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A General Look Into Genetic Disorsers
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Humans have 46 chromosomes, 22 pairs of which are autosomal chromosomes and 2 sex chromosomes. 23 of these 46 chomosomes come from the mother and the other 23 from the father. Gebetic material formed of DNS chains is present in the nuclei of the cells in the form of chromosomes. While human cells have 46 chromosomes, only germ cells (oocyte and sperm) have 23 chromosomes. The oocyte and sperm cells decrease from 46 chromosomes to 23 during their development and division processes. This type of division is called meiosis. Therefore, the chromosome number in the embryo, established with the fertilization of the oocyte with the sperm is again 46.
 Male (46, XY): 22 pairs of autosomal chromosome (1-22) = 44
X, Y sex chromosomes: 2
Female (46, XX): 22 pairs of autosomal chromosome (1-22) = 44
X, sex chromosomes: 2
The development and division stages in the oocyte and egg can be seen.
Chromosome number decreases to 23 from 46.
The sex of the baby is determined by the Y or X chromosome found in thesperm cell that fertilizes the oocyte. If the oocyte (23, X) is fertilized with a sperm carrying Y chromosome (23, Y), then the baby is male (46, XY). If the oocyte (23, X) is fertilized with a sperm carrying X chromosome (23, X), then the baby is female (46, XX). Therefore, the sex of the baby is not determined by the oocyte of the mother but the sperm of the father.
Genetic disorders are mainly divided into these groups:
Numerical abnormalities: Having less or more chromosomes than the normal 46. Numerical abnormalities are the most frequent genetic disorders detected in miscarriages and stillbirths. The most frequent numerical abnormality is aneuploidy. Aneuploidy is having less or more chromosomes than 2.
For example: If there are 3 of the 21st chromosome instead of 2:
47, XX +21 or 47, XY +21 (Down Sendromu)
If the sex chromosome is missing:
45, X0 (Monozomy X = Turner Syndrome)
Polyploidy is having 23 folds of the chromosome number that is normally 2 pairs (23x2) (Triploidy= 69, tetraploidy= 92).
Chromosomal disorders effecting pregnancy the most are:
• Turner Syndrome (45, X0)
• Trizomy 16
• Trizomy 21
• Trizomy 22
• Trizomy 18Trizomi 18
• Triploidy
• Tetraploidy
Chromosomal disorders affect the course of pregnancy negatively. Chromosomal disorders are especially responsible for an important part of the miscarriages occurring during the first half of the pregnancy.
50-60% of the miscarriages occurring during the first 3 months
30-40% of the miscarriages occurring during the second 3 months
5% of stillbirths during the third 3 months
0.6% of the live babies
Baby deaths under the age 1: 5-7%
Aneuploidy incidence in pregnancy increases with the increasing age of the woman. In women over 35, the chance to conceive decreases inversely with age and the risk of miscarriage and abnormal baby births increase significantly. Therefore, for pregnant women over 35, a process called amniosynthesis is applied at the 4th month and the genetic structure of the baby is examined. In man, the risk of aneuploidy does not increase with age, however, aneuploidy incidence increases in men having extreme sperm problems.
 Structural abnormalities: Has types as translocation (Resiprocal and robertsonian), deletion, inversion, microdeletion. The most important ones among these are translocations. Translocations can lead to recurrent miscarriages.
 Single gene diseases These are divided into 3.
Showing autosomal recessive transition Cystic fibrosis, ß-thalassemia
Rh ineptitude
Spinal muscular atrophy (type 1)
Tay –Sachs disease etc.
Being autosomal dominant Myotonic dystrophy
Huntington corea
Marfan syndrome
Charcot-Marie-Tooth disease etc.
The ones showing transition related to X (related to sex) Duchenne/Becker muscular dystrophy
Fragile X syndrome
Hemophilia
Lesch-Nyhan syndrome etc.
Diseases transmitted related to sex
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