1. Meiosis involves two divisions that reduce the number of chromosomes by half, leading to genetic variability. The key events are crossing over, separation of homologous chromosomes, and separation of sister chromatids. 2. Chromosomes are classified based on centromere location as metacentric, submetacentric, or telocentric. Aneuploidy occurs when the number of a particular chromosome is altered. 3. Nondisjunction of homologous chromosomes during meiosis can result in trisomy disorders like Down syndrome, where a third chromosome 21 is present.

4. Variability causes in Meiosis
1. Crossing over
3.Separation of sister
Chromatides during ANAPHASE II
2.Homologous Chromosomes
Separation during ANAPHASE I

7. **

8. p
q
Arm
Arm
The Centromere divides
each Chromatid in two
arms than can be equal
or different.
METAPHASIC CHROMOSOME STRUCTURE

9. Metacentric
p
q
Near the middle off Centre near one end at the end
Because the Centromere is not exactly in the centre of a Chromosome, each
chromosome has a sort arm and a long arm.
The short arm is designated with the letter p (for the French petite)
While the long arm is designated with q.

10. Metacentric
p
q
Near the middle off Centre near one end at the end
In Human Beings do not exist Telocentric Chromosomes

11. Metacentric
To classify the Human Chromosomes following the Centromeric location
we have to use The Chromosomic Indexes:

12. They are used to classify human chromosomes.

13. They are used to classify human chromosomes.
p
q

15. p
q

16. A photography of the
Metaphasic Chromosomes.
Without order.
Ordered Chromosomes
following Cytogenetic
Criteria.

20. G
BANDS

21. Types of banding
G-Banding is obtained with
Giemsa stain following
digestion of chromosomes
with trypsin. It yields a
series of light and dark
bands - the dark regions
tend to be
heterochromatic,
(DNA more condensed) and
AT rich. The light regions
tend to be euchromatic,
(Less condensed)and GC
rich. This method will
normally produce 300-400
bands in a normal, human
genome

25. PAINTING CHROMOSOMES USING SPECIFIC SOFTWARE

33. Phytohemagglutinin
In medicine this protein is useful and is used as a
mitogen to trigger lymphocytes cell division

34. BLOOD OR
BONE NARROW

36. Colchicine to stop Mitosis

37. Phytohemagglutinin
In medicine this protein is useful and is used as
a mitogen to trigger lymphocytes cell division

38. Colchicine is got from Colchicum autumnale.
Colchicine stops Mitosis in Metaphase.

39. To classify Chromosomes we use
three main criteria:
**
**
**
. For example,
.

40. Phytohemagglutinin
In medicine this protein is useful and is used as a
mitogen to trigger lymphocytes cell division

41. Amniocentesis
Chorionic villi Sampling

42. Amniocentesis
Chorionic villi Sampling
From fifteenth to seventeenth
week of pregnancy
From eighth to twelfth week of pregnancy
Risk of
Spontaneous
abortion 0.3%
Risk of
Spontaneous
abortion 0.8 %

43. Risk of Spontaneous
abortion 0.8 %

44. Risk of Spontaneous
abortion 0.8 %

45. From fifteenth to
seventeenth week of
pregnancy
Risk of
Spontaneous
abortion 0.3%

46. In human Beings , the Euploid
condition is two sets of 23
Chromosomes each; 46
Chromosomes.
The somatic cells are Diploid

47. Organisms with three or more sets of
Chromosomes are called POLYPLOID.
A Diploid organism is referred to as 2n, a Triploid
organism as 3n, a Tetraploid organism as 4n, and
so forth. Haploid individuals or cells are referred
as n.(only a set of chromosomes).
In Human beings Poliploids individuals are
extremely rare and only these individuals have
been got in abortions.
Observe the following figures because they are
cases of Triploid Human beings.

48. At A the Strawberry plant is exposed to a chemical called
Colchicine. At B due tho the effect of the drug, total no disjunction
produced Diploid gametes(2n). As a result, at C two Diploid gametes
fuse at fertilization . Finally at D a new polyploid plant is formed

49. Observe.........................
Julia Sáenz Hurtado
Servicio de Inmunología. Sección Genética
Hospital Infanta Cristina

50. Julia Sáenz Hurtado
Servicio de Inmunología. Sección Genética
Hospital Infanta Cristina
Observe again

51. The second way that chromosomes number varies
is a phenomenon called .
This refers to an alteration in the number of
particular Chromosomes, so that the total number
of chromosomes is not an exact multiple of a set.
In human beings Aneuploid individuals have a
chromosome number that is not an exact multiple
of the haploid number of 23.
The normal condition is the DISOMY (Two
chromosomes in a pair) but the abnormal condition
may be:

52. Normal Condition
Embryos missing a chromosome usually die; hence,monosomy of
an autosome is extremely rare in living persons. About 97 % of
embryos lacking a sex chromosome also die, but some survive
and develop characteristics of TURNER SYNDROME or
XO.

53. 45,X

54. TURNER SYNDROME

55. TURNER SYNDROME
LINDA HUNT

56. Meiosis Review
1. REDUCTION OF NUMBER
OF CHROMOSOMES BY
HALF
2.Meiosis only occurs
in germ cells.

57. Variability causes in Meiosis
1. Crossing over
3.Separation of sister
Chromatides during ANAPHASE II
2.Homologous Chromosomes
Separation during ANAPHASE I

58. 1. Crossing over
Exchange of Genetic Material between
Homologous Chromosomes

59. 2.Homologous
Chromosomes
Separation during
ANAPHASE I
Either
or
A B
A B

60. A1
A2
A1 A2
A3 A4 A3
A4
ANAPHASE II
3.Separation of sister Chromatides during
ANAPHASE II

61. p
q
Sister Chromatids
Each one is a
Molecule of DNA
Arm
Arm
The Centromere divides
each Chromatid in two
arms than can be equal
or different.
METAPHASIC CHROMOSOME STRUCTURE

62. Based on Centromere location, each chromosome
is classified as:
Metacentric
p
q
Comparison of centromeric locations
Near the middle off Centre near one end at the end

63. To classify Chromosomes we use
three main criteria:
**
**
**
. For example,
.

64. If three chromosomes are present instead of the usual pair.
The common cause of Trisomy is nondisjunction (no Homologous
separation) during the Meiosis(special division that suffer the
gametes).The result is a gamete (Ovum or Sperm) with 24
Chromosomes instead of 23 Chromosomes and, after the
fertilization the result is a Zygote with 47 chromosomes.
IMPORTANT: Autosomal Trisomies occur with
increasing frequency as maternal age
increases.

65. Nondisjunction DURING MEIOSIS

67. Translocation Down syndrome occurs when two
chromosomes, one of which must be a 21, join together
at their ends. Instead of having three independent,
separate number 21 chromosomes, a person with
translocation Down syndrome has two independent 21
chromosomes and a 21 chromosome that is attached to
another chromosome.
Observe as examples the Idiograms with a Tranlocation
21:21 and a Translocation 21:14.

75. Simian Crease

77. Simian Crease