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2020 MHS Genetics Unit2 > mendelian inheritance of single gene defects > Flashcards

mendelian inheritance of single gene defects Flashcards

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1
Q

causes of single-gene defects

A
  1. missense point mutation
  2. nonsense point mutation
  3. RNA synthesis/processing mutations
  4. small deletions/insertions (few bases)
  5. large deletions/insertions
  6. abnormal recombination
    1. recombination
    2. unequal crossover
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2
Q

a given DNA segment on a chromosome

A

locus

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3
Q

alternative versions of a gene

A

alleles

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4
Q

the most common allele in the population

A

wild-type

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5
Q

an allele that differs from the wild type by mutations

A

mutant/variant allel

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6
Q

there are two or more relatively common alleles in the population (>1% occurrence in population)

A

polymorphism

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7
Q

the set of alleles that constitute an individuals genetic make-up

A

genotype

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8
Q

the observable expression of an individuals genotype. In medicine, it means the expression of the disease

A

phenotype

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9
Q

What type of mutation is this an example of?

  1. sickle cell disease
    1. Glu6Val mutation
A

missense point mutation

  1. a mutation hat changes an amino acid in the primary structure
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10
Q

Duschenne muscular dystrophy

A

nonsense point mutation

  1. a mutation that creates a sttop codon - leading to a shortened proteins
    1. 250+ mutations of this type
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11
Q

what type of mutation is this

beta-thalassemia

  1. mutation in the promoter region
A

RNA synthesis/processing mutations

  1. mutations in the promoter of the gene
    1. beta thalassemia patients have mutations in the promoter region of hemoglobin beta genes, which leads to diminished (or lack of ) beta chain synthesis
  2. mutation that influences pre-mRNA splicing (exons are spliced out or a region of an intron becomes part of the coding sequence)
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12
Q

what type of mutation leads to a deletion/insertion divisible by 3.

A

small deletions/insertion (few bases)

  1. can lead to missing or extra amino acids.
    1. if the number of nucleotides deleted/inserted is divisible by 3
  2. can lead to chage in reading fram (frame shift mutation)
    1. if the number of nucleotides deleted/inserted is not dividable
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13
Q

May involve multiple exons or an entire gene is gone or moved

A

Large deletion/inserttions

  1. can involve multiple exons or an entire gene
    1. ~75% of the known defects in Duchenne muscular dystrophy
    2. BUI~90% of alpha thalassemeias are due tot thte deletion (loss) of at least one of the hemoglobin alpha genes
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14
Q

what are the causes of

  1. Duchenne muscular dystrophy
  2. betathalassemas
  3. alpha thalassemias
A
  1. duschenne muscular dystrophy
    1. large deletions/insertions
  2. beta thalassemias
    1. RNA synthesis/processing mutations
  3. alpha thalassemias
    1. large deletions/insertions
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15
Q

describe the two types of abnormal recombinatiton

A
  1. recombination
    1. exchange of genetic material between homologous sequences of chromosomes
  2. unequal crossover
    1. exhange of genetic material between mispaired sistter chromatids or chromosomes ( when they pair through non-identical regions)
    2. may lead to large deletions, duplications
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16
Q

hemophilia A is an examples of what type of single-gene defect?

A

adnormal recombination

  1. inversion
  2. abnormal recombination occurs between homologous sequencs on the same chromosome
  3. leading to a duplication or inversion

hemophilia A is due to the inversion of the coagulation factor 8 gene from exon 1 to exon 22

  • ~50 of sever hemophila A
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17
Q

describe the following variants

A
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18
Q

the visualization of the inheritance pattern of a phenotype with in the family

A
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19
Q

what genotype is a male with a variant allele on his X chromosome?

A

hemizygote

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20
Q

if not all individuals with a given genotype manifest the pheotype(disease), what does that say about its penetrance?

A

th variant allele has reduced penetrance

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21
Q
A
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22
Q

define and explain penetrance vs expressivity

A
  1. penetrance
    1. the probability that a variant allele has any phenotypec expression
  2. expressivity
    1. the severity of the expressed phenotype
    2. mild vs severe
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23
Q

what can influence penetrance and expressivitty of a disease?

A
  1. age
  2. environment factors
  3. other genes
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24
Q

what is the penetrance when 8 out of 10 people with same genotype has any kind of symptom of the disease?

A

the penetrance of the disease is 80%

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25
Q

what is mendelian inheritance pattern based on?

A
  1. based on
    1. the chromosomal location of the trait
      1. autosomal
      2. X-linked
    2. the dominance of the trait
      1. recessive
      2. dominant
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26
Q

usually affects males and females equally

A

autosomal

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27
Q

affects males and femals at different ratios

A

X-linked

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28
Q

the wild type allele is dominant, only homozygotes are affected.

A

recessive

29
Q

the variant allele can be both heterozygoud and homozygous.

A

dominant

  1. possibilities
    1. incomplete dominance
      1. homozygotes have more sever symptoms
    2. codominance
      1. at least two alleles expresses the phenotype
30
Q

with the given pedigree, determine the inheritance pattern.

A
  1. autosomal recessive disorders
    1. phenotype
      1. manifests only in homozygotes
      2. usually manifestes in siblings and not tparents or offspring
    2. both parents have o be att least heterozygote
    3. equally affects males and females
31
Q

what type of punnett square is this ineritance pattern, given that rr=disease presentation. determine the probabilty for an affected child. determine the probability the child will inherit the affected allele

A

autosomal recessive disorder

  1. R=wild
  2. r=recessive
  3. 25%
  4. 75%
32
Q

diagram and formulate the general concepts of inhertance probaility

A
33
Q

union between individuals who are second cousins or closer

A

consanguinity

  1. a factor that increases the incidence of autosomal recessive disorders
34
Q

what are two factors that increase the incidence of autosomal recessive disorders?

A
  1. consanguinity
    1. second cousin or closer
  2. founder effect
    1. isolated population
    2. inbreeding
35
Q

what is the term descriibing the retention of a variant allele at a high frequency?

A

founder effect - autosomal recessive genes are seen at higher penetrance, due to gene selection

  1. isolated population
  2. inbreeding
36
Q

what are the 5 autosomal recessive disorders associated with the founder effect?

A
  1. ellis-can crefeld syndrome-old amish
    1. dwarfism
    2. polydactily
  2. I-cell disease-quebec
    1. lysosomal storage disease
  3. type 1 tyrosinemia-quebec
    1. deficient Tyr degradation
  4. Tay-Sachs disease- ashkenazi jews
    1. shingolipidosis, lysosomal storage diseases
  5. Gaucher disease- ashkenazi jews
    1. shingolipidosis, lysosomal storage diseases
37
Q

How many alleles is necessary to maintain homeostasis in an autosomal recessive disease? what are these individuals?

A

One wild type allele in these cases is sufficient to maintain homeostasis, but they are not as well as wild type.

These individuals are heterzygotes, if its on the X and the person in male=hemizygote

38
Q

list four autosomal recessive diseases, generated from a loss of function mutation.

A
  1. autosomal recessive disorders
    1. diseases dur to loss of function mutations (such as enzyme deficinecies ) are mostly inherited by autosomal recessive manner except if the gene is on the C chromosome.
39
Q

categorize the genotype

A
40
Q

compare autosomal recessive vs Xlinked

A

autosomal recessive

  1. both genes need to present with tthe defective allele

Xlinked

  1. disease is cuased by a mutation in the X chromosome. In males, one altered gene in each cell can lead to disease manifestation
41
Q

list four well known autosomal recessive disorders related to loss of function mutations

A
  1. cystic fibrosis
    1. chloride ion channel deficiency
  2. sickle cells disease
    1. hemoglobin mutation
  3. phylketourina
    1. pheylalanine metabolism deficiency
  4. Tay-Sachs, Gaucher diseases
    1. lysosomal glycolipid degradation deficiency
42
Q
A

Autsomal dominanat disorders

  1. highlights
    1. phenotype usually occurs in every generation
    2. one parent of an affected child usually is affected
      1. except nonpenetrant cases and new mutations
    3. equally affects males and females
43
Q

a heterozygous motther and homozygous recessive father have a child. Draw the punnet square and state the probability for child carrying and being affected by an autosomal dominant disorder.

A

autosomal dominant disorders only need one copy of the gene to show a phenotype. So, heterozygous and homozygous dominant are affect. = no carriers.

probability is at 50% for the child to be affected

44
Q

the phenotype in heterozygous is still present but is less severe than in homozygotes.

give an important clinical correlate

A

incomplete dominance

achondroplasia

  1. gain of function mutation in fibroblast growth factor receptor 3
45
Q

what disease and type of inheritance is associated with a gain of function mutation in fibroblast growth factor receptor 3

A

incomplete dominant inheritance- phenotyp in heterozygotes is less severe that in homozygotes

  1. achondroplasia
    1. heterozygotes
      1. abnormal bone growth
      2. short stature
      3. large head, characteristic face
      4. usually normal intelligence
      5. usually normal life span
    2. homozygotes
      1. more severe bone deformities
      2. early death
46
Q

Achondroplasia affects the following family. Explain the pedigree

A

solid blue = affected = heterzygous

line through it = incompatable with life = homozygous dominant

47
Q

show a pedigree with a new mutations in autosomal dominant inhertiance

A

new mutations are frequent causes of autosomal dominant diseases.

  1. new mutations occured in the gamete of one of the parents
48
Q

about 50% of neurofibromatosis cases are due to

A

new mutations in autosomal dominant inheritance

49
Q

neurofibromatosis type 1

  1. explain how affected parents can produced offspring that are unaffected
A

affected individuals can have unaffected parents if the penetrance is not 100%

50
Q

a patient appears with cafe au lait spots and is conscerned about them. Should this person be worried?

A

less obvious phenotypes might misdiagnose a disease. This has been associated with neurofibromatosis type 1

  1. new mutations could occur in the parents ~50% of cases
  2. not 100% penetrance
  3. may be age dependent
  4. phenotype-expressivity
    1. usual
      1. cafe aua lait spots
      2. fleshy benign tumors on skin (neurofibromas)
      3. small benign tumors on the iris of eye
    2. more sever (less frequent)
      1. learning disabilityes
      2. CNS tumors
      3. malignant peripheral nerve sheath tumors
51
Q

list seven autosomal dominant diseases

A
  1. familial hypercholesterolemia (LDL-receptor mutation)
  2. achondroplasia (dwarfism
  3. Huntington disease(neruodegenerative disorder)

below are hereditray tumors/cancers

  1. hereditary breast and ovarian cancer
  2. familial adenomatoru polyposis (colon cancer)
  3. retinoblastoma
  4. neruofibromatosis type 1
52
Q

what is the difference between narrow expressivity and variable expressivity?

A
53
Q

much higherincidence in males and heterozygous females may be affected.

A

x linked recessive disorders

54
Q

what type of inheritance pattern does this disease have? Draw a punnet square and list the probabilities for carrier and affected.

A
  1. X-linked recessive disorders
    1. disease
      1. males
      2. heterzygous females
    2. no male to male transmission
    3. daughters of affected fathers are obligate carriers
    4. isolated cases can be caused by new mutations
55
Q

describe the inheritance pattern and what happens to the cousins offspring

A
  1. consanguinity
    1. increases the chances for a female to become homozygous and express the phenotype
    2. notice the father generates obligate daughter carriers
      1. wildtype boys mate generating
        1. one male affected
        2. one female carrier
          1. 25% carrier (including boy and girl)
          2. 50% chance female carrier
          3. 50% affected boy
          4. 0% carrier boy
  2. manifesting female heterozygotes
    1. if the normal allele is inactivatd during X-inactivation, than the female will express only the variant allele in some of her cells (mosaicism) and will have the phenotype (usually less severe)
56
Q

explain how a female can be affected by an X-linked inherited disease

A

manifesting female heterozygotes

  1. if the normal allele is inactivated during X-inactivation, than the female will express only the variant allele in some of her cells (mosacism) and will have the phenotype (usually less severe)
57
Q

what are four well known X-linked recessive diseases. Explain there phenotype

A
  1. hemophilia A and B
    1. coagulation factor 8 and 9 deficiency)
  2. glucose 6-phosphate dehydrogenase deficiency
    1. acute hemolytic anemia
  3. ornithine transcarbamoylase deficiency
    1. urea cycle
  4. duchenne muscular dystrophy
    1. dystrophin gene defects
58
Q

what type of inheritence pattern is seen in the following pedigree.

what can be seen in the females for these inherited disorders?

A

X-linked dominant disorders

  1. what tto look for in the pedigree
    1. no male to male = x-linked
    2. females are affected, NOT CARRIERS
      1. this makes it dominant
    3. affected females transfer to male and female children
      1. frequency is twice as high for occuring in females compared to males
  2. in affected females
    1. some cells inactivate the mutant gener (mosaicism), so the females will have MILDER SYMPTOMS
59
Q

generate a punnett square for an unaffected father and a mother with Rett syndrome.

  1. what is the probability for their child to be affected?
  2. what is the probability for their daughter to be affected/carrier?
  3. what is the probability for their son to be affected/carrier?
  4. what is tthe probability for theri child to be a carrier?
A

rett syndrome

  1. leathal in males
  2. females probably survive this condition because of X-inactivation
  3. developmental and intellectual disability, characteristic flapping hands.
  4. what is the probability for their child to be affected?
    1. 50%
  5. what is the probability for their daughter to be affected/carrier?
    1. 50%/0%
  6. what is the probability for their son to be affected/carrier?
    1. 0%/0%- lethal
  7. what is the probability for their child to be a carrier?
    1. 0%
60
Q

a child presents with the follow ing signs and symptoms, what should the child be tested for?

female: developmental stagnation, intellectual disability, flapping movement of the hands and gastrointestinal issues.

Describe the inheritance pattern.

How is the mother not affected?

A

rett syndrome

61
Q

inheritance of variant alleles in the pseudregions of sex chromosomes.

what is an example of this inheritance pattern?

A

pseudoautosomal inheritance

inheritance of variant alleles in the pseudoautosomal regions of the sex chromosomes (Xp and Yp)

  1. what to look for in the pedigree
    1. male to male transfer
      1. distinguishing feature from x-linked inheritance
      2. due to meiotitc recombination between the pseudoautsomal regions on the X and Y chromosomes
62
Q

what type of inheritance pattern is involved with the pseudoautosomal regions on the short arm of chromosomes X and Y?

A
63
Q

whatt is the presence of cell populations with different genetic make up in an individual?

when does the mutation take place?

A

mosaicism

  1. the presence of cell populations with different genetic make up in an individual
  2. mutation takes place before or after conception
    1. before, X-inactivation- female only
    2. after, male and female
64
Q

explain the difference between somatic mosaicism and germline mosaicism

A
  1. somatic
    1. can affect large segments of the body (segmental neuro fibromatosis)
    2. the cause of sporadic cancers
    3. cannot be inherited ( a single affected individual in pedigrees)!!!!
  2. germline
    1. produced in germline during development
    2. can be inherited(the disease passed down in thep edigree
65
Q

explain the inheritence pattern

A
  1. germline masaicism
    1. same father, different children
    2. the father does not have the mutation in his somatic cells
    3. the father must have germline mosaicism
      1. the germline cells developed a mutation that leads to the disease
66
Q

what is the bases of inheritance?

A

reproduction

67
Q

transmission of a phenotype to future generations depends on the _________ ________.

explain

A

transmission of a phenotype to future generations depends on the REPRODUCTIVE FITNESS

  1. if the phenotype involves infertility or death before reproductive age, it cannot be transmitted
68
Q

what is the effect of reproductive fitness on inheritance and pedigrees?

A

reproduction = basis of inheritance

transmission of a phenotype to future generations depends on the reproductive fitness = if the phenotype involves infertility or death before reproductive age, it cannot be transmitted

2020 MHS Genetics Unit2 (5 decks)

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