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Genetic variation in Gametes
- the complicated mechanism of cell division may
occasionly go wrong.
- when this happen there may be alteration in
structure or change in number of chromosomes.
I) Abnormal Chromosome Number :
- changes in chromosome number are known as
( Polidy ).
- Euploidy :
the basic set of chromosomes is the haploid set
( n ) which is the number of chromosomes present
in the gametes ,n=23.
the diploid set ( 2n ) is the number of chromosones
present in the normal somatic cells, 2n = 46.
1- Polyploidy
- when akaryotype has any multiple of the basic haploid
( n ) other than the normal diploid ( 2n ) it is referred
to as Polyploidy.
- it may be further designated as Triploid ( 3n ) ,
, Tetraploidy ( 4n ) , Pentaploid ( 5n ) ..... etc
- Triploidy: is the case in which the number of
chromosome is 3 folds the haploid number
i.e. 23 x 3 = 69 chromosome.
- mechanism of Triploidy:
i- failure of one of the maturation division of the
ovum or sperm.
in 23% of cases ahaploid ovum ( n ) + adiploid
sperm ( 2 n ).
in 10% of cases adiploid ovum ( 2 n ) + ahaploid
sperm ( n ).
ii- fertilization of the ovum by 2 sperms in 66% of
cases , triploid zygotes are caused by dispermy.
- Tetraploidy:
is the case in which the number of chromosome is
4 folds the haploid number i.e. 23 x 4 = 92
mechanism:
failure of completion of the first cleavage division of
the zygote.
- Triploidy & Tetraploidy are incompatible with life
leading to early abortion.
- Polyploidy is a result of failure of the spindle
mechanism after the chromosome have duplicated,
but before cell division is complete
this results in the formation of a( 2 n ) gamete which
if fertilized by an ( n ) gamete , ( 3 n ) zygot would
result.
2- Aneuploidy
- in this condition the number of chromosomes is not
the exact multiple of the haploid number.
- one or more chromosome may be added or missed.
- the case in which there is extra one chromosome
is called Trisomy ( 2 n + 1 = 47 ).
examples of Trisomy:
1- Down syndrome (= monglism= Trisomy 21 ).
2- Edward syndrome (= Trisomy 18 ).
3- Patau syndrome (= trisomy 13 ).
4- Trisomy of the X chromosome ( XXX )
- if only one chromosome is missing, the condition is
called Monosomy ( 2 n - 1 = 45 ).
examples of Monosomy:
Turner syndrome ( monosomy X chromosome ).
- mechanism of Aneuploidy:
is non disjunction i.e. failure of the homologus pair of
chromosomes to separate ( disjoin ) in Anaphase
stage of either the first or second meiotic
division.
II- Abnormal chromosomal structure
1- Translocation.
2- Deletion.
3- Duplication.
4-Inversion.
5- Ring chromosome.
6- Isochromosome.
7- Chromosomal Fragile sites.
1- Translocation
mechanism:
breaks occurs in both non- Homologus chromosomes
& then exchange of chromosomal material occurs.
( if this occurs between Homologous chromosome
chromosome it is called Crossing Over which occurs
normally during Meiosis ).
2- Deletion:
mechanism:
loss of aportion of achromosome mostly due to
breakage of the chromosome by viruses, irradiation
or any chemical agent.
e.g. - Cri Du Chat syndrome.
- Wolf syndrome.
3-Duplication
mechanism :
the presence of an extra piece of chromosome due to
unequal cross over.
4- Inversion
breaks of chromosome followed by reconstriction but
with the middle part ( the part of the chromosome
between the 2 breaks ) inverted.
5- Ring chromosome
breaks of both ends of the chromosome occur, with loss
( deletion ) of the 2 terminal ends & union of the 2
ends to form aring.
6- Isochromosome
- the chromosome divides transverselly at the
centromere instead of the normal longitudinal
separation.
- so, the chromosome divides into 2 arms ( each goes
to one cell ) instead of the division into 2 chromatids
of both arms.
e.g. Isochromosome X.
7- Chromosomal Fragile sites
- afragile site is aparticular site of chromosome showing
atendancy towards breakage, so producing
structural chromosomal abnormalities.
- Fragile X-syndrome:
manifests itself in males having abreak or agap at
the distal segment of the long arm of the
X-chromosome.
