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GEN > Genetics in Clinical Practice 1 > Flashcards

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
Q

Does a specific sex have higher chance of transmitting autosomal dom disorder to sons or daughters?

A

No either sex can to both sons and daughters

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26
Q

What is a key feature of transmission of autosomal dom disorders?

A

Male to male transmission

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27
Q

Name 2 important characteristics of autosomal dom cond

A
  1. Variable (reduced) penetrance

2. Variable expression

28
Q

Define reduced penetrance

A

Prop of gene mutation carriers who manifest/present phenotypic signs/features of a cond

29
Q

Define fully penetrant disorder and give e.g.

A

If you have gene mutation for it, you will develop it e.g. HD

30
Q

Define not fully penetrant disorder and give e.g.

A

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
Q

How is penetrance usually expressed?

A

As a %

32
Q

Name 2 types of penetrance and e.g.

A
  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
Q

Define variable expression

A

Extent of clinical manifestation variable within and between families where gene carriers affected in diff degrees from mils to severe

34
Q

What is variable expression mostly due to and give e.g?

A

Complexity of genetic makeup/pathways that work together to gen how genes and proteins work
Marfan syndrome, achondroplasia, neurofibromatosis 1 + 2

35
Q

How do autosomal dom cond dev?

A

Either inherited/de novo (new mutation in gamete)

36
Q

Why do some cond have higher mutation rate?

A

Large no. of mitotic div of male gamete stem cells during man’s lifetime

37
Q

What do autosomal dom cond show through gens?

A

Genetic anticipation (cond gets worse as it goes down gens)

38
Q

Define Genetic Mosaicism

A

Parent carries small prop of gametes (germ line cells) that have same mutation

39
Q

How can normal healthy parents have a children affected by achondroplasia?

A

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
Q

Give e.g. of autosomal dom disorders that show genetic mosaicism

A

Osteogen imperfecta, DMD

41
Q

How are M + F affected by autosomal rec cond

A

Equally

42
Q

Which family members are generally affected by autosomal rec cond?

A

Members of single sibship (group of children from 2 parents) or cousins (unless multiple consanguinity)

43
Q

What is the effect of consanguinity on resulting offspring?

A

Inc risk of rec disorders

44
Q

What is chance of child affected by autosomal rec disorder if both parents carriers?

A

1 in 4

45
Q

What is probab of future sibling of affected indiv also being affected?

A

1 in 4

46
Q

What is probab of future sibling of affected indiv being a carrier?

A

1 in 2

47
Q

What is probab that existing unaffected sibling is also a carrier and why?

A

2 in 3 (denom changes from 4 to 3 as affected indiv excluded)

48
Q

Define compound heterogeneity and give e.g?

A

Someone who has diff allelic mutations at same locus in same gene
e.g. CFTR
Beta thalassaemia/Sickle cell anaemia

49
Q

Define homozygosity

A

Same allele on both chromosomes e.g. SCA, Hbs p.E6V/E6V

50
Q

How are M + F affected by X-linked rec disorders?

A

Only M related via F line usually affected

F usually asymptomatic

51
Q

What is chance of affected son born to F carrier of X-linked rec disorder?

A

1 in 2

52
Q

What is chance of carrier daughter born to F carrier of X-linked rec disorder?

A

1 in 2

53
Q

What is chance of carrier daughters of M by X-linked rec disorder?

A

100%

54
Q

What is chance of unaffected sons of M affected by X-linked rec disorder and why?

A

100% as No male to male transmission (son only inherits Y)

55
Q

In what case can F be affected with X-linked rec disorders?

A

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
Q

Explain how female tortoiseshell cats come about and why

A

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
Q

What happens as result of X-inactivation?

A

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
Q

How is random X-inactivation caused?

A

XIST gene works in both directions to inactivate X early in blastogen by methylation

59
Q

What is result of random X-inactivation on cells?

A

All of daughter cells have same X inactivated (clones with same pattern of X-inactivation)

60
Q

How are M + F affected by X-linked dom disorders?

A

F < M
Affected F show mosaic pattern in tissues e.g. skin
Cond may be lethal in males

61
Q

What is chance of affected offspring of mothers affected by X-linked dom disorders?

A

50%

62
Q

What is chance of daughters inheriting X-linked dom cond from affected M?

A

100%

63
Q

What is chance of sons inheriting X-linked dom cond from affected M?

A

0%

64
Q

What do lines of Blaschko represent?

A

Territories of clonal cell pops

65
Q

What causes lines of Blaschko?

A

Some clonal cells pops expressed normal X, some expressed mutated X which destroyed by apoptosis

66
Q

In what X-linked dom disorders are lines of Blaschko manifested?

A

Incontinentia pigmenti, dermal hypoplasia

GEN (6 decks)

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