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Genetics Flashcards

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1
Q

From which parent is the mitochondrial DNA inherited?

A

Mother

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2
Q

What is an allele?

A

A variation of the same gene

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3
Q

What is meant by Dominant Autosomal?

A

Characteristic is dominant if it manifests in a heterozygote
This characteristic is then expressed in the phenotype
Certain genes being activated (dominant gene), triggers a change in the phenotype

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4
Q

What is a dominant autosomal disorder?

A

Single gene disease or trait passed down to offspring with multiple generations affected
Each affected child normally has one affected parent
Each child of an affected person has a 1 in 2 chance of being affected

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5
Q

What is meant by a vertical pedigree pattern?

A

Males and females are equally affected and equally likely to pass on the condition

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6
Q

How could you make a genetic mutation dominant?

A

Mutations resulting from a DNA expansion of a CAG repeat on chromosome

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7
Q

What are the three consequences a dominant autosomal disorder can incur?

A
  • *Gain of function** - Gene now makes a protein with a new function, e.g - longer lifespan or new location to increase effect
  • *Dominant negative effect** - Mutated form interferes with the activity of proteins it binds to, e.g - dimers reduce activity
  • *Insufficient** - Mutant in one gene results in 1/2 amount of a protein that is not enough for normal function
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8
Q

What is meant by an Autosomal Recessive Disorder?

A

2 copies of recessive allele must be present in order for trait/disease to develop
Tend to be ‘loss of function’ mutations

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9
Q

What do carriers of autosomal recessive disorders have?

A

Lost a single copy of a gene but the normal one is sufficient enough to maintain normal function

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10
Q

What is the lowest probability that a sibling of an affected child will be affected?

A

25% (1 in 4)

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11
Q

What does a horizontal pedigree pattern show?

A

Affected individuals are often seen in more than one child in a family and not in all generations

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12
Q

What effect does consanguinity have on the occurrence of autosomal recessive disorders?

A

Increases probability of occurrence

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13
Q

Which sex do X-linked disorders (recessive) mainly affect and why?

A

Males - only one X chromosome so it is effectively dominant

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14
Q

If a father is affected by X-linked dominant disorder, what will be the effects on the sons and daughters?

A

No sons will be affected

All daughters will be affected

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15
Q

Why might X-linked dominant disorders affect males and females differently?

A

Females can be less affected because the normal X gene may be active in some cells rather than the mutant gene
The process is called X inactivation and is random

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16
Q

What is usually an effect of X-linked dominant disorders?

A

Increase of current function

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17
Q

What is meant by a Y-linked disorder?

A

Only affects males

All sons of an affected father are affected

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18
Q

If a father is affected by a mitochondrial disease, what is the effect on the children?

A

None are affected
Mitochondrial DNA will not be passed down by the father, however, if the mother was affected, all the children would be affected

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19
Q

What causes mitochondrial disease variability and what does the severity of symptoms depend on?

A

Random segregation of genes during binary fission so some mutant forms may be removed from a mitochondrion
Depends on the number of affected mitochondria as in the future more mutant mitochondria may accumulate to become a larger proportion of the mitochondria in the organism

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20
Q

How you would show a carrier of an autosomal and one of an x-linked trait on a pedigree diagram?

A

Autosomal - square/circle half-shaded

X-linked - circle with a black dot in the center

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21
Q

How you would show someone who is deceased on a pedigree diagram?

A

With a diagonal line across the square/circle (top part of line on right side and bottom on left)

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22
Q

How you would show the first person who sought medical attention on a pedigree diagram and what are they called?

A

They’re called proband. Shown as an arrow pointing to the bottom left corner of their symbol (circle/square)

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23
Q

How are monozygotic twins drawn on a pedigree diagram?

A

Their lines have the same origin (like the top vertex of a triangle) and there is a horizontal line connecting them.

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24
Q

How are dizygotic twins drawn on a pedigree diagram?

A

Their lines have the same origin (like the top vertex of a triangle) with no horizontal line connecting them.

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25
What are the conditions for a pedigree diagram to show an Autosomal Dominant trait?
Vertical Transmission Not all offspring affected usually Males and Females can be affected At least one affected parent - can be mother or father
26
What are the conditions for a pedigree diagram to show an X-linked recessive trait?
Not all generations are affected Not all offspring are affected Only females affected if father is affected
27
What are the conditions for a pedigree diagram to show a Mitochondrial-inherited trait?
``` Vertical transmission All generations affected All offspring affected Males and females affected Inherited from mother ```
28
What are the conditions for a pedigree diagram to show an Autosomal Recessive trait?
Usually no history of family disease / Distant relative affected Horizontal Transmission Males and females affected
29
Describe the steps involved when calculating the risk that a person is a carrier of an inherited disease?
Work with the closest relatives on either side of the proband's family Multiply risks of each person carrying the disease to find the probability that the proband is a carrier
30
Is Type VIIII more or less severe than Type I Osteogenesis Imperfect?
More severe
31
In Multiple Endocrine Neoplasia type 1 (MEN1), why do not all people with a mutation in the MEN1 tumor suppressor gene develop the same type of adenoma at the same time?
A second event has to occur to promote tumor formation, which is based on environmental differences
32
Why do only 10% of people with hereditary hemochromatosis have clinically relevant iron accumulation?
Dietary load of iron can vary considerably | Lower levels of intake are associated with improved disease prognosis
33
What causes the difference in disease progression between Duchenne and Becker muscular dystrophy?
In DMD there is a frameshift deletion in the dystrophin gene and so no active dystrophin is produced In Becker Muscular dystrophy the mutation does not result in a frame shift and so active dystrophin is produced whilst in shorter form, retains activity
34
What is a trinucleotide repeat expansion?
Mutation in which a region of 3 repeated nucleotides in the genome increases in number during DNA replication As the number of repeats increases, it reaches a threshold above which they are no longer stable during DNA replication and the number of repeats increases each subsequent round of DNA replication This alters protein function Greater number of repeats results in a more severe phenotype. Therefore, usually, future generations have more severe phenotypes.
35
What is meant by karyotype?
An individual's collection of chromosomes
36
How do you prepare a karyotype?
1. Collect ~5ml ** heparinized venous blood** (can use amniotic cells or chronic villus sample - CVS) 2. Isolate **white cells** 3. Culture in presence of **phytohemagglutinin** (stimulates T-lymphocyte growth and differentiation) 4. After 48 hours add **colchicine** (causes mitotic arrest - do at metaphase or prophase) 5. Place in **hypotonic saline** 6. Place on the slide 7. Fix and stain (Gisema stain or fluorescent stain) 8. Cut out the individual chromosomes and arrange them in a karyotype.
37
What is meant by chromatin?
A substance within a chromosome consisting of DNA and protein
38
What are the 5 common structures, identified by the G-banded architecture of a chromosome?
``` Telomere on either end of the chromosome p-arm (short) q-arm (long) G-dark and G-light bands Centromere ```
39
How do you refer to the small aberrations within chromosomes?
``` State chromosome number State whether they are on the p or q arm State their band number (based on position from centromere) State their sub-band number If possible state Sub-sub-band feature ```
40
What does bphs stand for?
Bands per haploid set
41
What do the darker regions of a chromosome karyotype mean?
Dark = heterochromatin - more compact, fewer genes
42
What do the lighter regions of a chromosome karyotype mean?
Light = euchromatin - more open, more genes
43
Why are karyotypes performed more in Prophase rather than Metaphase?
Because the chromosomes are less compact and therefore you get more detail from the karyotype
44
What is meant by Aneuploidy?
An abnormal number of chromosomes
45
What is the purpose of meiosis?
To achieve reduction from diploid (2n = 46) to haploid (n = 23) To ensure genetic variation in the gametes
46
What method of genetic variation does meiosis enable?
Random assortment of homologs | Recombination
47
What is non-disjunction and what does it result in?
When chromosomes are not split properly between daughter cells Always results in either 1+ (trisomy) or 1- (monosomy) chromosomes
48
Can non-disjunction occur in meiosis I or meiosis II?
Both
49
What is the most common form of chromosomal abnormality?
Sex chromosome aneuploidy
50
Why is sex imbalance tolerated?
X-inactivation of excess X chromosomes = Only 1 X chromosome active Low gene content of Y chromosome
51
Why, if inactivated, does abnormal number X have an effect?
Both X and Y chromosome have a PAR (pseudo-autosomal region) | PAR still produces genes in this region
52
What is the second most common chromosomal abnormality in live births?
Trisomy 21 (Down Syndrome)
53
What is this abnormality due to?
3 copies of chromosome 21 | Non-disjunction during maternal meiosis (85-90%)
54
What is the correlation between maternal age and cases of trisomy per 1000 births?
As maternal age increases, the number of cases rises exponentially
55
Why is it then, that 75% of children with Down syndrome are born to mothers under 35?
Because 90% of children are born to mothers at this age
56
Why does this correlation exist?
The older the mother, the longer the oocyte has been in the state of meiosis So chromosomes do not line up as neatly during metaphase Likely due to the degradation of factors that hold homologous chromatids together
57
Why does non-disjunction occur in spermatogenesis?
Primary spermatocyte undergoes ~ 23 mitotic divisions per year and potentially accumulate defects
58
Is paternal age a risk factor of aneuploidy?
No
59
What is the risk factor increasing the chances of paternal involvement in aneuploidy?
Smoking
60
Monosomy in which chromosome resulted in the most miscarriages?
21
61
Trisomy in which chromosome resulted in the most miscarriages?
16
62
What do the terms qter and pter refer to?
Tip of the long arm of the chromosome | Tip of the short arm of the chromosome
63
What is 'der' an abbreviation of?
Derivative chromosome (contains extra material)
64
What are 'ins' and 'inv' abbreviations for?
Insertion (unidirectional transfer of genetic material) Inversion (breaks in the chromosome where the bit of chromosome that broke off rotates and stays upside down inside the chromosome)
65
What is the abbreviation for translocation?
t (bidirectional transfer of genetic material)
66
What does a +/- before a chromosome number tell us about it?
Gain/loss whole chromosome
67
What does a +/- after a chromosome number tell us about it?
Gain/Loss of part of a chromosome
68
In what phase does the crossing over of chromosomes occur?
Prophase I
69
What is the purpose of Crossing Over?
To increase genetic diversity
70
How does Crossing Over occur?
Pairs of chromosomes align Chiasma form Crossover occurs
71
How often does Crossing Over occur?
1-3 times per chromosome per meiosis
72
Explain how Unequal Crossing Over occurs?
Chromosomes do not align correctly at centromeres Unequal lengths of chromatids are exchanged Leading to **deletion** of genes in one chromosome And **duplication** of genes in the other chromosome
73
What are the reasons for the deletion of genes in single chromosome abnormalities?
Unequal crossing over Breaks in chromosome Can occur at the ends
74
What are the 2 types of inversion in single chromosome abnormalities?
Paracentric inversion - Breaks in chromosome occur in the middle of the chromosome arm & that section of the chromosome is inverted, creating a reinvented piece of DNA Pericentric inversion - Breaks occur around the centromere & that section of the chromosome is inverted, creating a reinvented piece of DNA
75
Explain how the insertion occurs in 2-chromosome abnormalities
A section of chromosome from one chromosome is inserted into another chromosome Resulting in **duplication** in one chromosome And **deletion** in the other
76
Describe how translocation can occur in 2-chromosome abnormalities and what this means for the patient
Section of one chromosome exchanged with the section from another chromosome If this occurs in a balanced fashion, the carrier is unaffected May cause problems in offspring if they inherit one or the other The derivative chromosomes in an unbalanced fashion can result in partial trisomy or monosomy
77
What results from the derivative chromosome during translocation in an unbalanced fashion?
Trisomy | Or monosomy
78
What is the theoretical % chance that the child of a parent with translocation of chromosomes will be affected?
50% as there is a 2 in 4 chance that there will be an unbalance
79
What is meant by a Microscopic chromosomal deletion?
Deletion of chromosome can be detected easily in a microscope
80
What is meant by Chromosomal Microdeletion?
Deletion can be seen in high-resolution banding
81
What causes William's syndrome?
7q11.23 deletion
82
What are the symptoms of William's syndrome?
``` Long philtrum Slightly upturned nose Lack social anxiety Arched eyebrows Supravalvular aortic stenosis ```
83
What are the phenotypes of William's syndrome caused by?
Imbalance of genes that are unrelated apart from their genomic location
84
What are some of the symptoms of 7q11.23 duplication syndrome?
``` Delayed speech development Autistic behaviors Dilation of aorta Flat eyebrows Broad nose and short philtrum ```
85
Compare the general phenotypes of duplication and their reciprocal deletion
Duplications usually have a milder phenotype than the reciprocal deletion
86
Describe the 3 classes of chromosomes
Metacentric - Short arm p and long arm q Submetacentric - short arm considerably shorter than the long arm Acrocentric - short arm has been reduced to a short stump
87
Which chromosomes are generally acrocentric in humans?
``` 13 14 15 21 22 ```
88
What type of translocation can only acrocentric chromosomes undergo?
``` Robertsonian translocation (the p-arms of both chromosomes are lost and the q-arms of both chromosomes translocate to form a single chromosome) Results in them having a normal version of both chromosomes and one translocated chromosome ```
89
Can this type of translocation be homologous or non-homologous?
Both
90
Explain the mechanism behind this type of translocation?
Both p arms of the 2 different acrocentric chromosomes are removed The remaining long arms join together - Robertsonially translocating to form a derivative chromosome
91
If this occurs during the development, most people are unaffected but why might their offspring still be affected?
Because the parents are carriers

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