (Tt) condition
gametes T t T t F2 TT Tt Tt tt
tall
dwarf
ratio :

31. Common terms in genetics
Recessive
tall
dwarf 
parents
a recessive gene can only express itself in a homozygous condition TT tt
gametes T t
e.g. t represents a recessive gene which causes the plant to be dwarf when in a homozygous (tt) condition  F1
Tt (tall)
Tt (tall)
gametes T t T t F2 TT Tt Tt tt
tall
dwarf
ratio :

32. Common terms in genetics
Hybrid
tall
dwarf 
parents
an individual which results from crossing two homozygous parents which are genetically different TT tt
gametes T t  F1
Tt (tall)
Tt (tall)
e.g. the F1 generation
gametes T t T t F2 TT Tt Tt tt
tall
dwarf
ratio :

33. Examples of monohybrid inheritance
Alleles
Phenotype
Dominant
Recessive
Characteristic
In maize plants Colour of the seeds in a cob
dark-coloured e.g. purple
light-coloured e.g.yellow
In fruit flies Length of wing Appearance of abdomen
long
vestigial
broad
narrow
In mice Coat colour Ear size
black
brown
normal
short

34. Examples of monohybrid inheritance
Dominant characteristics
Recessive characteristics
brown eye
blue eye
Eye colour
lobed ear
lobeless ear
Ear lobes
tongue can be rolled
tongue cannot be rolled
Tongue rolling
normal pigment
Skin pigment
no pigment (albinism)
straight thumb
Thumb shape
curved thumb
Examples of human characteristics that are inherited in a Mendelian manner

35. 22.5 How to solve problems involving monohybrid inheritance?
We are often provided with the phenotypes(P) and genotypes(G) of the parents, and then asked to:
deduce P and/or G of the offspring and their P and/or G ratios
determine the probability that an offspring will have a certain P or G
predict the number of offspring that will have a certain P or G
all can be solved by genetic diagrams and Punnett squares

36. Genetic diagrams
Consider a monohybrid cross between two humans which are heterozygous for the presence of ear lobes.
Let
L = allele for lobed ear (dominant)
l = allele for lobeless ear (recessive)
Choose a symbol to represent the alleles of the character of the parents.

37. Genetic diagrams a State the genotypes and phenotypes of the parents.
Let
L = allele for lobed ear (dominant)
a State the genotypes
and phenotypes of
the parents.
l = allele for lobeless ear (recessive)
Lobed ear
Lobed ear 
Parents Ll Ll
b Label the genotypes
‘Parents’.
c Write ‘’ between
the phenotypes and
genotypes of the
parents to stand for
a cross.
always write dominant allele first

38. Genetic diagrams a Write down the possible alleles of
Let
L = allele for lobed ear (dominant)
a Write down the
possible alleles of
the male and female
gametes.
l = allele for lobeless ear (recessive)
Lobed ear
Lobed ear 
Parents Ll Ll
Label them ‘Gametes’.
Gametes L l L l
b Add lines as
shown.

39. Genetic diagrams a Show the results of possible random crossing of the
Let
L = allele for lobed ear (dominant)
a Show the results of
possible random
crossing of the
gametes by using
lines as shown.
l = allele for lobeless ear (recessive)
Lobed ear
Lobed ear 
Parents Ll Ll
Label the offspring ‘F1’ generation.
Gametes L l L l
b State the
phenotypes of the
offspring
underneath the
genotypes. LL Ll Ll ll F1
Lobed ear
Lobeless ear

40. Punnett squares
Draw four boxes as shown.

41. Punnett squares
Write down the possible alleles from the female along the top, one above each box.
Also do the same along the left side for the male, one next to each box.
Label them
(female)
and
(male). L l L l

42. LL Ll Ll ll Punnett squares
The products of the various possible combinations after fusions are written in the appropriate boxes. L l LL Ll L Ll ll l

43. LL Ll Ll ll Punnett squares
State the phenotypes and genotypes of the offspring. L l LL Ll L
lobed ear
lobed ear Ll ll l
lobed ear
lobeless ear

44. from heterozygous couples, 3/4 and 1/4 of offspring will have lobed ears and lobless ears respectively
You should note that:
The outcome of any particular cross is totally unrelated to that of any other.
It is possible to predict the proportion of offspring that will have a certain phenotype or genotype.
The use of genetic diagrams or Punnett squares only gives the expected results.

45. first parent  second parent
Types of monohybrid crosses
up to 6 types for any pair of alleles
e.g. eye colour in human (B: dominant allele for brown eyes, b: recessive allele for blue eyes)
first parent  second parent
Offspring 1 2 3 4 5 6
phenotypic ratio
genotypic ratio BB 
all brown eyes
all BB Bb BB 
all brown eyes
BB : Bb = 1 : 1 Bb 
brown : blue = 3 : 1
BB : Bb: bb = 1 : 2 : 1 bb BB 
all brown eyes
all Bb Bb bb 
brown : blue = 1 : 1
Bb : bb = 1 : 1 bb 
all blue eyes
all bb

46. Types of monohybrid crosses
If you are familiar with the 6 types of crosses, you should be able to:
Find the phenotypes or genotypes of parents if result of certain crosses is provided.
State the dominant or recessive characters for a certain phenotype if the phenotypes of the parents and offspring are provided.

47. How to find out the genotype of an organism with a particular dominant phenotype
a dominant phenotype
2 possible genotypes
homozygous dominant
heterozygous dominant
How can we find out the genotype accurately ?
By test cross.

48. How to find out the genotype of an organism with a particular dominant phenotype
Example: To identify the genotype of a tall plant
Case 1
If the genotype is TT
organism to be tested
homozygous recessive organism tt TT
Gametes T t
F1 genotype phenotype Tt
tall
(all the offspring are tall)

49. homozygous recessive organism
How to find out the genotype of an organism with a particular dominant phenotype
Example: To identify the genotype of a tall plant
Case 2
If the genotype is Tt
organism to be tested
homozygous recessive organism tt Tt
Gametes T t t tt
F1 genotype phenotype Tt
tall
dwarf
ratio :

50. How to find out the genotype of an organism with a particular dominant phenotype
Example: To identify the genotype of a tall plant TT T
organism to be tested
homozygous recessive organism tt t
Gametes
F1 genotype phenotype Tt
tall
(all the offspring are tall) Tt
organism to be tested
homozygous recessive organism tt T t
Gametes
F1 genotype phenotype
tall
dwarf
ratio :
if both tall and dwarf offspring are obtained
if all offspring are tall
unknown organism
= homozygous dominant
unknown organism
= heterozygous dominant

51. Observation of maize cobs with grains of different colours
Carry out Practical 22.2
Observation of maize cobs with grains of different colours

52. 22.6 Sex determination in humans
sex is determined by the sex chromosomes
the 23rd pair of chromosomes
Cell of a male
Cell of a female
has one X chromosome and one Y chromosome
has two X chromosomes Y X X
all eggs carry
one X chromosome
50% sperms carry X and 50% carry Y X X Y XY XY
XY become boys X
XX become girls XX XX

53. 22.7 How to study human inheritance?
by studying pedigree or family tree
and tracing the pattern of inheritance of some easily recognizable characters
male
female
individual 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 15
brown eye
blue eye
male
female
generation
An example of a pedigree showing the inheritance of eye colour

54. 22.8 Variation among individuals of the same species
Have you ever met
two people who
are exactly alike ?
The differences in characteristics among individuals of the same species
variations
continuous variations
discontinuous variations

55. Continuous variation
There is a continuous range of intermediate phenotypes between extremes of a quality.
e.g. weight, height
The characters are controlled by many genes and may be affected by environment.

56. number of people in each height
Continuous variation
a normal distribution curve can be obtained
100
200
300
400
500
600
700
800
900
1000
1100
1200
140
145
150
155
160
165
170
175
180
185
190
number of people in each height
height (cm)

57. Discontinuous variation
e.g. tongue rolling, ear lobes
There are no intermediates.
The characters may be controlled by one pair of genes and are less easily affected by environment.
No normal distribution curve can be produced.

58. Carry out Practical 22.3
Observation of variations in humans, e.g. height variation, tongue rolling

59. 22.9 Why are we all different?
variation is a result of
Heredity
Environment or
interacts with

60. Heredity Genetic variation
caused by heredity
Genetic variation
is a result of
independent assortment of chromosomes at meiosis
random fertilization
mutation
a sudden, relatively permanent inheritable change in the DNA of a gene or more than one gene
only the one occurs in gamete cells can be passed to the next generation

61. zygote divides into two cells each cell continues to divide
Environment
environment can affect the expression of certain genes
results in
variation
an egg fertilized by a sperm
zygote divides into two cells
each cell continues to divide

62. two balls of cells develop into two genetically identical individuals
Environment
environment can affect the expression of certain genes
results in
variation
two balls of cells develop into two genetically identical individuals

63. Environment environment can affect the expression of certain genes
results in
variation
brought up in a poorly-nourished environment
brought up in a well-nourished environment
identical twins

64. Environment Other examples light
e.g. the gene for chlorophyll production
chlorophyll produced
no chlorophyll produced

65. Environment Other examples 2 temperature
e.g. the gene for curly wings in fruit flies nn
develop at 25°C
develop at 16°C
parents n
gametes
fertilized eggs F1

66. 22.10 Significance of variations
Variations exist in the length of necks in a population of giraffes, some with long necks, some with short necks.
When food supply is enough, neck length is not a determining factor in the survival of the giraffes.

67. 22.10 Significance of variations
When the weather becomes dry for a long time, no grass grows on the ground.
Short-necked giraffes will die.
Long neck length becomes a favourable variation.
It allows giraffes to get leaves on tree tops, hence to survive.

68. 22.10 Significance of variations
Long-necked giraffes survive and pass on this characteristic to their offspring.

69. 22.10 Significance of variations
Variations cause some individuals to be better adapted to the environment than others.

70. discontinuous variation
Concept diagram
Genetics
is the study of
inheritance
occurs by action of
explains
characteristics
genes
may show
continuous variation
discontinuous variation

71. heterozygous condition
Concept diagram
genes
are made up of segments of
can be
are located on
occur as pairs of
DNA
dominant
recessive
and some proteins make up
character expressed in both
character expressed only in
chromosomes
alleles
heterozygous condition
homozygous condition
with different numbers in
determine the

72. Concept diagram chromosomes alleles diploid cells haploid cells
with different numbers in
determine the
diploid cells
haploid cells
genotype
phenotype
formed by
formed by
represents each cell’s
represents an organism’s
mitotic cell division
meiotic cell division
genetic make-up
observable features
(in relation to the gene studied)
(in relation to the character studied)