Chromosomal Abnormalities Flashcards
What is cytogenetics?
Study of the genetic constitution of cells through the visualisation and analysis of chromosomes
Why would someone be referred to cytogenetics?
- Prenatal diagnosis
- Birth defects
- Abnormal sexual development
- Infertility
- Recurrent fetal loss (3 or more)
What is involved in chromosome analysis?
- Count the number of chromosoms
- Identify each chromosome pair
- Assess if there is any missing/extra material
What is aneuploidy?
Loss/gain of whole chromosomes - abnormal number of chromosomes in a cell
How does aneuploidy arise?
Errors in meiosis
What is Down syndrome?
Trisomy 21
What is Patau syndrome?
Trisomy 13
What is Edwards syndrome?
Trisomy 18
What is Turner syndrome?
45, X
Only one normal X chromosome, rather than two
What is polyploidy?
Gain of a whole haploid set of chromosomes
Triploidy 3n
Usually spontaneously aborts
What is the main cause of polyploidy?
Fertilisation of an egg by more than one sperm
How is aneuploidy caused?
Originates from non-disjunction at one of the meiotic cell divisions, forming gametes with a missing chromsome and an extra chromosome
What happens when aneuploidy occurs during mitotic cell division?
Causes mosaicism (two cell populations in an individual)
What is anaphase lag?
Chromosomes can be left behind at cell division because of defects in spindle function or attachment of chromosomes leading to the loss of a chromosome
What are some characteristics of individuals with Down syndrome?
Characteristic facial features, intellectual disability, heart defects, early alzheimers
What are some characteristics of individuals with Edward syndrome?
Small lower jaw, rocker bottom feet, overlapping fingers
What are some characteristics of individuals with Patau syndrome?
Multiple congential abnormalities, polydactyly, holoprosencephaly
Why is a single X chromosome a problem?
Turner syndrome patients will be monosomic for genes in the PARs. SHOX gene (within PAR) associated with short stature.
What are some characteristics of individuals with Turner syndrome?
Short sstature, heart defects, mild learning disabilities, neck webbing, infertility
What is mosaicism?
Presence of 2 or more cell lines in an individual, usually caused by mitotic non-disjunction
What does the degree of mosaicism depend on?
When mitotic error occured: First post zygotic division -no mosaicism looks like a meiotic event Subsequent divisions -3 cell lines, monosomy cell line usually lost
What are reciprocal translocations?
Reciprocal translocations are usually an exchange of material between nonhomologous chromosomes. No loss of genetic material.
What chromosome mutations are there within one chromosome?
Deletion, duplication, inversion, ring chromosome, isochromosome
What is an isochromosome?
Creation of two non identical chromosomes, one is a combination of the two short arms and the other is a combination of the two long arms
What is a ring chromosome?
Loss of telomeres or ends of both arms and formation of a ring
What chromosome mutations are there within two chromosomes?
Inversion, reciprocal translocation and robertsonian translocation
What are robertsonian translocations?
Rearrangement of genetic material between two chromosomes; the q arm of two acrocentric chromosomes combine to form one ‘super chromosome’ with the loss of both p arms
What is the largest and smallest chromosome?
Largest = 1 Smallest = 22
On which chromosomes can robertsonian translocations occur?
13, 14, 15, 21, 22
What is microarray comparative genomic hybridisation?
Examine the whole genome at high resolution
