The transmission of genetic
information from generation
to generation
How genetic is transmitted to the next
generation?
FERTILISATION
GENETICS
The study of inherited
characteristics and the way
they are passed on from one
generation to another
Gregor Mendel (1824- 1884)
The Father of Modern Genetics
Basic terminology in Genetics
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Gene
Allele
Dominant allele
Recessive allele
Homozygous
Heterozygous
Pure breed
Phenotype
Genotype
Cell
Nucleus
Chromosome
-made up of DNA
Chromosomes
DNA
centromere
Gene
Nucleotide
Histone
DNA
-long coiled strand
Gene
-short section of DNA
- Code information for
production of protein
There is about 2m of DNA in each
human nucleus. The total DNA in a
human body is enough to stretch to
the moon and back
What are chromosomes?
• A thread-like structure of DNA, carrying genetic
information in the form of genes
• Is found inside the nucleus of a cell
• Made up of DNA & protein (histone)
• Each cell in each type of organism has a definite number
of chromosomes
• Each cell has Diploid Number of chromosomes:
1 set of Paternal Chromosomes + 1 set of Maternal
Chromosomes
Chromosomal number in different
organisms
What are DNA?
• Main constituent of chromosomes, made up of
nucleotides
• DNA is responsible for telling the cell to make the
right type of protein and this determine the type
of cell
• Consists of two polynucleotide strands coiled
together by hydrogen bond between the pair of
bases to form a double helix
• Pair of bases: Adenine – Thymine
Cytosine - Guanine
What are DNA?
DNA as gene
Control production of
Protein
Responsible for
Characteristic
Cell characteristics depends on
protein
Cell specialisation
depends on proteins:
• All body cells in an
organism contain the
same genes but
many genes in a
particular cell are not
expressed
• The cell only makes
the specific protein it
needs to carry out its
specialised function
What are DNA?
ladder
Bases as the rung
of the ladder
What are DNA?
• Adenine always pairs
with thymine
• Guanine pairs with
cytosine
What are DNA?
Certain forces causes the ladder to twist around itself to form a
shape similar to a spiral, called a double helix
The discovery of structure of DNA
Rosalind Franklin
• She made many
measurements
on DNA, using
patterns
obtained by
directing a beam
of X-rays onto
crystals of this
molecule
• She died before
her part in the
discovery of
DNA structure
was properly
recognised
Double helix structure of DNA
• They used the measurement
made by Franklin, and
results of the chemical
analysis of DNA which
showed that the number of
bases (A, G)was always
equal to the number of bases
(T, C)
• Produce a working model of
DNA, double helix
• Watson and Crick were
awarded the Nobel prize (the
highest scientific award) for
their work of DNA
James Watson & Francis Crick
What are gene?
gene controlling • A length of DNA that codes
for a protein
skin colour
• A basic unit of inheritance
that determines a particular
gene controlling
characteristic in an
tongue rolling
organism/a short segment of
DNA
chromosome
• Sequence of bases in a gene
is the genetic code for putting
gene controlling
together amino acids in the
eye colour
correct order to make a
gene controlling
specific protein
blood group
• Control particular traits in an
organism
• “factor”
The number and structures of chromosomes present in
a cell nucleus
Male Sex Chromosomes
Male sex chromosomes of XY are different size. X chromosome is longer than
Y chromosomes. Y chromosomes only carries genes which determine sex
characteristics.
Down Syndrome
Turner Syndrome
Klinefelter Syndrome
Homolgous chromosomes
- partner chromosomes that carry same genes & same
size
Each
chromosome
in a pair may
carry
alternative
forms of the
same gene/
allele
Allele
H
h
T
T
HOMOZYGOUS
b
RECESSIVE
b
HETEROZYGOUS n
N
DOMINANT
ALLELES
GENE
ALLELES
Dominant allele
 Is an allele which always shows
its trait when it is present, and
suppresses the effect of
recessive allele; represented by
a capital letter (R)
• The allele that produces the phenotype of
the organism/ the one that shows up in the
phenotype
• E.g. T is dominant allele
• TT (homozygous)/ Tt (heterozygous)
Recessive allele
 Is an allele that only expressed
when there is no dominant allele
present/ the one that is hidden
in the phenotype
 represented by a lowercase
letter (r)
• E.g. t is recessive allele
Characteristics and Traits
Charateristic & Trait
Characteristic
Trait
Phenotype
 Phenotype - the physical feature
resulting from a genotype
• The observable characteristic of an organism
• Can be affected by the environmental factors
• (e.g. red, white, tall)
Genotype
 Genotype – genetic make-up of
an organism in terms of the
alleles present/gene
combination for a trait
• The genetic constituent of an organism
• (e.g. RR, Rr, rr)
Genotypes
 Homozygous genotype – When
the two alleles are same
• a pair of homologous chromosomes are
identical
 (2 dominant genes or 2 recessive
genes)
 e.g. TT or tt; also called pure
breed
• E.g. T T - homozygous dominant
•
t t - homozygous recessive
Genotypes
 Heterozygous genotype – When
the 2 alleles are different- one
dominant & one recessive allele
• a pair of homologous chromosomes are
different
(e.g. Tt); also called hybrid
Genotypes
 Heterozygous genotype – Tt
 Homozygous genotype – TT
– tt
H
h
T
T
HOMOZYGOUS
b
RECESSIVE
b
HETEROZYGOUS n
N
DOMINANT
ALLELES
GENE
ALLELES
Purebreeds and Hybrids
Types of Genetic Crosses
 Monohybrid cross - cross
involving a single/one trait
e.g. flower color (phenotype)
 Genotype- Ff / FF/ ff
 Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
(phenotype)
 Genotype- FfHh / FFHH / ffhh
P1 Monohybrid Cross
Parents
phenotype
genotype
Round
seeds
RR
R
Gamete
F1
generation
Rr
genotype
phenotype ?
x
x
Wrinkled
seeds
rr
R
r
r
Rr
Rr
Rr
P1 Monohybrid Cross
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Round seeds x Wrinkled seeds
RR
x
rr
Gamete
R
r
r
Rr
F1
Rr
generation
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
P1 Monohybrid Cross
Parents phenotype
Parents genotype
x
x
(diploid)
meiosis
Gamete
(haploid)
fertilisation
F1
generation
genotype
phenotype
(diploid)
Law of Segregation / Mendel’s First law
• One character are determined by
alleles which occur in pair.(Height-Tt)
• During the formation of gametes (eggs
or sperm), the two alleles responsible
Tt
for a trait separate from each other
during meiosis. (2n
n)
T t
• Only one of each pair of alleles in
present in one gamete.(1 allele in 1 gamete)
• Alleles for a trait are then
"recombined" at fertilization,
producing the genotype for the traits
of the offspring.
DRAW A SCHEMATIC
DIAGRAM
Homozygous black male mouse is cross
with a homozygous white female mouse.
Allele B is for black fur is dominant and b for
white fur is recessive.
Draw a schematic diagram to determine the
phenotypic ratio of the off springs in the
cross.