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BMS2042 > WEEK 6 CHROMOSOME ABBERATIONS > Flashcards

WEEK 6 CHROMOSOME ABBERATIONS Flashcards

1
Q

What do autosomal aneuploidies arise from?

A
  • Errors in MEIOSIS –> Monosomy (one chromosomal homologue missing) and trisomy (extra chromosome)
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2
Q

Why are autosomal aneuploidies generally not well tolerated compared to sex aneuplodies?

A

-Autosomes have more genes so gene dosage compensatory

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3
Q

In humans, what happens to autosomal monosomic foetuses?

A
  • They die in utero
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4
Q

Where are autosomal monosomies better tolerated (which type of organisms)?

A
  • Plant kingdom

e. g. Monosomic tobacco and Jimson Weed –> but less viable and often sterile

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5
Q

What do autosomal monosomies unmask?

A
  • Recessive alleles

- Lethal alleles can be tolerated in the host IF an intact, non lethal homologue is available.

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6
Q

In what condition can lethal alleles be tolerated in the host for autosomal monosomies?

A
  • If an intact, non lethal homologue is available
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7
Q

What is haploinsufficiency and an example?

A
  • When a single copy of an allele is not sufficient to perform the biological function
  • DiGeorge Syndrome (22q11.2 deletion)
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8
Q

What is the concentration of the protein indirectly associated with?

A
  • The gene number
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9
Q

What is the formula for a trisomy?

A

-2n +1

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10
Q

What are trisomies?

A
  • The accumulation of an additional chromosome
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11
Q

Do trisomy in all chromosomes occur?

A
  • Yes BUT only 3 survive –> 35% of spontaneous abortions
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12
Q

Is the survivability increased if the trisomy is in a small chromosome?

A
  • YES
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13
Q

Are trisomic plants viable and fertile?

A
  • They are viable but NOT fertile (infertile)
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14
Q

What is the genotypic formula for Down Syndrome (Trisomy 21)?

A
  • 47, (XX or XY), +21
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15
Q

What is the DSCR?

A

-The Down Syndrome Critical Region –> hypothetical region on chr.21 THOUGHT to be involved in the phenotype observed –> 21q22.2 = DSCR

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16
Q

Which two candidate genes were identified in the mouse DS model?

A
  1. DYRK–> reproduces DOSAGE SENSITIVE learning defects in Drosophila and mice
  2. DSCAAM–> reproduces heart and Nervous System defects
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17
Q

What is the relationship between maternal age and Down Syndrome prevalence?

A
  • Prevalence of DS births INCREASES with maternal age at conception
  • IN 95% of cases, the ovum is the source of extra Chr.21
  • Most ND events occur at anaphase I
  • PATERNAL AGE HAS NOTHING TO DO WITH IT!
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18
Q

WHY IS DS MORE COMMON IN OLDER MOTHERS?

A
  • In human FEMALES–> meiosis starts in foetus but STOPS at Diplotene (Prophase I)
  • Meiosis only continues upon OVULATION (So women in late 30s/early 40s make eggs that are older “been arrested at prophase I’ for longer)
  • Also may be that spindle fibres are LESS EFFECTIVE at older ages
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19
Q

What occurs in Pauau syndrome?

A

TRISOMY 13 –> 47, +13
1/5000 live births (higher proportion DIE in utero)
-Risk INCREASES with maternal age
-Few survive beyond 1 yr
-Survivors–> Severe learning difficulties, psychomotor difficulties, cardiac abnormalities

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20
Q

What occurs in Edwards Syndrome?

A

TRISOMY 18–> 47, +18

  • 1/8000 live births
  • MOST foetuses result in SPONTANEOUS ABORTION
  • Few survive BEYOND 1 YR
  • Common phenotypes–> Failure to thrive, microcephaly scull (small) deformities, born with clenched hands
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21
Q

Does paternal age play a role in Down syndrome?

A
  • NO
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22
Q

An individual has diploid somatic (“body”) cells with 2n = 20 chromosomes. At the end of mitosis in somatic cells, each daughter cell would have ____ chromosomes. In germline cells, at the end of meiosis I, each daughter cell would have_____ chromosomes. At the end of meiosis II, each daughter cell would have ____ chromosomes.

A
  • 2n=20
  • 1n=10
  • 1n=10
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23
Q

If non-disjunction of chromosome 21 occurred at anaphase I of meiosis, the two chromosome 21s present in a gamete would be ______?

A
  • Genetically different
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24
Q

What does meiosis allow for?

A
  • The generation of genetic variation among offspring
  • The generation of new combinations of alleles on the same chromosome
  • The transmission of equivalent genetic information from generation to generation
  • The genetic contribution of two individual parents to each offspring
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25
Q

The human autosomal trisomies that can result in live births are trisomies for which three autosomes?

A

Autosomes 13, 18 and 21

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26
Q

What best describes human sex determination?

A
  • Individuals with a Y chromosome are male
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27
Q

Nondisjunction in the first meiotic division in a male human could result at fertilization in which two syndromes?

A
  • turner syndrome and kleinfelter syndrome
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28
Q

Of the four common human sex chromosome aneuploidies, which two are fertile?

A
  • Triple X

- Double Y

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29
Q

Of the four common human sex chromosome aneuploidies, which two are fertile?

A

SRY

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30
Q

Which of the following is the karyotype of a human male with an extra copy of chromosome 18?

A

47 XY, + 18

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31
Q

In humans, normal sex chromosome number imbalance is corrected by dosage compensation which:

A
  • down regulates gene expression on one of the X chromosome in females
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32
Q 
Which of the following karyotypes would create cells with two Barr bodies?
47, XXY
47, XXX 
48, XXXX
46, XX
A

47, XXX

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33
Q

The type of chromosome rearrangement where there are two breaks in a chromosome and the internal region rotates 180o is called a(n) ________, whereas the type where a segment of one chromosome becomes attached to a different chromosome is called a(n) _______.

.

A

INVERSION, TRANSLOCATION

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34
Q

What do reciprocal translocations occur between?

A
  • Any two non homologous chromosomes
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35
Q

A mother heterozygous for a small deletion on one of her chromosomes wants to have a child with a male with a normal chromosome distribution. What is the likelihood that the child will be heterozygous for the same small deletion?

A

50%

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36
Q

Which of the following translocations would be most likely to have no consequence to the host?

A

Reciprocal translocations

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37
Q

An inversion where the centromere is inside the inverted region is called _____ inversion, whereas one where the centromere is outside the inverted region is called _______.

A
  • Pericentric

- Paracentric

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38
Q

A single crossover has occurred within the inversion loop of a person heterozygous for a pericentric inversion. What is the proportion of normal versus abnormal gametes?

A

50% normal

50% abnormal

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39
Q

The main type of mutagen that causes chromosome rearrangements is:

A

Radiation

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40
Q
  1. A person is a carrier for a large paracentric inversion, what proportion of her gametes will be unbalanced, assuming that a crossover will occur within the inversion loop?
  2. In this individual, unbalanced gametes are a result of the formation of a(n)__________ bridge while the _____
    chromosomal fragment would be lost during meiosis.
A
  1. Dicentric bridge

2. Acentric

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41
Q

Human adult haemoglobin consists of four subunits, two alpha subunits and 2 beta subunits. These two subunits share a high degree of similarity between each other. What is the most likely reason these two genes have evolved?

A

Duplication

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42
Q

If you saw this karyotype for chromosomes 1 and 2 in humans what type of chromosomal rearrangement does it indicate?

1, der(1), 2, der(2)

A

Reciprocal translocation

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43
Q

A heterozygote for a translocation will produce how many balanced gametes during meiosis if the chromosomes segregate via the alternate pattern?

A

100% balanced

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44
Q

The structure formed when the chromosomes concerned align at meiosis in a heterozygote for a Robertsonian translocation is a:

A

trivalent

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45
Q

What are polyploidies?

A

-Term used to describe instances where MORE THAN 2 COMPLETE SETS of the genome are available

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46
Q

Why are STABLE polyploidies un-common in animals?

A
  • Reproduction of a polyploid leads to aneuploidy

- It interferes with GENE DOSAGE

47
Q

Which species are polyploidies much more stable in?

A

-Polyploidies are much more stable in PLANTS! Also seen in FISH, LIZARDS AND AMPHIBIANS (Batura Toad)

48
Q

WHAT IS AUTOPOLYPLOIDY?

A

-More than 2 sets of chromosomes all derived from one ancestral species

49
Q

How can auto polyploidy happen nautrally?

A
  1. Fusion of NON-DIPLOID gametes

2. Cell RE-ENTERS interphase after prophase I (mitosis) in early embryonic development

50
Q

How formation of autopolyploids be induced experimentally?

A

-Colchicine (inhibits chromosome segregation in mitosis)

51
Q

What is ALLOPOLYPLOIDY?

A

-Generation of a polyploid individual from the fertilisation of two closely related but different species

52
Q

What are two reasons as to when the progeny of allopolyploids will be fertile?

A

THE PROGENY WILL BE FERTILE IF:

  1. Progeny has DIPLOID number of chromosomes AND
  2. There is sufficient similarity between genes for synapsis to occur
53
Q

Why is Allopolyploidy done?

A

To create progeny with the most desirable characteristics from each parent (one whole set of chromosomes from each) –> AB

54
Q

What is an example of a successful allotetraploid?

A
  • Triticale (hybrid of wheat and rye)

28chr * 14chr= 42 chr

55
Q

What process is an advantage in commercial plant growing?

A
  • Autopolyploidy
56
Q

What is the process that is thought to have contributed to our evolution from an Amphioxus (Primitive chordate) to a complex human?

A
  • Multiple stages of polyploidy
57
Q

What are examples (2) of common plant polyploidies?

A
  • Bananas (got rid of the black and yuck seeds)–> but now can not plant banana naturally
  • and Strawberries (made them much larger 8n)
58
Q

Can there be new species by autopolyploidy?-

A

YES

  • e.g. Salmon
  • Tetraploid is both viable and fertile –> isolated from its diploid ancestor
59
Q

Is a Diploid organism viable and fertile in context of autopolyploidy?

A
  • YES

- Viable and fertile

60
Q

Is a Triploid (Diploid * Tetraploid) organism viable and fertile in context of autopolyploidy?

A
  • Viable BUT not fertile (STERILE)
61
Q

Is a tetraploid organism viable and fertile in context of autopolyploidy?

A
  • YES

- Viable and Fertile

62
Q

What is endopolyploidy?

A

-Situation where certain cells within diploid organism become polyploidic (type of tissue mosaicism–> In cancers, solid tumours, leukemias)

63
Q

What is endopolyploidy alsk known as?

A
  • Somatic polyploidy
64
Q

How can endopolyploidy (AKA Somatic Polyploidy) occur? (2 reasons)

A
  1. Cells RE-ENTERING mitotic cycle (prophase) WITHOUT progressing through the other stages (thus re entering interphase)
  2. Cell can progress through the NORMAL steps of mitosis except that a single nuclear membrane will form over ALL DNA during telophase
65
Q

What is an example of endopolyploidy in humans?

A
  • Human LIVER cells –> 3n, 4n or 8n

- Chromosomes can still remain attached “polytene chromosomes”

66
Q

Why can polyploidy occur? (2 possibilities)

A
  • Producing high levels of gene products
  • To generate larger cells
  • (these are both POSSIBILTIES but not definite)
67
Q

Why are variations in autosome number not tolerated?

A
  • Unmasks recessive alleles and haploinsufficiency
  • Same as polyploidies –> not tolerated in most animals
  • Can be tolerated in plants (commercial importance)
68
Q

What are 3 things that the fragile regions in chromosomes can do?

A
  • Break off (and sometimes rejoin to another chromosome)
  • Invert
  • Duplicate
69
Q

What can changes in chromosome structure lead to in phenotypes?

A
  • Varied phenotypes
  • Sometimes NOTHING happens (if they are all balanced)
  • Sometimes disease can occur
70
Q

What are chromosome fragile sites and are they prone to spontaneous break?

A
  • Tiny gaps or ‘pinches’ that tend to break

- They are usually NOT prone to spontaneous breaks unless other factors influence chromosomal instability

71
Q

What are the 4 (with an extra one) main types of chromosomal abberations?

A
  1. Deletions
  2. Duplications
  3. Inversions
  4. Translocations (reciprocal or non-reciprocal)
    - There are also ring chromosomes, but we don’t count them as we hardly see them
72
Q

What occurs in a chromosomal deletion?

A
  • Chromosome breaks in one or more places –> chromosome portion is lost
73
Q

What are the two types of chromosomal deletions?

A
  1. Terminal deletion (on the end)

2. Intercalalry deletion (aka interstitial deletion) –> portion in the middle is lost and forms a loop

74
Q

In meiosis or mitosis with deletion chromosomes, why does a deletion loop form?

A
  • Bc. partial chromosomes can’t pair properly

- the deletion loop (compensation loop) forms and this allows synapsis to occur

75
Q

What is an example of a disorder with a partial monosomy and what are the symptoms?

A
  • Cri Du chat syndrome
  • Partial deletion of Chr.5 46, 5p-
  • Anatomical deformities in glottis and larynx
  • Mental retardation
  • Generally NORMAL life expectancy
76
Q

What occurs in a duplication event?

A
  • Portion of the chromosome is duplicated
77
Q

What are duplications produced as a result of with an example?

A
  • Uneven crossing over OR errors in DNA replication
  • There is a HIGH degree of phenotypic variation (not always bad thing as long as overall genes are BALANCED)
  • e.g. of gene duplication (like in alpha and beta subunits of Hb)
78
Q

Is there a high degree of phenotypic variation in duplications?

A
  • YES

- this is not always a bad thing

79
Q

What are the two positive effects of duplication?

A
  1. Gene redundancy

2. Evolution

80
Q

What is an example of how gene redundancy is useful from a duplication event?

A
  • Ribosomes are required in high numbers in metabolically active cells
  • Having MULTIPLE copies of the rRNA gene allows for SIGNIFICANT numbers to be generated
81
Q

How did gene duplication contribute positively to gene duplication?

A
  • Paralogous genes arose from genetic duplication event
82
Q

What is an example of a negative effect of gene duplication?

A

MECP2 DUPLICATION SYNDROME: Duplication of a region on X, q-arm
-X linked inheritance
Symptoms: Profound intellectual disability
- Hypotonia
- Predisposition to infections
- Epileptic seizures

83
Q

What are the two types of inversions and what do these involve?

A
  1. Paracentric inversion–> Centromere is OUTSIDE the inverted region
  2. Pericentric inversion –> Centromere is INSIDE the inverted region
84
Q

In general, what happens in gene inversions?

A
  • The gene order is reversed!
85
Q

How do inversions arise and are the genes in balance?

A
  • Unusual looping of chromosome
  • Genes are IN BALANCE! So MINIMAL EFFECT ON INDIVIDUAL–>BUT consequences for offspring IF inversion interferes with the expression of other genes like ONCOGENES
86
Q

If there is a HOMOZYGOTE for two chromosomes that exhibit an inversion, what would happen?

A
  • Meiosis would continue normally

- Genes would pair up during prophase as normal

87
Q

If there is a homozygote for an inversion, will that inversion be passed onto the offspring?

A
  • YES
88
Q

If there is a HETEROZYGOTE for an inversion, what will happen if there is no crossover event?

A
  • Inversion loop forms to allow genes to pair during prophase
  • 50% will have inverted chromosome and 50% will have normal chromosome
89
Q

If there is a HETEROZYGOTE for a PERICENTRIC inversion, what will happen if there IS A crossover event?

A
  • 50% of NORMAL gametes (one inverted BUT balanced)

- 50% of ABNORMAL GAMETES (carrying deletions–> BUT NOT BALANCED –> infertility

90
Q

If there is a HETEROZYGOTE for a PARACENTRIC inversion, what will happen if there IS A crossover event?

A
  • Gametes produced : ½ normal; ½ ABNORMAL
  • Acentric fragment (with no centromere) can get lost –> CANNOT ATTACH to the spindle
  • Dicentric chromosome forms DICENTRIC BRIDGE –> fragment is lost
91
Q

What are dicentric chromosomes?

A
  • Chromosome breaks during separation  fragment lost
  • Leads to 2 normal gametes 1 with BALANCED INVERSION
  • There are 2 deletion chromosomes (if fused with normal gamete foetus it is NOT VIABLE)
92
Q

WHEN DO RING CHROMOSOMES FORM?

A
  • When breaks occur on both arms and the ends re-join

- Effects are SEVERE –> ring chromosome 14 syndrome (very unstable and lost in early stages of meiosis)

93
Q

In general, what are translocations?

A
  • The transfer of genetic materials from one location to another
  • If occurs within the SAME homologous pair: Intrachromosomal
  • If occurs between NON-HOMOLOGOUS pairs: Interchromosomal
94
Q

What is a reciprocal translocation?

A

Exchange of genetic material WITH replacement

95
Q

What is a non-reciprocal translocation?

A

Exchangqe of genetic material with NO REPLACEMENT

96
Q

What are the two origins of translocations?

A
  1. Chromosomal break and re-joining

2. Abnormal crossing over

97
Q

If there is an individual that is homozygous for a translocation, does meiosis continue normally?

A
  • YES
98
Q

If there is an individual that is heterozygous for a translocation, does meiosis continue normally?

A
  • NO

- Pair via a translocation cross –> “quadrivalent” (4 sister pairs of chromatids)

99
Q

What is the result of alternate segregation?

A
  • 2 normal gametes

- 2 gametes with a balanced translocation (100% balanced)

100
Q

What is the result of Adjacent-1 segregation?

A
  • 100% of gametes are UNBALANCED
101
Q

What happens when an unbalanced gamete (from adjacent segregation) fuses with a normal gamete?

A
  • Zygote will be unviable

- Thus REDUCED fertility in heterozygotes –> recurrent miscarriages

102
Q

What is the result of adjacent-2 segregation?

A

-All 4 gametes unbalanced (100%)

103
Q

Is adjacent-1 segregation of adjacent-2 segregation more rare?

A
  • Adj-2 segregation is more rare
104
Q

What does Der(1) mean?

A
  • Based on the position of the centromere
  • Der(1) means derivative of chromosome 1–> centromere comes from chromosome 1 (Based on the centromere location) while the other part comes from chromosome 2.
105
Q

What are Roberstonian Transloactions and an example?

A

-Breaks can occur on the P arms of acrocentric chromosomes –>Reduces the chromosome number by 2
-P-arms lost and TWO q arms FUSE (Only tolerated IF P-arms contain NON ESSENTIAL (redundant) genes)
E.g. Familial Down Syndrome

106
Q

What is familial down syndrome?

A
  • Heritable form of Down Syndrome –> 3% of Down Syndrome Births
  • Robertsonian Translocation b/w Chr. 14 and 21
107
Q

What two things can inversions and translocations promote in evolution?

A
  • Speciation if spread through population

- Reproductive isolation

108
Q

If there is a normal gamete with normal gamete mating what is the fertility of the offspring?

A
  • Fertile
109
Q

If there is a normal gamete with translocation gamete mating what is the fertility of the offspring?

A
  • Reduced fertility
110
Q

If there is a translocation gamete mating with a translocation gamete what is the fertility from the offspring?

A
  • Fertile
111
Q

What is chronic Myelogenous Leukemia (CML)?

A
  • Translocation between chr.9 and chr. 22  Philadelphia chromosome
  • Translocation creates CHIMERIC protein BCR-ABL
112
Q

What does chimeric mean (in terms of chimeric protein)

A
  • fusion proteins

- proteins created through the joining of two or more genes that originally coded for separate proteins

113
Q

What is AML (Acute Myeloid Leukemia)?

A
  • Symptoms similar to CML BUT caused by t(18:21) Causes chimeric protein RUNX1-RUNX1T1 (ETO)
  • Also caused by inv(16), or t(15:17)

BMS2042 (11 decks)

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