Karyotype analysis Flashcards
1
Q
Outline karyotype analysis
A
- Karyotyping is a test to** examine chromosomes** in a sample of cells
- This test can help identify genetic problems as the cause of a disorder or disease
2
Q
What is a karyotype?
A
- The general appearance of the complete set of chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes
3
Q
How are chromosomes prepared for karyotype determination?
A
- Metaphase spread
- The cell membrane is ruptured to help visualize the chromosomes
4
Q
Outline the process of metaphase spread
A
- An appropriate cell sample is taken (usually blood or amniocentesis)
- Culture and arrest cells at metaphase (using colchicine to inhibit mitosis)
- “Spread” chromosomes
- Fix cells and stain chromosomes (C and G bands; fluorescent ‘paints’)
- Count and classify
- Assemble an ordered karyotype
- Chromosomes are arranged in homologous pairs for analysis
5
Q
Describe the typical human karyotype
A
- 46 chromosomes in total
- 2 sex chromosomes, X and Y
- XX : female
- XY: male
- 44 autosomes
- 23 homologous pairs
- (X is much longer than Y; hemizygous pairing)
6
Q
How are chromosomes typically classified?
A
- Initially based on gross morphology
- Autosomal chromosomes are placed into** seven groups (A-G)**
- Placed in order of decreasing length, A = longest and G = shortest
- X = Indistinguishable from group C
- Y = similar to group G
7
Q
Describe chromosome banding
A
- In any given group, A-G, individual chromosomes are difficult to distinguish as they are all approximately the same length
- Identification of these within any given group is therefore achieved with band staining techniques
- C bands (Centromere): selectively stain centromeres (C banding uses dye which just picks up just the centromeres to show different types)
- G (uses Giemsa) bands: heterochromatin tends to stain more than euchromatin (specific binding patterns)
- R bands (Reverse): approximately the reverse of G bands
8
Q
How are chromosomes classified based on centromere position?
A
- p arms (top part of chromosome) = short
- q arms (bottom part, below centromere) = long
- q/p = ratio characteristic (arm ratio)
Thus,
- Metacentric = (ratio) 1
- Submetacentric = (ratio) 1.5-2.5
- Acrocentric = (ratio) 2.5-10
- Telocentric = Typically absent in humans
9
Q
What does FISH stand for?
A
- Fluorescent
- in
- Situ
- Hybridisation
10
Q
What is FISH used for?
A
- Uses fluorescently labelled probes (short lengths of synthetic DNA complimentary to chromosome specific sequences)
- Probes are labelled with differently coloured fluorescent dyes
- It provides researchers with a way to visualize and map the genetic material in an individual’s cells, including specific genes or portions of genes
- This may be used for understanding a variety of chromosomal abnormalities and other genetic mutations
11
Q
What is the tern used to describe the number of complete sets of chromosomes in a cell?
A
Ploidy
12
Q
Outline a human diploid cell
A
- It has two complete sets of chromosomes
- Most cells in humans are diploid, comprising 23 chromosome pairs, so 46 chromosomes in total
- This is 22 pairs of autosomes and a pair of sex chromosomes
- The total number of chromosomes in diploid cells is described as 2n, which is twice the number of chromosomes in a haploid cell
13
Q
Outline a common human haploid cell
A
- Gametes, a sex cell containing only one set of different chromosomes, or half the genetic material necessary to form a complete organism (i.e., haploid)
- Referred to as n
- Due to meiosis
- 2n regained upon zygote formation (n from sperm and n from egg = 2n zygote)
14
Q
What is reductive division?
A
- The first cell division in meiosis, the process by which gamete cells are formed
- A unique event in which the chromosome number is reduced from diploid (46 chromosomes) to haploid (23 chromosomes)
- Also called first meiotic division or first meiosis
15
Q
Outline Polyploidy
A
- The heritable condition of possessing more than two complete sets of chromosomes
- Relatively common in plants
- 3n = triploid, 4n = tetraploid, etc.
- Rare in the animal kingdom
- Usually lethal in humans, less than 1% of human conceptions are triploid (but most that are tend to be miscarried early into the pregnancy - 99%)
16
Q
Outline aneuploidy
A
- The condition of having an abnormal number of chromosomes in a haploid set
- Changes in chromosome number involving LESS than the entire complement of chromosomes
- Usually plus or minus one
- Example in humans: 46 + 1 = 47 - Trisomy, 46 -1 = 45 - Monosomy
- Arises due to non-disjunction of chromosomes and is the basis of most human chromosomal disorders
- Resulting gametes are n+1 and n-1
- After fertilization, offspring are either 2n+1 trisomic or 2n-1 monosomic
17
Q
What is nondisjunction of chromosomes?
A
- Nondisjunction is the failure of the chromosomes to separate, which produces daughter cells with abnormal numbers of chromosomes
- It can be either when homologous chromosomes do not separate during meiosis I, or when sister chromatids do not separate during meiosis II
18
Q
Outline Down syndrome
A
- Flat facial profile, down slanting eyes with epicanthal skin folds, low set ears, protruding tongue, low nose bridge
- Mental retardation
- An IQ under 60
- Generally a reduced life expectancy (but this has increased by around 20 years since 1990)
- Over 80 associated physical and mental disorders - congenital heart defect occurs in around 60% of all cases
- Down syndrome is due to trisomy at chromosome 21 (unknown cause)
19
Q
Describe Trisomy 21
A
- Where chromosome 21 has an extra chromosome
- Chr 21 contains around 1.5% DNA
- Not many genes there, but those that are are ones linked with symptoms to the disorder
- It was the first fully sequenced genome as a part of the Human Genome Project
20
Q
Outline other chromosomal caused disorders
A
- Edward’s syndrome: Trisomy 18, few children live longer than 10 years, small birth weight and clenched fists at birth
- Patau syndrome: Trisomy 13, cleft lip/palate, defects in digit development, 70% die within a year of birth
21
Q
What are the effects of aneuploidy of sex chromosomes?
A
- A variety of conditions are produced by the non-disjunction of sex chromosomes
- Genetic ‘balance’ is less disturbed than if it involved autosomes due to Y chromosomes contain around 1.5% genome and few genes, and X chromosomes containing around 5% genome and many genes
- Extra X chromosome copies are inactivated; this mechanism exists to compensate for the potential difference in ‘dosage’ of X-linked genes
- This is called X-inactivation
22
Q
What hypothesis did Mary Lyon propose?
A
- The Lyon Hypothesis
- First suggestion of X-inactivation
- Suggested that in any given female cell, only one X functions
- The other is thus inactive and can be observed as a ‘Barr body’
- HOWEVER, not all are turned off; females are mosaics and composed of different cell lines- thus, whether it is the maternal one or paternal one that is inactivated is a random process
23
Q
Outline Klinefelter’s syndrome
A
- XXY
- Male external genitalia
- Normal life span
- Possible infertility
- Barr bodies present in cells
- Additional copy of an X chromosome may result in mental retardation (such as XXXY or XXXXY - and multiple Barr bodies)
24
Q
Outline “Super-female” syndrome
A
- Xn
- 47, XXX = female, healthy and indistinguishable from XX females except by karyotype (which reveals an extra Barr body)
- 48, XXXX = female, usually mild mental retardation or learning difficulties
- 49, XXXXX = female and typically results in severe disability
25
Q
Outline “Super-male” syndrome
A
- XYY
- Not well classified
- Tall, muscled, but poorly coordinated
- Increased hormone production means acne is often severe and may be problematic long term
26
Q
Outline Turner syndrome
A
- X0
- Only viable human monosomy
- Short stature, webbed neck and shield shaped chest
- Usually normal intelligence
- NO Barr bodies
27
Q
State four types of chromosomal structural changes
A
- Deletion
- Duplication
- Inversion
- Translocation
28
Q
Outline DiGeorge/Velocardiofacial syndrome
A
- 22q 11.2 del (deletion of band in long arm of chromosome 22)
- Suspected by karyotype analysis, confirmed by FISH
- Cleft palate and cardiac defects
- Immune system deficiencies
- Moderate mental retardation
29
Q
Outline Fragile X syndrome
A
- Occurs in males
- Major cause of male mental retardation
- Characteristic facial phenotype (‘long’ face and large ears)
- Hypogonadism (reduced functional activity of gonads)
- Associated with fragile site on X chromosome
- Break occurs at FMRI (Fragile X mental Retardation-1) gene
- Functions of this gene are currently emerging
30
Q
Outline Robertsonian translocation
A
- 45 XX t (9,22)
- Originally thought to be a deletion on chromosome 22, but determined to be a translocation to chromosome 9
- Occurs in around 90% of chronic myelogenous leukaemia (CML) cases
- It is acquired, not congenital
31
Q
Outline chromosomal inversion
A
- E.g., inversion 16 (45, XY, inv 16))
- Inversions do not involve a lot nor gain of chromosomal material
- If the breakpoints of the inversion do not disrupt the genes they may not have an associated phenotype
32
Q
Explain the XY system of sex determination
A
- It was noted that female cells had 20 large chromosomes but an odd number and appearance of chromosomes in male cells (19 large and 1 small)
- This showed that chromosomes paired (homologous) and x and y paired too (hemizygous)
- Determined that unfertilised eggs contained an X chromosome, and sperm contained EITHER an X or Y chromosome
- Thus, sex is determined by which sex chromosome is in the sperm cell
33
Q
Explain the X0 system of sex determination
A
- Only one type of sex chromosome = X
- Sex is determined on whether or not sperm contains an X chromosome
- For example, in crickets, females are 22+XX
34
Q
Explain the ZW system of sex determination
A
- Found in birds, fish, and butterflies
- Sex chromosome is present in the ova, not the sperm
- Designated Z and W
- Male = ZZ, female = ZW
35
Q
Explain the Haplo-diplo system of sex determination
A
- Found in most species of bee and ant
- No sex chromosomes
- Female develop from a fertilised ova and are diploid (2n)
- Male are developed from an unfertilised ova and are haploid (n)
- Unfertilised eggs form via parthenogenesis
