Embryo

Mary Ann Dinglasan HUB 42

1. What is the most common cause of abnormal chromosome number? Give an example of a clinical syndrome involving abnormal numbers of chromosomes. Chromosome abnormalities usually happen as a result of an error in cell division. Meiosis is the name used to describe the cell division that the egg and sperm undergo when they are developing. When fertilization occurs, the normal 46 total number of chromosomes results in the fetus. If meiosis does not occur properly, an egg or sperm could end up with too many chromosomes, or not enough chromosomes. In addition to the cell division, mitosis begins in the fetus after fertilization.This process repeats itself, until the entire baby is formed. Error in mitosis can occur, just like the error previously described in meiosis. If the chromosomes do not split into equal halves, the new cells can have an extra chromosome (47 total) or have a missing chromosome (45 total). This is another way why chromosome abnormality can occur. As for the maternal age, the risk for a baby to be born with a chromosome abnormality increases. This may be attributed to the aging of the egg cells that causes the incorrect number of chromosomes at fertilization. Examples of clinical syndrome involving abnormal numbers of chromosomes are: • Down's syndrome or trisomy 21

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Edward's syndrome or trisomy 18 Patau syndrome or trisomy 13 Cri du chat syndrome or 5p minus syndrome (partial deletion of short arm of chromosome 5) Wolf-Hirschhorn syndrome or deletion 4p syndrome Jacobsen syndrome or 11q deletion disorder Klinefelter's syndrome or presence of additional X chromosome in males Turner syndrome or presence of only a single X chromosome in females XYY syndrome and XXX syndrome

2. In addition to numerical abnormalities, what types of chromosomal alterations occur? Aside from the abnormalities in numbers of chromosomes, aberrations can also happen in the structures. Here are the following examples: •

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Deletions: A portion of the chromosome is missing or deleted. Known disorders in humans include WolfHirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q deletion disorder. Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Known human disorders include Charcot-Marie-Tooth disease type 1A, which may be caused by duplication of the

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gene encoding peripheral myelin protein 22 (PMP22) on chromosome 17. Translocations: A portion of one chromosome is transferred to another chromosome. There are two main types of translocations: Reciprocal translocation: Segments from two different chromosomes have been exchanged. Robertsonian translocation: An entire chromosome has attached to another at the centromere - in humans these only occur with chromosomes 13, 14, 15, 21, and 22. Inversions: A portion of the chromosome has broken off, turned upside down, and reattached, therefore the genetic material is inverted. Insertions: A portion of one chromosome has been deleted from its normal place and inserted into another chromosome. Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material. Isochromosome: Formed by the mirror image copy of a chromosome segment including the centromere.

Chromosome instability syndromes are a group of disorders characterized by chromosomal instability and breakage. They often lead to an increased tendency to develop certain types of malignancies.

3. What is mosaicism, and how does it occur?

It is the property or state of being composed of cells of two genetically different types. Mosaicism happens because a mutation occurs at some point after the zygote is created. In general, the later in embryonic development that the mutation occurs, the more restricted in distribution the resulting phenotype is, because the cells derived from a mutated founding cell all carry its mutation. Many of the mutations may be neutral; some may not. They may thus confer some selective advantage (or disadvantage) to the cell and may therefore contribute to disease. If the mutation affects cell growth mechanisms, it may be “self-promoting” in the affected tissues,favoring (or hindering) expansion of the mutant population.