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genetic disease
a genetic disorder is a disease that is caused by a change or mutation in an individuals DNA sequence. A condition where there is an observable / detectable genetic component
diseases caused by chromosomal abnormalities
Down syndrome (+21)
Patua syndrome (+13)
Edwards syndrome (+18)
turners syndrome (x chromosome is missing/partially missing affects females only)
Burkitts lymphoma (body makes abnormal B lymphocytes)
karyotype analysis
involves blocking cells in mitosis with colchine, digesting with trypsin, and staining the condensed chromosomes with dye such as giemsa. these stain regions of chromosome that a re heterochromatic, gene poor, AT rich producing a dark band. thinnest bands contains over a million base pairs and potentially hundreds of genes. also stains for specific bands or structures
chromosomes nomenclature
chromosomes are arranged chart arm on top labelled p. long arm on bottom labelled q
normal chromosome nomenclature males: 46XY females: 46XX
numerical chromosome abnormalities
change in number with no breakage
there are three types
1. polyploidy
2. aneuploidy
3. mixoploidy
abnormality of an entire chromosome set
some cells are naturally polyploidy
e.g., regenerating liver
aneuploidy
abnormal number of specific chromosomes
monosomy - loss of one copy of chromosome pair this is Lethal
triploidy - additional copy of a chromosome
nullisomy - loss of both homologues this is preimplantation lethal
aneuploidy causes
cancer cells often show extreme aneuploidy. aneuploid cells arise from
non-disjunction: failure of paired chromosomes to separate at meiosis I. one daughter cells inherits both conjoined chromosomes and the other none
anaphase lag: failure to incorporate a chromosome into a daughter cell at cell division this chromosome is lost
mixoploidy
an individual has cells with different chromosomal composition occurs due to
mosaicism: individual has two or more genetically different cell lines derived from a single zygote
chimera: as above but cell lines derived from different zygotes. this Is very rare
aneuploidy mosaics are common e.g., normal/trisomic mixture due to non-disjunction in cell of early embryo
caused by trisomy 21 which is caused by the abnormal cell division during the development of the sperm cell or the egg cell. greatly increased incidence with increasing maternal age. 95% due to trisomy 21 5% due to unbalanced translocation between long arm of 21 and usually but not exclusively long arm of 14. chromosome recombination is different in Down syndrome more recombination occurs towards the tops of the chromosome rather than the centre suggesting abnormal pairing
Down syndrome clinical features
growth retardation, intellectual disability, abnormal facial features, small ears, heart defects, increased incidence leukaemia, premature ageing, short necks
karyotype
the number, shape, and size of the chromosome of a somatic cell arranged in a standard manner. human karyotype has 46 chromosomes. 23 form each parent. males are XY and females are XX. the sex of offspring determined by the sex chromosome carried in the sperm
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