Cytogenetics - human chromosomes and disorders Flashcards
what are microsatellites?
repeated units of DNA
what is an aneuploidy?
one or more individual chromosomes in extra copy or missing
how many chromosomes do humans have?
46 chromosomes
(23 from mom and 23 from dad)
in a normal karyotype picture, what do the dark bands represent? What do the light bands represent?
dark bands= heterochromatin (AT rich and gene poor region)
light band = euchromatin (GC rich and gene rich)
describe the banding nomenclature of chromosomes
ex) 1p13.3
1p13.3 refers to the
1st chromosome
p= short arm of chromosome
13= region of the chromosome
3= sub-band of region
what are the two basic types of chromosome abnormalities?
- numberical aberrations
- like trisomy (47 chromosomes) or monsomy (45 chromosomes)
- structural aberrations
- involving 1 chromosome- deletion , duplications, inversion, ring chromosome
- involving 2 chromosomes - translocations
how does aneuploidy arise?
from chromosomal non-disjunction
- failure of homologous chromosomes to separate and segregate normally in either mitosis or meiosis
- when it occurs in meiosis, non-disjunction results in imbalances gametes - should one of those gametes fertilize, all cells derived from the resulting embryo will have a chromosome imbalance
- when it occurs in mitosis - nondisjunction produces two cell-lineages derived from a single zygote (known as mosaicism)= less severe syndrome
when does non-disjunction occur in meiosis?
non-disjunction can occur in either the first or second meiotic division and consequences are different
first = gametes contain either both parental chromosomes which failed to separate or neither
second = gametes contian two identical copies of the same chromosome or neither
zygotes will either bet monosomic or trisomic
what are some clinical features of down syndrome?
- developmental delay
- variable intellectual disability
- characteristic facial features
- congenital heart defects
- premature ageing
- risk of leukaemia
what is the incidence of trisomy 21?
1/650 live-births (about 50% spontaneous abortions)
risk increases with maternal age
30 yr= 1/900
36 year= 1/300
40 yr= 1/100
44 yr = 1/40
What is trisomy 18?
Ewards Syndrome= rocker-bottom feet, usually die within 1 year
What is Trisomy 13?
Patau syndrome = polydactyly, death usually within 1 year
what is monosomy X?
Turner syndrome
what is kleinefelter syndrome?
extra x chromosome male
what is a reciprocal translocation of chromosome?
where two chromosomes swap genetic material