Chromosomal abberration Flashcards

Question

Uniparental diploidy

Answer 1

``` • Uniparental diploidy – generation of diploid set of chromosomes from a single parent • i.e. sperm carries 46 chromosomes; egg carries 0 • Very rare event (requires multiple errors in meiosis in both parents) • Foetuses don’t develop correctly • Possibly due to genetic imprinting? ```

Answer 2

``` • Uniparental disomy – inheritance of both chromosomes from a single parent e.g. normal sperm and egg which has • A chromosome is lost during an early mitotic division in the foetus • Only a 1/3 chance • Range of symptoms • Many go undiagnosed • Abnormalities arise – imprinting errors? Uniparental disomy (UPD) occurs when a person receives two copies of a chromosome, or part of a chromosome, from one parent and no copies from the other parent. ```

Answer 3

• Deletion of paternal 15q11-13 • Or uniparental disomy where both copies of Chr 15 are inherited from the mother • Maternal copies of this region are silent due to imprinting • Conversely, maternal deletion of the same region – Angelman syndrome • Symptoms • Poor muscle tone • Insatiable appetite - obesity • Cognitive delays

Answer 4

* Arise due to errors in meiosis * Monosomy – one chromosomal homolog missing * Trisomy – extra chromosome * Normally not well tolerated * Compared to sex aneuploids (Triple X, Turner) * Why?

Answer 5

(2n – 1) • Autosomal monosomies are not tolerated in humans • Exception Turner syndrome (but that’s a sex aneuploid which is tolerated better due to X-inactivation) • In humans, monosomic foetuses die in utero • Generally better tolerated in the plant kingdom • Monosomic tobacco & Jimson weed have been isolated • Tend to be less viable and often sterile

Answer 6

* Lethal alleles can be tolerated in the host, if an intact non-lethal homolog available * Similar to X-linked inherited traits being more common in males

Answer 7

• When a single copy of an allele is NOT sufficient to perform biological function Example: • DiGeorge Syndrome (22q11.2 deletion syndrome) • TBX1 mutations

Answer 8

* Accumulation of an additional chromosome * Generally better tolerated in humans than monosomy * i.e: trisomies more likely to survive (0.3% live births) * Survivability increased if trisomy is in small chromosome * Trisomy in all chromosomes occur, but only 3 survive * Trisomy accounts for 35% spontaneous abortions

Answer 9

* Trisomic plants viable, but infertile * Typically associated with phenotypic differences * Trisomics of Jimson weed (Datura) * Diploid number 24 * Trisomy of each chromosome results in a different capsule phenotype

Answer 10

``` • Occurs in 1/800 live births • Phenotypically variable • Commonly presents with: • Short stature • Mental retardation • Epicanthic fold (give eye characteristic almond-shape) • Heart and nervous system abnormalities • Life expectancy not common beyond 60s ```

Answer 11

• A hypothetical region on chromosome 21 thought to be involved in the phenotypes observed • 21q22.2 = DSCR • In a mouse DS model, identified candidate genes 1. DYRK – reproduces dosage-sensitive learning defects in Drosophila and mice 2. DSCAAM – reproduces heart and nervous system defects

Answer 12

Prevalence of Down Syndrome births increases with maternal age (at conception) • In human females, meiosis starts in the foetus, but stops at diplotene (prophase I) • Meiosis only continues upon ovulation • Therefore women in the late 30s early 40s produce eggs that are older (been arrested in prophase I) for longer • Spindle fibres less effective at older ages?

Answer 13

``` • 1/15000 live births • A higher proportion die in utero • Risk increases with maternal age • Poor prognosis • Few survive beyond 1 year • Survivors affected with severe learning difficulties, psychomotor difficulties, cardiac abnormalities ```

Answer 14

``` • 1/8000 live births • Most foetuses result in spontaneous abortion • Few (5-10%) survive beyond 1 year • Common phenotypes: • Failure to thrive, microcephaly skull deformities, born with clenched hands ```

Answer 15

Term used to describe instances where more than 2, complete sets of the genome are available

Answer 16

``` • Stable polyploidy un-common in animals 1. Reproduction of polyploid leads to aneuploidy 2. Interferes with gene dosage • But polyploidy is seen in fish, lizards and amphibians • Batura toad(Bufotes baturnae) • Much more stable in plants ```

Answer 17

• More than 2 sets of chromosomes all derived from one ancestral species • Can happen naturally if: 1. Fusion of non-diploid gametes 2. The cell re-enters interphase after prophase I (mitosis) in early embryonic development • Can also be induced experimentally using colchicine • Tolerated in plants and of important commercial significance

Answer 18

Once produced, an autotetraploid is an “instant” new species So tetraploid is reproductively isolated from its diploid ancestor Diploid X Tetraploid = Triploid (sterile)

Answer 19

• Generation of a polyploid individual from the fertilisation of two closely related but different species • Progeny will be fertile if: 1. The progeny has a diploid number of chromosomes AND 2. There is sufficient similarity between genes for synapsis to occur

Answer 20

Inter-species hybrid • Often done to create progeny with the most desirable characteristics of both parents • The hybrid has one set of chromosomes from each parent (e.g. AB) • Allopolyploidy is tolerated well in plant species, used frequently in commercial crops • Rarer in mammals and other animals mule Donkey = 62 chromosomes Horse = 64 chromosomes Mule = 63 chromosomes (infertile)

Answer 21

• Interspecies hybrids can be made fertile if made polyploidic • Use colchicine to arrest mitosis during embryogenesis, then remove (chromosome doubling) at fertlization AAABB after inducing it to re-enter interphase AA AA AA BB BB • Generates a fertile amphidiploid (or an allotetraploid)

Answer 22

* The fusion of two somatic cells (from different species) into a single hybrid cell * Generates a hybrid that contains the genetic material of BOTH species * Valuable technique for mapping genes and determining genetic function

Answer 23

``` • Mouse cell line defect in Thymidine kinase (TK gene) • Generate SCH with human cell line with functional TK gene • Redundant human chromosomes lost • Only chromosomal segment that complements mouse mutation remains ```

Answer 24

• The situation where certain cells within a diploid organism become polyploidic • A type of tissue mosaicism • Can happen by: 1. Cells entering the mitotic cycle (prophase), without progressing through the other stages – re-enter interphase 2. The cell can progress through the normal steps of mitosis, except that a single nuclear membrane will form over all DNA during telophase

Answer 25

``` • Humans liver cells can be polyploidic • 3n, 4n or 8n • Sometimes chromosomes remain attached – called polytene chromosomes • Unclear why polyploidy happens • Produce high levels of gene products? • To generate larger cells? ```