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Genetics 1 > Bocian Mendelian > Flashcards

Bocian Mendelian Flashcards

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1
Q

Describe Autosomal dominant inheritance

A

Autosomal Dominant Inheritance
A. The gene for the disorder is found on an autosome (i.e., not on the X or the Y or in the
mitochondrial genome)
B. The phenotype is expressed in heterozygotes (usually) or in homozygotes
1. aa is not affected
2. Aa is affected

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2
Q

Marfan syndrome inheritance?

A

Autosomal Dominant

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3
Q

Neurofibromatosis inheritance

A

Autosomal Dominant

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4
Q

Describe Autosomal Recessive inheritance

A 
Autosomal Recessive Inheritance
A. The phenotype is expressed only in those who are homozygous for the mutation
1. AA is not affected
2. Aa is a carrier but still normal
3. aa is affected
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5
Q

Cystic Fibrosis Inheritance?

A

Autosomal Recessive

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6
Q

What is Codominant Inheritance?

A

Codominant Inheritance
A. Expression of each allele can be detected, even in the presence of the other
1. Example: ABO and MN blood groups. The system consists of three alleles A, B, and
O. Both A and B are dominant in relation to O, so blood group A can have the genotype
AA or AO. Blood group B can have the genotype BB or BO. However, neither A nor B
shows dominance over the other, so individuals with the genotype AB have the
phenotypic characteristics of both blood group A and blood group B.

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7
Q

What is Incomplete Dominant (Semidominant) or Incomplete Recessive Inheritance?

A

Expression of heterozygote (Aa) is different from both homozygous types (AA and aa)
and is intermediate between them
1. Example: Achondroplasia (AD) – homozygotes are much more severe than
heterozygotes; however, achondroplasia is considered to be a dominant disorder.

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8
Q

Achondroplasia inheritance?

A

Autosomal Dominant

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9
Q

Describe 3 general characteristics of autosomal dominant diseases

A

A. The normal gene product is usually a non-enzymatic structural protein

B. The normal gene product is required in normal amount and with normal function

C. Abnormal phenotypes result from a mutant gene that produces no protein, a reduced amount of protein, reduced or increased activity or function of the protein, an abnormal form of the protein which may be toxic to the cell or have a new function, protein expression at the wrong time or place, or occasionally an increased amount of the protein.

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10
Q

Ehlers-Danlos syndrome inheritance?

A

Autosomal Dominant

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11
Q

Describe 3 general characteristics of Autosomal Recessive Disorders

A

A. The gene product is usually an enzyme

B. Abnormal phenotypes result from absent or very low levels of the enzyme/protein

C. There is a margin of safety wide enough to allow normal function in heterozygotes
1. An individual can have only half the normal amount of enzyme and still have
normal amounts of substrate converted

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12
Q

Describe PKU heterozygotes

A

PKU heterozygotes (“carriers”) (Pp) have only half the normal amount of enzyme but are normal clinically because half the enzyme is still enough to do the job

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13
Q

Describe genetic heterogeneity

A

Genetically different disorders may appear clinically (phenotypically) identical, and
identical disorders may be due to different genes………..

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14
Q

What is allelic (mutational) heterogeneity?

A

Different mutations in the same gene causing

the same disorder

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15
Q

What is Locus (non-allelic) heterogeneity?

A

Mutations in different genes causing the same

disorder

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16
Q

What is phenotypic heterogeneity?

A

Different mutations in the same gene causing different disorders

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17
Q

What is allelic homogeneity?

A

For some disorders, all or nearly all affected individuals have the same mutation

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18
Q

Mutation in achondroplasia?

A

Allelic homogeneity

99% have mutation in the FGFR3 gene

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19
Q

Describe allelic heterogeneity in recessive and dominant disorders

A
  1. In recessive disorders, e.g., cystic fibrosis, different mutations at the same locus (compound heterozygote) can account for variable severity of the disorder
  2. For many dominant disorders, all or most affected families have unique (“private”) mutations
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20
Q

Severity of NF-1?

A

Allelic heterogeneity

Severity and/or expression may differ depending on the type of mutation or where in
the gene the mutation occurs

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21
Q

Describe adult PKD in the context of locus heterogeneity

A

Autosomal dominant (“adult”) polycystic kidney disease can be caused by either of two genes, PKD1 (85% of cases) or PKD2 (15% of cases)

Same phenotype

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22
Q

What is variable expressivity?

A

Expressivity relates to the degree to which an affected person manifests a particular
disorder

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23
Q

Phenotype in variable expressivity?

A

The phenotypic manifestations may differ among people who have the same genotype

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24
Q

What is intrafamilial and interfamilial variability?

A

Intrafamilial variability (variability within families) may reflect the action of modifying genes, but interfamilial variability (variability between/among families) is more likely to be due to allelic heterogeneity at a single locus.

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25
Q

Describe variable expression in the context of a family with a disease

A

Affected individuals, even in the same family, can have different manifestations of the condition

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26
Q

What is variable severity?

A

Affected individuals, even in the same family, can be mildly, moderately, or severely affected

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27
Q

What is a congenital disorder?

A

A congenital disorder is one that is present at birth; it may or may not have a genetic basis

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28
Q

What is a genetic disorder?

A

A genetic disorder is one that is determined by genes; it may be present at birth or may have later onset

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29
Q

Describe late-onset genetic disorder

A

Range of age of development of phenotype. Can be later in life

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30
Q

What is pleiotropy?

A

The phenomenon in which a single gene is responsible for a number of distinct and
seemingly unrelated phenotypic effects - a single cause results in abnormalities in more
than one organ system in different parts of the embryo or in multiple structures that arise
at different times during embryologic development

31
Q

What are sex-limited phenotypes?

A

Autosomally transmitted but expressed only in one sex

32
Q

What are sex-influenced phenotypes?

A

Expressed in both sexes but with different frequencies

33
Q

What is penetrance?

A

The percentage of people with a particular genotype who are actually affected clinically

34
Q

What is reduced penetrance or non-penetrance?

A

Some individuals who have the
appropriate genotype fail to express it

a) May be misinterpreted as “skipping a generation”

35
Q

Describe the characteristics of autosomal dominant inheritance. What is meant by vertical transmission?

A

Only one copy of the mutated gene is required for expression of the trait

The trait appears in each generation (vertical transmission)

36
Q

What is meant by new mutation? How does this relate to autosomal dominant mutations?

A

An affected child may not necessarily have an affected parent

37
Q

What is germ-line mosaicism? How does this explain an autosomal dominant disease “carrier”?

A

Both parents may be clinically normal, but one may have the mutation in only some of
the body’s cells—not enough to be clinically (i.e., obviously) affected

  1. But, if the mutation is in some of the germ cells, i.e. eggs or sperm, more than one
    child may be affected
38
Q

Describe autosomal dominant mutation in regards to variable expression.

A

Autosomal dominant traits may vary widely in their phenotypic expression and/or severity, even among affected individuals in the same family

39
Q

Describe incomplete penetrance in the context of autosomal dominant inheritance

A

Some individuals who carry the mutation may not show any clinical abnormality

40
Q

Describe delayed age of onset in an autosomal dominant inheritance

A

In some conditions, e.g. Huntington disease, affected individuals may not show
manifestations until a later age

Diagnosis may not occur until after they have reproduced, depriving them of some
important options for reproductive decision-making

41
Q

Give some examples of a negative family history with an autosomal dominant disease

A

A. The patient represents a new mutation
B. A parent has germ-line mosaicism
C. A parent carries the mutation but with non-penetrance or very mild expression which was
not recognized
D. Genetic heterogeneity; the patient really has another, similar disorder with different
inheritance, e.g. autosomal recessive
E. Delayed age-of-onset disorder, and a parent had the mutation but died from another cause
before onset and recognition of the genetic disorder
F. Non-paternity

42
Q

What to tell patient in an autosomal dominant disease

A
  1. Offspring are at a 50% risk to inherit the gene
  2. The condition may be more or less severe, or may have different manifestations in the
    child
43
Q

What to tell couple with child with autosomal dominant disease

A
  1. Recurrence risk will depend on whether disorder was inherited from a parent or arose
    as a new mutation
  2. Parents may need to be carefully examined and/or have special studies to determine if
    one of them carries the gene
44
Q

Characteristic of autosomal recessive inheritance? What is horizontal transmission?

A

The trait usually appears only in siblings and not in their parents, offspring, or other relatives (horizontal transmission)

Two copies of the mutation are required for expression of the trait

45
Q

What is a compound heterozygote?

A

Compound heterozygote has a pair of genes with a different mutation in each copy (really is a type of homozygote – the term is used to distinguish this individual from one who has the same mutation in each copy of the gene)

46
Q

What is a double heterozygote?

A

Double heterozygote carries mutations in two different genes

47
Q

What are obligate carriers?

A

An individual who may be clinically unaffected but who must carry a gene mutation based on analysis of the family history; usually applies to disorders inherited in an autosomal recessive and X-linked recessive manner (but could also apply to someone with an autosomal dominant disorder and incomplete penetrance)

48
Q

What is consanguinity? Risk?

A
  1. Genetic relatedness between individuals descended from at least one common
    ancestor
  2. They are more likely to carry identical alleles inherited from this common ancestor
  3. Both could transmit an identical allele to their offspring, who would then be
    homozygous for that allele and, therefore, affected
  4. A consanguineous couple have an increased risk that their offspring will be affected
    with a recessive disorder; the magnitude of the risk depends on where in the family the
    common ancestor is
  5. The more rare the disorder, the more likely that the parents are consanguineous
49
Q

Genetic heterogeneity and clinical manifestation of disease?

A

Some genetically different disorders may appear clinically identical

50
Q

What is pseudodominance?

A

Either consanguinity or a high gene frequency in the population may make a recessive pedigree falsely appear dominant

51
Q

What determines gender in humans?

A

Either consanguinity or a high gene frequency in the population may make a recessive pedigree falsely appear dominant

52
Q

The majority of genes on X chromosome?

A

The majority of genes on the X have nothing to do with sexualcharacteristics.

53
Q

What is the Lyon hypothesis?

A

Single active X hypothesis

all but one X chromosome in each cell to be almost entirely turned off (inactivated). (The “almost” is very important – there are some genes on the X that
escape inactivation.)

54
Q

Why is Turner phenotype not too different from normal?

A

patients missing an entire X chromosome (i.e., Turner syndrome) or those with an entire extra X chromosome (e.g., XXY or XXX individuals) are
not very different overall from normal individuals because all but one X chromosome is inactivated.

55
Q

What is XIST?

A

There is a gene called XIST (X-inactivation-specific transcript) on the X chromosome (at Xq13.2) that is the master regulatory switch locus for Xinactivation.

56
Q

Describe 6 major characteristics of X-linked recessive inheritance

A
  1. Males are affected
  2. Heterozygous (carrier) females are (usually) clinically normal or show only mild
    clinical expression
  3. All sons of affected males are normal
    a) A son inherits his fathers’ Y chromosome, not his X, so he won’t inherit the
    mutation
  4. All daughters of affected males are carriers
    a) Daughters inherit their fathers’ X chromosome; since it has the recessive
    mutation, they will all be carriers and (usually) unaffected
  5. Half the sons of a heterozygous female are affected
    a) Women have two X’s; since their sons get either one or the other X, half will be
    affected and half normal
  6. Half the daughters of a heterozygous female are carriers
    a) The trait may be passed through a series of carrier females
    b) Affected males are always related through females
57
Q

What is skewed X-inactivation?

A

X-inactivation doesn’t occur at the 2-cell stage – it occurs after the early cleavage stage; therefore, there are multiple cells present, each of which
randomly inactivates one of the X’s

58
Q

What is the distribution of X inactivation? Consequences?

A

think of Xinactivation the female population as a bell-shaped curve

Therefore, some carrier females may express the disorder because of skewed Xinactivation,
in which the majority of cells have the normal X inactivated and the X with the mutation active – therefore, there is much less of the normal product of that gene than there would be if the normal X were active in at least half the cells.

59
Q

Diseases that are X linked but women may still have?

A

some women might have a low-enough level of clotting factor VIII to give them symptoms of hemophilia A, or some women might
have symptoms of Duchenne muscular dystrophy (both of these are classical X-linked recessive disorders in which males are affected and females are unaffected (usually) carriers

60
Q

Describe sex chromosome abnormalities in women

A

Sex chromosome abnormalities may cause a carrier female to express the disorder

(1) 45,X women (Turner syndrome) may manifest X-linked recessive disorders
(2) A man with 47,XXY (Klinefelter syndrome) may carry an X-linked recessive disorder but not express it

61
Q

Describe homozyogosity of a mutant X linked gene in an affected female

A

Homozygosity for the mutant gene in an affected female will result in expression of the disorder in the female

(1) XX carrier sister of XY man with hemophilia-A meets another XY man with hemophilia-A in the waiting room of the hemophilia clinic……if they have children, a daughter could inherit the X with the mutation from each of them and have hemophilia (XX x XY -> XX)

62
Q

New sex linked mutation?

A

Not every affected male necessarily has a carrier mother

~1/3 of males with X-linked recessive disorders have new mutations

63
Q

Describe Germ-line mosaicism and X-linked disease

A

In general, in X-linked recessive disorders, ~2/3 of cases are inherited from a carrier mother and ~1/3 of cases are new mutations.

A proportion of these mothers are mosaics

So, if the mother of a boy with a Duchenne muscular dystrophy mutation tests negative for the mutation in her blood, she still has a ~15% chance of carrying the mutation in her germline (empiric risk) and could have another affected
son or carrier daughter.

64
Q

What is Fragile X syndrome inheritance?

A

X linked recessive

The most common inherited cause of intellectual disabilities

65
Q

What is the most common inherited cause of intellectual disability?

A

Fragile X syndrome

66
Q

Can Fragile X occur in females?

A

YES (remember skewed X inactivation)

67
Q

Describe Fragile X in males and females with a full mutation

A

Nearly all males with a full mutation are affected (usually moderately)

Half of females with a full mutation are affected (usually mildly)

68
Q

Mutation in Fragile X?

A

Fragile X is due to unstable DNA tri-nucleotide repeat sequences

Unstable DNA tracts of tandemly-repeated 3-base-pair units increase in length (allelic expansion) and cause methylation and loss of gene function

69
Q

Describe repeat classification in Fragile X

A

Normal gene: 5 - 44 copies of the CGG repeat
(1) Phenotypically normal

Intermediate: 45 – 54 repeats
(1) Phenotypically normal; might expand into a premutation but not into a full mutation

Pre-mutation (“carrier”): 55 – 200 repeats
(1) Can occur in females or in males

Full mutation: >200 repeats
(1) Are methylated and silenced

70
Q

When can pre-mutations expand to full mutations in Fragile X?

A

Pre-mutations expand to full mutations only when passed on by females

71
Q

Characteristics of X-linked dominant inheritance. 2 Trends. How common?

A

Affected males have all daughters affected and all sons normal

Affected females transmit the condition to half of their children of either gender

a) Affected females are twice as common than affected males
b) Affected males are more severe than affected females

72
Q

Y-linked (Holandric) inheritance? What is associated with it?

A

The only trait that is consistently associated with the Y chromosome is maleness

73
Q

Is it ok to refer to an affected patient as a mutant?

A

No

Genetics 1 (7 decks)

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