Download Homologous chromosome

Genes & Inheritance
Prepared by Ms W.S.Kwan
What are
chromosomes?
• Chromosomes appear as very thin & thread-like
structures called Chromatins
• Chromosomes are made up of DNA & protein
• 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 •
Gene
A gene is a short length of
DNA on a chromosome
It is a unit controlling an
inherited character
• DNA controls the formation
gene controlling of proteins, such as
eye colour
enzymes & hormones
gene controlling
blood group
chromosome
• Hence, genes control all
the metabolic activities of
the cells
Homologous chromosome
- Chromosomes always
exist in pairs ‘Diploid’
in body cells (somatic
cells]
- Each human body cell
has 46 chromosomes
(i.e. 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 in
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
e.g. dark skin / white skin
In sexual reproduction
Gametes (sex cells)
Male gamete
Sperm
Fertilization
Female gamete)
Egg
Zygote
Role of Gametes: transfer the genetic information from
the parents to the offspring
What would happen if gametes
(sex cells)are formed by mitosis?
Mitosis
Fertilization
Mitosis
Male (2n)
Female (2n)
Sperm (2n)
Egg (2n)
Zygote (4n)
Sperm/Egg (4n)
Fertilization
Zygote (8n)
Meiosis
Another type of nuclear division
Meiosis
 Producing cells with chromosome
number half of the parent cell
 Two nuclear divisions
– Meiosis I and meiosis II
 Four haploid daughter cells are
produced
First Meiotic Division
• Chromosomes
become visible
• Nuclear
membrane
disappears
Homologous
chromosomes
pair up &
duplicate
• Homologous
chromosomes
pair up &
duplicate
• Crossing-over
may occur
between
homologous
chromosomes
(not in mitosis)
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
 Equal separation of
chromatids of each
chromosome
 4 daughter cells
form
 Each daughter cell
has a half number
of chromosome
as the
parent cell
Summary of meiosis:
One diploid parent cell produces 4 haploid daughter cells
Occurrence of meiosis
 Plants: anthers and ovules
 Mammals: testes and ovaries
Formation of pollen grains by meiosis
Comparison between Mitosis & Meiosis
Mitosis
Meiosis
Number of division
1
2
No. of daughter cells
produced
2
4
Somatic (body) cells
Gametes (sex cells)
Growing tissues
Reproductive
tissues (gonads)
Type of cells produced
Occurrence
Comparison between Mitosis & Meiosis
Mitosis
Pairing of homologous
chromosomes
No
Chromosome number
of daughter cells
Same as parent cell
diploid [2n)
Role
For Growth,
Replacement &
asexual reproduction
Meiosis
Yes
Half of parent cell
haploid [n)
For Gamete
Formation
Significance of meiosis
 Produce haploid gametes which
combine to restore the diploid
conditions after fertilization.
 Allow genetic variations in the
offspring
Sources of genetic variation
• Random assortment of chromosomes
in meiosis
• Random fusion of gametes during
fertilization
• Cross fertilization
• Crossing-over in meiosis
~END~