MEIOSIS
Institute of Biology and Medical Genetics of the First
Faculty of Medicine and General Teaching Hospital
REDUCTION OF THE
CHROMOSOME NUMBER AND
SEPARATION BY MEIOSIS
paternal
maternal
A
a
ANAPHASE I
a
A
ANAPHASE II
A
A
A
aa
a
COMPARISON MITOSIS - MEIOSIS
DRAW THE EXPECTED DISTRIBUTION OF
2 PAIRS OF CHROMOSOMES (2n = 4)
DURING MEIOSIS (NO-CROSSING
OVER IS SUPPOSED)
HOW MANY DIFFERENT TYPES OF
GAMETES CAN ORIGINATE WHEN:
2n = a) 4, b) 6, c) 46?
2n = 4
ANAPHASE I
ANAPHASE II
M
P
HOW MANY DIFFERENT TYPES OF
GAMETES CAN ORIGINATE WHEN:
2n = a) 4, b) 6, c) 46?
a) 4
b) 8
c) 223
In general 2n
MEIOSIS – PROPHASE I
TETRAD
SYNAPSE
BIVALENT
LEPTOTENE
DIAKINESIS
DESYNAPS
E
DIPLOTENE
ZYGOTENE
2n = 4
CROSSING-OVER
PACHYTENE
MEIOSIS I – CONT.
MEIOSIS II
Task 7, p. 29
WHICH GENETIC MECHANISMS COULD
CAUSE THE GENETIC VARIABILITY OF
NEXT GENERATIONS?
RECOMBINATIONS
• INTRACHROMOSOMAL
• OF MATERNAL AND PATERNAL
CHROMOSOMES DURING
GAMETOGENESIS
• CHROMOSOME SETS DURING
FERTILIZATION
MUTATIONS
1. Nondisjunction in Down, Turner,
and Klinefelter syndrome
p. 80/Task 6e, a, b (preserve this order)
Remaining c) and d) - syndromes XXX
and supermale – home work,
as selfstudy!
N.B.: Fill in all possibilities of the
nondisjunction events for a particular
syndrome could originate. This is asked in
the text, exactly as in final exam test. 11
General schedule of disjunction
of chromosomes in the meiosis
normal
and fertilisation
gamete
normal gamete of
the second parent
Germinal cell
(pair of
chromosomes)
Chromosome
disjunction
in meiotic division I.
No.No.
or XX
or XY
No.
No.
After the meiosis I
(single chromosome)
Chromatid disjunction
No.
No.
No. No.
zygote
in meiotic division II.
After the meiosis II
(monochromatidal No.
chromosome)
N.
o
N.
o
N.
o
Normal
karyotype
General schedule of nondisjunction
normal gamete of
(principle, example)
aberrant
the second parent
gamete
(monosomic)
Germ cell
(pair of
chromosomes)
No.No.
or XX
or XY
No.
No. No.
After the meiosis I
(single chromosome)
Chromatid
nondisjunction
in meiotic division II.
No.
After the meiosis II o o
(monochromatidal N . N .
chromosome)
No.
No. No. No.
zygote
No.
No.
cytogenetic
diagnosis
Down syndrome (simple trisomy)
Nondisjunction in the meiosis I
aberrant
in the father or in the mother
normal gamete of
the second parent
gamete
(monosomic)
21 21
21 21
21 21
21
21 21 21
zygote
21 21 21 21
47,XX,
+2147,XY,
Down syndrome (simple trisomy)
Nondisjunction in the meiosis II
normal gamete of
aberrant the second parent
in the father or in the mother
gamete
(monosomic)
21 21
21
21 21
21
21
21 21 21
zygote
21 21
21
21
47,XX,+21
47,XY,+21
Turner syndrome
Nondisjunction in the
paternal meiosis I
mother’s normal
aberrant
gamete
gamete
(monosomic)
XY
X
X
XY
zygote
XY
XY
45,X
Turner syndrome
Nondisjunction in the maternal
meiosis I
aberrant
gamete
father’s normal
gamete
(monosomic)
XX
XX
X
X
zygote
XX
XX
45,X
Turner syndrome
Nondisjunction in the
paternal meiosis II
mother’s normal
aberrant
gamete
gamete
(monosomic)
XY
X
X
Y
X
zygote
X
X
or:
XX
45,X
Y
Y
Y
Y
Turner syndrome
Nondisjunction in the maternal
meiosis II
aberrant
father’s normal
gamete
gamete (monosomic)
XX
X
X
X
X
zygota
X
X
XX
45,X
Klinefelter syndrome
Nondisjunction in the paternal
meiosis I
aberrant
mother’s normal
gamete
gamete
(monosomic)
XY
XY
XY
X
XXY
zygote
XY
XY
47,XXY
Klinefelter syndrome
Nondisjunction in the maternal
meiosis I
father’s normal
aberrant
gamete
gamete (monosomic)
XX
XX
Y
XXY
XX
zygote
XX
XX
47,XXY
Klinefelter syndrome
Nondisjunction in the maternal
meiosis II
aberrant
father’s normal
gamete
gamete (monosomic)
XX
XX
X
X
Y
XXY
zygote
XX
X
X
47,XXY