Abnormal number of chromosomes in a cell. - gain (trisomy) - loss (monosomy)

Cytogenetics - Inheritance Flashcards by Eleanor Barnard

What is cytogenetics?

Study of chromosomes / anything bigger than a gene.

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Which chromosomes are autosomes?

1-22.

23 = X/Y

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Which chromosomes are acrocentric? (5)

13-15, 21, 22.

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What is conventional cytogenetics?

Metaphase chromosome analysis.

-G banding

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What is molecular cytogenetics?

Analysis at all stages of cell cycle.

not just metphase, like conventional

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What are the main techniques of molecular cytogenetics?

FISH
Microarray CGH
Next generation sequencing
MLPA
QF-PCR
qPCR

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At which stage of the cell cycle are chromosomes most visible?

Metaphase.

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What is G-banding used for?

Karyotyping.

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Which chromosome is generally the largest?

Chromosome 1.

-decrease in size from that

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How many genes does each band contain?

~50.

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What is the short arm of a chromosome called?

-p1, p2

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What is the long arm of a chromosome called?

q1, q2, q3

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How do cytogenetic abnormalities produce an abnormal phenotype? (5)

Dosage effect (gain / loss)
Disrupt gene
Genomic imprinting
Position effect
Unmask recessive disorder

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What generally produces a more severe phenotype; sex chromosome imbalance or autosomal?

Autosomal imbalance.

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What are the main numerical chromosome abnormalities? (4)

Diploidy
Aneuploidy
Polyploidy
Mosaicism

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What is diploidy?

2 copies of each chromosome.

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What is aneuploidy?

Abnormal number of chromosomes in a cell.

gain (trisomy)
loss (monosomy)

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What is polyploidy?

Gain of whole sets of chromosomes.

-triploidy / tetraploidy

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What is mosaicism?

Presence of 2+ populations of cells.

-aneuploidy + diploidy

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When do chromosomal abnormalities originate? (3)

Gametogenesis (meiosis)
Fertilsation
Early cleavage

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What are the risks of increased maternal and paternal ages?

^ maternal age - aneuploidy.

^ paternal age - no significant risk.

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What is non-disjunction?

Failure of chromosome / chromatid to separate.

What are the different effects caused by non-disjunction at meiosis I and II?

Meiosis I - 2 disomic, 2 nullisomic gametes.

Meiosis II - 1 disomic, 1 nullisomic, 2 normal gametes

What are the 3 chromosome that trisomy occurs at, and why?

13, 18, 21.

-relatively small chromosomes (can just about cope with extras)

What proportion of foetuses with trisomy 21 spontaneously abort?

75%.

What effect does trisomy 21 have on fertility?

Females - normal. | Males - usually infertile.

What condition does trisomy 18 cause?

Edwards syndrome.

What are the main features of Edwards syndrome?

``` Microcephaly Cleft lip Low birth weight Severe mental retardation Heart problems ```

What condition does trisomy 13 cause?

Patau syndrome.

What are the main feature of Patau syndrome?

``` Small birth weight Microcephaly Sloping forehead Holoposencephaly Cleft lip Polydactyly ```

What is holoprosencephaly?

Forebrain fails to develop into two hemispheres.

What age does female meiosis begin?

5 months. - the longer the eggs are unfertilised, the more likely they are damaged - male meiosis start at puberty

What are the main sex chromosome aneuploidies? (2)

``` Turner's syndrome (45, X) Klinefelter syndrome (47, XXY) ```

What are the main features of Turner's syndrome?

Infertility Lymph obstruction (webbed neck, swelling) Short stature Coarctation of aorta

What are the main feature of Klinefelter syndrome?

Infertile Hypogonadism Long arm and legs

What errors occur at fertilisation? (2)

- Polyploidy (e.g. triploidy) | - Molar pregnancy (double paternal, no maternal)

What is the genetic make up of trisomy, and what proportion spontaneously abort?

69, XXY / XYY / XXX. -99.9% spontaneously abort

What are the origin of triploidy? (3)

- Digyny (diploid egg fertilised) - Diplospermy (diploid sperm) - Dispermy (2 sperm to one egg)

What are the main features of double maternal triploidy?

- Small placenta and foetus - Large head - Growth retardation

What are the main features of double paternal triploidy?

- Growth retardation | - Large, cystic placenta

What are the conclusions from double maternal / paternal triploidy?

Maternal genome >> foetus. | Paternal genome >> placenta.

What is a molar pregnancy?

Placenta develops without a foetus. - empty egg - double paternal genome >> massive cystic placenta

What errors occur at early cleavage?

Mosaicism. | -mitotic non-disjunction

What are the main consequences of mosaicism?

- Variable phenotype | - Recurrence risk (gonadal)

Summarise numerical abnormalities.

MEIOSIS - gain / loss of single chromosomes. FERTILISATION - gain of whole chromosome sets. POST-FERTILISATION - gain / loss during mitosis.

What are the 2 types of chromosomal rearrangements?

Balanced | Unbalanced

What are the main types of balanced rearrangements? (3)

- Translocation - Inversion - Insertion

What are the 2 types of translocation?

- Reciprocal (break and exhange) | - Robertsonian (whole arm fusion, no phenotype risk)

Which type of translocation occurs with acrocentric chromosomes?

Robertsonian translocation.

What is the process of inversion?

2 breaks, rotation, then rejoining. | - pericentric / paracentric

What are unbalanced rearrangements?

Overall loss / gain of chromosomes. - commonest = deletions / duplications - mainly sporadic

What is the difference between interstitial and terminal deletions?

INTERSTITIAL - middle of chromosome. 2 breaks >> segment lost. TERMINAL - end of chromosome. 1 break >> segment lost.

What are the 2 types of duplications?

- Direct | - Inverted (segment reversed)

What causes a ring chromosome?

Breakage then circularisation of a chromosome.