2. Chromosomes
●Genetic materials found inside the nucleus of a cell
●Made up of DNA & protein
●Appear as very thin & thread-like structures called
Chromatins
●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
3. chromosom
e
gene controlling
skin colour
gene controlling
tongue rolling
gene controlling
eye colour
gene controlling
blood group
●A gene is a short
length of DNA on a
chromosome which is
a unit determining an
inherited character
●It consists of a
chemical substance
called
deoxyribonucleic acid
(DNA)
4. chromosom
e
gene controlling
skin colour
gene controlling
tongue rolling
gene controlling
eye colour
gene controlling
blood group
●DNA is responsible for
telling the cell to make
the right type of protein
& control the metabolic
activities of the cells
5. ●Chromosomes always exist in pairs in the body
(somatic) cells
●Each human somatic cell has 46 chromosomes
○23 pairs of homologous chromosomes
Homologous Chromosomes
6. ●Members of homologous
chromosomes carry the
same genes (controlling the
same character) in same loci
●But the genes on the
members of homologous
chromosomes may be of
different forms (effects)
●Alleles
allele for
white skin
colour
allele for
dark skin
colour
7. ●Alternative forms of genes on the same
position of the homologous chromosomes
which control the same character but have
different expressions
Alleles
8. What would happen if gametes are
formed by mitosis?
Male (2n) Female (2n)
Sperm (2n) Egg (2n)
Zygote (4n)
Sperm/Egg (4n)
Zygote (8n)
Mitosis
Fertilization
Mitosis
Fertilization
9. Meiosis
●Producing cells with chromosome number half of
the parent cell
●Two nuclear divisions
○Meiosis I and meiosis II
●Four haploid cells are produced
10. First Meiotic Division
●Chromosomes become visible
●Nuclear membrane disappears
●Homologous chromosomes pair up (not in mitosis)
●Crossing-over (not in mitosis) may occur between
homologous chromosomes
●Homologous chromosomes line up in the middle of cell
randomly
●The 2 members of each homologous chromosome
separate from each other and move towards the opposite
poles of the cell
●Nuclear membrane reforms
11. Second Meiotic Division
●Separation of chromatids of each
chromosome
●4 daughter cells with half of the chromosome
number of the parent cell are formed
●Occurrence of meiosis
○Plants: anthers and ovules
○Mammals: testes and ovaries
12. Comparison between Mitosis
& Meiosis
Mitosis Meiosis
Number of division 1 2
No. of daughter cells
produced
2 4
Type of cells produced Somatic (body) cells Gametes (sex cells)
Chromosome number of
daughter cells
Same as parent cell
(diploid)
Half of parent cell
(haploid)
Pairing of homologous
chromosomes
No Yes
Occurrence Growing tissues Reproductive tissues
(gonads)
Role For growth and
replacement
For gamete formation
13. Significance of meiosis
●Produce haploid gametes which,
after fertilization, can restore
normal diploid conditions
●Produce genetic variations
15. Heredity: tendency of an individual to
resemble his parents
Variation: the differences between
individuals of the same species
Genetics: the study of how characters
are transferred or inherited from one
generation to the next
The pattern of
inheritance and variation
17. Mendel’s experiment
●A number of
different traits that
can be studied
●Plant should be self
fertilizing and have a
flower structure that
limits accidental
contact.
●Offspring should be
fully fertile
28. Terms
●Genotype - the genetic composition
●Phenotype - the external appearance
●Homozygous (pure-breeding)
○with the same alleles in the same character
●Heterozygous (hybrid)
○with different alleles
●Dominant gene (or character)
○which expresses its effect even in heterozygous condition
●Recessive gene (or character)
○which expresses its effect only in homozygous condition
●Pedigree
○it shows the inheritance of one or more characters in different generations
●Test cross
○cross the organism with dominant character to an organism with
homozygous recessive character
○to test whether the organism with the dominant character is homozygous
or heterozygous
29. Sex Determination
●in human: by sex chromosomes
○male : XY
○female : XX
●chromosomes other than sex chromosomes somatic
chromosomes which control normal body characters
●22 pairs are identical in both sexes (human)
○autosomes
●The 23rd pair is different in male and female
○sex chromosomes
30. Twins
●identical twins
○one embryo separates into two and each develops into a
new individual
○they should have same characters and same sex
●non-identical twins
○formed from two different zygotes from two
separate fertilizations
○they usually have different characters
○they may have different sexes
32. Types of Variations
●discontinuous variation
○the character is clear-cut & not showing intermediates
○eg. tongue rolling, ear lobes and colour of corn
albinis
m
33. ●Meiosis
○homologous chromosomes separate independently from each other
and pass into a different gamete ( INDEPENDENT ASSORTMENT).
As a result, a great variety of gametes are produced
●Random Fertilization
○since fertilization is a random process, there are many possible
different combinations of genes in a zygote
●Mutation
○genetic make-up may suddenly change
○sometimes occur naturally
○rate may greatly increased if the organism is exposed to radiation,
certain chemicals or neutron bombardment
○most mutation are harmful
●Environmental Factors
○cause variation in characters with continuous variation
Causes of Variations
34. ●Deoxyribose Nucleic Acid
●A very long, thin molecule made up of DNA
nucleotides
●2 nucleotide strands in the form of double
helix
●DNA together with proteins made up the
chromosomes
●Chromosomes are mainly found inside the
nucleus in human cells
DN
A
35. DNA is a polymer of DNA nucleotides
Each nucleotide consists of ...
Pentos
e
Phosphate Organic bases
4 types of
bases :
A
T G
C
38. ATG GTT AAA ATC CCC CTG GGT ATG GTG ATT
Transcription &
Translation
AA 1 AA2 AA3 AA
4
AA
5
AA6 AA
7
AA8 AA
9
AA10
Polypeptide
Proteins serve as
●enzymes
●Hormones
●Structural proteins
Genetic information stored as triplet codes of bases in
DNA
