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Genetics in Clinical Practice 1 Flashcards

1
Q

Name both Mendel Laws of Genetics

A

Law of Segregation

Law of Independent Assortment

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2
Q

State the Law of Segregation

A

Each indiv has 2 characteristics (alleles) for each characteristic and only 1 is transmitted to each offspring

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3
Q

State of the Law of Independent Assortment

A

Genes at diff loci segregate independently (not if 2 loci closely linked)

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4
Q

What are single gene disorders caused by?

A

Mutation in single gene

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5
Q

Define Mutation

A

Pathogenic heritable alteration in gene that affects protein structure/function

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6
Q

Define Allele

A

Diff form of a gene at a locus

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7
Q

Define locus

A

Pos of a gene on a chromosome

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8
Q

Define Genotype

A

Indiv genetic constitution (particular alleles) at a specified locus/loci

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9
Q

Define Phenotype

A

Clinical effect/presentation of expressed gene/genes

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10
Q

Define Heterozygote

A

Indiv with diff alleles at a specified locus

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11
Q

Define Hemizygous

A

Presence of only a single copy of a chromosome/gene e.g. X chromosome in males

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12
Q

Where are autosomal genes located?

A

On autosomes

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13
Q

Where are X + Y linked genes located?

A

On X/Y chromosome

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14
Q

What can X-linked conditions sometimes show?

A

Sex-linked inheritance

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15
Q

Define monogenic heterogeneity and give e.g.

A

Caused by 1 gene e.g. CF

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16
Q

Define phenotypic heterogeneity

A

Caused by 1 gene but diff mutations in that gene give rise to diff phenotypes

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17
Q

Define genetic heterogeneity

A 
Same/similar phenotypes caused by mutations in multiple genes
Either allelic (diff alleles at locus produce varying expressions of cond) or locus (mutations at diff loci produce same phenotype)
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18
Q

Describe how a pedigree is drawn

A
  1. A4 landscape paper
  2. Rule 4 horizontal lines across (3cm apart)
  3. Draw consultand symbol in middle of 3rd line
  4. Record DOB and relevant medical details
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19
Q

What are autosomal dominant conditions caused by?

A

Mutation in single autosomal gene

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20
Q

How can indiv affected by autosomal dominant conditions be described?

A

Heterozygous for mutation i.e. have wild type and mutant copy of gene

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21
Q

What is the chance of a parent affected by autosomal dom cond passing on normal/mutant allele to sons and daughters?

A

50%

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22
Q

What does a parent unaffected by autosomal dom cond always pass on?

A

Normal allele

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23
Q

How can the transmission of autosomal dom cond be described?

A

Vertical transmission (transmitted from gen to gen)

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24
Q

How are M + F affected by autosomal dom cond?

A

Equally

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25
Does a specific sex have higher chance of transmitting autosomal dom disorder to sons or daughters?
No either sex can to both sons and daughters
26
What is a key feature of transmission of autosomal dom disorders?
Male to male transmission
27
Name 2 important characteristics of autosomal dom cond
1. Variable (reduced) penetrance | 2. Variable expression
28
Define reduced penetrance
Prop of gene mutation carriers who manifest/present phenotypic signs/features of a cond
29
Define fully penetrant disorder and give e.g.
If you have gene mutation for it, you will develop it e.g. HD
30
Define not fully penetrant disorder and give e.g.
Not all indiv who inherit dom disease mutation show signs of cond e.g. BRCA 1/2 causes 80% chance of dev breast cancer and 60% of ovarian cancer
31
How is penetrance usually expressed?
As a %
32
Name 2 types of penetrance and e.g.
1. Sex specific e.g. cherubism has 100% M penetrance and 50 - 75% female penetrance (2:1 male predom) 2. Age specific e.g. bowel cancer - 5% of gen pop dev by 80 but risk inc to 80% by 80 for lynch syndrome mutation carriers
33
Define variable expression
Extent of clinical manifestation variable within and between families where gene carriers affected in diff degrees from mils to severe
34
What is variable expression mostly due to and give e.g?
Complexity of genetic makeup/pathways that work together to gen how genes and proteins work Marfan syndrome, achondroplasia, neurofibromatosis 1 + 2
35
How do autosomal dom cond dev?
Either inherited/de novo (new mutation in gamete)
36
Why do some cond have higher mutation rate?
Large no. of mitotic div of male gamete stem cells during man's lifetime
37
What do autosomal dom cond show through gens?
Genetic anticipation (cond gets worse as it goes down gens)
38
Define Genetic Mosaicism
Parent carries small prop of gametes (germ line cells) that have same mutation
39
How can normal healthy parents have a children affected by achondroplasia?
Embryo begins normal but few mitotic div later, new mutation occurs and cell with mutation will pass onto its daughter cells but all other cells normal
40
Give e.g. of autosomal dom disorders that show genetic mosaicism
Osteogen imperfecta, DMD
41
How are M + F affected by autosomal rec cond
Equally
42
Which family members are generally affected by autosomal rec cond?
Members of single sibship (group of children from 2 parents) or cousins (unless multiple consanguinity)
43
What is the effect of consanguinity on resulting offspring?
Inc risk of rec disorders
44
What is chance of child affected by autosomal rec disorder if both parents carriers?
1 in 4
45
What is probab of future sibling of affected indiv also being affected?
1 in 4
46
What is probab of future sibling of affected indiv being a carrier?
1 in 2
47
What is probab that existing unaffected sibling is also a carrier and why?
2 in 3 (denom changes from 4 to 3 as affected indiv excluded)
48
Define compound heterogeneity and give e.g?
Someone who has diff allelic mutations at same locus in same gene e.g. CFTR Beta thalassaemia/Sickle cell anaemia
49
Define homozygosity
Same allele on both chromosomes e.g. SCA, Hbs p.E6V/E6V
50
How are M + F affected by X-linked rec disorders?
Only M related via F line usually affected | F usually asymptomatic
51
What is chance of affected son born to F carrier of X-linked rec disorder?
1 in 2
52
What is chance of carrier daughter born to F carrier of X-linked rec disorder?
1 in 2
53
What is chance of carrier daughters of M by X-linked rec disorder?
100%
54
What is chance of unaffected sons of M affected by X-linked rec disorder and why?
100% as No male to male transmission (son only inherits Y)
55
In what case can F be affected with X-linked rec disorders?
Non-random X-inactivation leading to chance expression of mutant allele in certain tissues e.g. heart (DMD carriers), brain (Fragile X syndrome), Turner syndrome
56
Explain how female tortoiseshell cats come about and why
Gene controlling fur colour on X 2 alleles - 1 codes for black and 1 codes for orange M only have 2 poss genotypes (XbY = black/ XBY = orange) as only have 1 X F have 3 (XBXB = orange/ XbXb = black/ XBXb = tortoiseshell) For F, if 1 of 2 X inactivates, only 1 active X with its alleles expressed In patch of skin where skin tissues arise from cell with orange allele X inactivated, get black fur and vice versa creating patchwork fur
57
What happens as result of X-inactivation?
1 X in each cell switched off before blastocyst implants in F embryos Only 1 X used per cell in 46XX, 47 XXY All F are mosaics expressing either maternal/paternal X but not both in any cells
58
How is random X-inactivation caused?
XIST gene works in both directions to inactivate X early in blastogen by methylation
59
What is result of random X-inactivation on cells?
All of daughter cells have same X inactivated (clones with same pattern of X-inactivation)
60
How are M + F affected by X-linked dom disorders?
F < M Affected F show mosaic pattern in tissues e.g. skin Cond may be lethal in males
61
What is chance of affected offspring of mothers affected by X-linked dom disorders?
50%
62
What is chance of daughters inheriting X-linked dom cond from affected M?
100%
63
What is chance of sons inheriting X-linked dom cond from affected M?
0%
64
What do lines of Blaschko represent?
Territories of clonal cell pops
65
What causes lines of Blaschko?
Some clonal cells pops expressed normal X, some expressed mutated X which destroyed by apoptosis
66
In what X-linked dom disorders are lines of Blaschko manifested?
Incontinentia pigmenti, dermal hypoplasia

GEN (6 decks)

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