Download GENES_AND_INHERITANCE__ARI

What are 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
gene controlling
skin colour
gene controlling
tongue rolling
chromosome
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)
gene controlling
skin colour
gene controlling
tongue rolling
chromosome
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
• Chromosomes always exist in pairs in the
body (somatic) cells
• Each human somatic cell has 46
chromosomes
– 23 pairs of homologous chromosomes
allele for
white skin
colour
allele for
dark skin
colour
• 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
• Alternative forms of genes on the same
position of the homologous
chromosomes which control the same
character but have different
expressions
What would happen if gametes
are formed by mitosis?
Male (2n)
Female (2n)
Sperm (2n)
Egg (2n)
Mitosis
Fertilization
Mitosis
Zygote (4n)
Sperm/Egg (4n)
Fertilization
Zygote (8n)
Meiosis
 Producing cells with chromosome
number half of the parent cell
 Two nuclear divisions
– Meiosis I and meiosis II
 Four haploid cells are produced
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
genetic materials exchanged
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
 Followed by
cytoplasmic
cleavage
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
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)
Comparison between
mitosis & meiosis
Mitosis
Pairing of homologous
chromosomes
Occurrence
Role
Meiosis
No
Yes
Growing tissues
Reproductive
tissues (gonads)
For Growth &
Replacement
For Gamete
Formation
Significance of meiosis
 Produce haploid gametes which, after
fertilization, can restore normal
diploid conditions
 Produce genetic variations
Sources of genetic
variation
• Different combination of the genes of
each gamete
• Random fusion of gametes during
fertilization
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