Njilla steeve . aneuploidy presentation

ANEUPLOIDY
MDE 605
PRESENTED BY
NJILLA STEEV-BECCA NJISSIC
(SC17P276)
MDE 605. Njilla steeve 15/5/2019

OUTLINE
 Introduction
 Causes of aneuploidy
 Disease conditions
 Conclusion.

INTRODUCTION
 It is a category of chromosomal aberration in which the chromosome number
is abnormal.
 It involves changes in chromosome number by additions or deletions of less
than a whole chromosome set .
 Organism with this conditions are called aneuploids.

 Aneuploid nomenclature (see table 1) is based on the number of copies of
the specific chromosome in the aneuploid state. For autosomes in diploid
organisms, the aneuploid (2n +1) is trisomic, (2n -1) is monosomic, and
(2n- 2) is nullisomic. In haploids, ( n+1) is disomic.
 Special notation is used to describe sex chromosome aneuploids because it
must deal with the two different chromosomes. The notation merely lists
the copies of each sex chromosome, such as XXY, XXY, XXX or XO (O
stands fors absence of a chromosome).

Table 1: Partial List of Terms to Describe Aneuploidy, Using Drosophila as
an Example (Eight Chromosomes: X, X, 2, 2, 3, 3, 4, 4)

Causes of Aneuploidy
 The primary cause of aneuploidy is nondisjunction during mitosis or
meiosis.
 Nondisjunction is a process by which homologous chromosomes or
chromatids fail to segregate normaly, that is all the chromosomes move to
one end of the pole and none to the other.

Figure 1:The origin of aneuploid
gametes by nondisjunction at the first
or second meiotic division
MESSAGE: Crossovers are
needed to maintain the intact
tetrad until anaphase I. If
crossing-over fails for some
reason, first-division
nondisjunction occurs.

Diseases conditions resulting from aneuploidy.
Trisomics (2n+1)
Down syndrome; (47,XX, XY+21)
 Down syndrome was the first chromosomal aberration to be discovered in
humans by the physician John Langdon Down in 1886.
 It affects about 1 in 800 live births. Approximately 95% of all children
with Down syndrome have three individual copies of chromosome 21

 Most affected individuals are mildly to moderately mentally retarded,
have congenital heart defects, and a high (1/100) risk of acute
leukemia. They are usually short, have a broad, short skull, and
possess hyper flexible joints, mental retardation, females may
reproduce but not males. Life expectancy is 17 years, however, 8%
make it past 4o years.

 About 95% of infected individuals have an extra copy of chromosome 21
caused by nondisjunction of chromosome 21 in a parent who is
chromosomally normal. In this sporadic type of Down syndrome, there is no
family history of aneuploidy.
 An interesting aspect of this syndrome is its increased incidence among
children born to older mothers, a fact known more than 25 years before the
discovery of its genetic cause

 Recently, molecular genetic techniques identified the origins of the
three chromosome 21 copies in a large sample of individuals with
Down syndrome. As expected, 95% of the extra copies of
chromosome 21 were of maternal origin. Of these cases,
approximately 70% resulted from errors in meiosis I and 25% were
errors in meiosis II. Less than 1% of the Down syndrome children
resulted from nondisjuntion in meiosis II in the father.

 The remaining 5% of Down syndrome individuals that contain three
free copies of chromosome 21 were produced by mitotic nondisjunction
in the zygote or embryo. In these cases, the extra copy of chromosome
21 originated equally from either parent and there was no increased
incidence with older parents.
 About 5% of individuals with Down syndrome do not have trisomy 21,
but rather have a translocation of the long arm of chromosome 21 to
usually chromosome 14 or 15. This type of Down syndrome is termed
familial down syndrom), based on the pattern of inheritance from
generation to generationMDE 605. Njilla steeve 125/5/2019

Figurer 2: characteristic of down syndrome Figure 3: Karyotype of a male with down syndromeMDE 605. Njilla steeve 135/5/2019

Trisomy 18 ( Edwards syndrome);
 Edwards syndrome affects 1 in 10 thousand live births. The karyotype for this
syndrome is 47, XX or XY,+18. Most affected fetuses fail to survive to birth.
Those that do survive are usually female, with 80–90% mortality by 2 years
of age.

 An affected infant is usually born with major heart defects and a displaced
liver. In addition, the infant usually has a small nose and mouth, a
receding lower jaw, abnormal ears, and a lack of distal flexion creases on
the fingers. The distal joints have limited motion, and the fingers display a
characteristic posturing in which the little and index fingers overlap the
middle two. The syndrome is usually accompanied by severe mental
retardation.

Trisomy 13 (Patau syndrome);
 Patau syndrome, karyotype 47,XX or XY,+13, affects 1 in 20 thousand
live births. A very high percentage of fetuses with trisomy 13 fail to
survive until birth.
 Individuals who are born with trisomy 13 often have small or missing
eyes, although in rare cases they exhibit a single fused eye structure in
the center of the face. Additional features include cleft palate, cleft lip,
congenital heart defects, polydactyly, and severe mental retardation.
Mortality is very high in the first year of life.

XXY (Klinefelter syndrome);
 The incidence of Klinefelter syndrome (47,XXY) is about 1 in 1000 live births.
 The phenotypic expressions include midly impaired IQ, tall stature,
underdeveloped testes, and infertility. Some problems with behavior and
speech development are associated with this syndrome.
 If discovered early, testosterone injections during adolescence can help to
induce the secondary sexual characteristics (pubic and facial hair) that are often
reduced in Klinefelter individuals.

Figure 4: Characteristic of Klinefelter syndromeMDE 605. Njilla steeve 185/5/2019

XYY Karyotype;
 Approximately 1 in 1000 live male births is an individual with a 47,XYY
karyotype.
 There is some indication that XYY men may have some speech and reading
problems, which could lead to lower test scores and lowered intelligence test
results. Criminal tendencies could therefore be attributed to lower intelligence,
rather than to the extra Y chromosome. But this idea is highly speculative. Males
with XYY are usually fertile.

Tripple X chromosomes;
 A triple-X female (47,XXX) appears in about 1 in 1000 female live births.
Fertility can be normal, but these individuals are usually mildly mentally
retarded. Delayed growth, as well as congenital malformations, are also
sometimes present.

Figure 5: XYY karyotype Figure 6: Karyotype of a tripple-X female.

MONOSOMICS (2n -1)
X0 (Turner Syndrome);
 Turner syndrome occurs in about 1 in 2000 live female births, with
approximately 99% of Turner syndrome fetuses failing to survive until
birth. The karyotype designation is 45,X .
 Individuals with Turner syndrome usually exhibit normal intelligence
but underdeveloped ovaries, infertility, abnormal jaws, webbed necks,
and shieldlike chests. They are often short in stature.

Figure 7: Characteristic of turner syndrome

Conclusion
Aneuploidy refers to abnormal number of chromosomes. It is
primarily caused by nondisjunction in mitosis or meiosis but more
predorminant in meiosis. Aneuploidogens like colchicine, tobacco
smoke, benzene can also cause aneuploidy. The resulting disease
conditions are dentrimental to affected individuals and most aneuploid
states in human fetus lead to dead. Aneuploidy is important in the
determination of tumorigenic cells like colorectal cancer

References
 Griffiths AJ, Miller JH, Suzuki DT. 2000. An introduction to genetic analysis.
8th edition. pp 490- 495, chapter 15.
 Sens S. 2000. “ Aneuploidy and cancer”. Current opinion in oncology. 12(1):
82- 88.
 Templado C, Uroz L, Estop A . 2013. “New insights on the origin and
relevance of aneuploidy in human spermatozoa”. Molecular human
reproduction. 19 (10):634-643.
 Yang AH, Kaushual D, Rehen SK. 2003. Chromosome segregation defects
contribute to aneuploidy in normal progenitor cells. The journal of
neuroscience. 23(32). MDE 605. Njilla steeve 255/5/2019

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Njilla steeve . aneuploidy presentation

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