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Medicine 1 > 7. obsterics genetics > Flashcards

7. obsterics genetics Flashcards

1
Q

Define autosomal dominant

A

heterozygotes with one copy of abnormal gene are affected

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2
Q

define autosomal recessive

A

homozygotes with two copies of the abnormal gene are affected

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3
Q

define x linked recessive

A

males with one copy of the abnormal gene of X chromosome are affected

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4
Q

rarer inheritance patterns

A

x linked dominant mitochondrial inheritance

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5
Q

x linked recessive

A
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6
Q

autosomal recessive inheritance carrier risk rate

A

2/3 sibling carrier risk

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7
Q

what type of inheritance if CF and what is the carrier rate in a general population

A

AR

1 in 25

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8
Q
A
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9
Q

What do you do if you don’t know the carrier rate for a recessive condition?

A

Hardy-Weinburg principle

•The Hardy-Weinberg principle allows the calculation of carrier rates once the incidence of a condition is known as long as the gene frequency is in equilibrium

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10
Q

The Hardy-Weinberg Principle

A
  • In a large, randomly mating population the relative proportion of different genotypes remains constant
  • This only holds true if there are no outside influences eg selection or assortative mating
  • If you have 2 alleles for an autosomal condition: A and a, with a frequency of p and q, p+q=1

p2 = homozygous unaffected

2pq = carriers

q2 = affecteds

p2+ 2pq+q2= 1

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11
Q

type of inheritance of phenylketonia and carrier rate

A

AR

1/50

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12
Q

Factors that can disturb the Hardy-Weinberg Principle

A
  1. Non-random mating: Assortative mating – tendency to choose a mate with similar characteristics eg height, IQ
    * *2. Consanguinity** – relationships between close relatives can lead to an increased carrier risk within a family
  2. The mutation-selection equilibrium: New mutations are arising all the time. Different genes have different new mutation rates according to their size and structure

Usually this is balanced by loss of alleles due to reduced reproductive fitness in affected individuals

An alteration in this balance will affect the equilibrium

  1. Selection (heterozygote advantage)
    For some AR conditions, carriers seem to have a reproductive advantage. This has lead to certain genes being very common in a particular population
  2. Small population size (the founder effect)
    One allele can be transmitted to a large proportion of children purely by chance leading to an increased incidence of a certain condition in a population

6. Migration (gene flow)
Migration and intermarriage can introduce new alleles into a population

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13
Q

Targeted tests - non-invasive

A
  • Ultrasound scan- Surveillance eg growth, 3D scan, Fetal echocardiogram
  • Fetal MRI scan
  • Molecular testing on fetal cells/DNA in maternal blood for gender or other conditions
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14
Q

Targeted tests - invasive

A
  • Chorionic villus sampling (CVS)
  • Amniocentesis
  • Cordocentesis (Fetal blood sampling)
  • Fetal tissue biopsy

–liver

–muscle

–skin

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15
Q

Chorionic Villus Sampling

A

11-13 weeks

transabdominal approach

Chromosomes

  • Direct preparation - 1-2 days
  • Long-term preparation

–results in 2 weeks

Good yield of fetal DNA for molecular tests

1% miscarriage risk (0.5-1%)

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16
Q

Amniocentesis

A

15-16 weeks gestation

Amniocytes derived from fetal tissues and amnion

Both amniocytes and supernatant can be used for prenatal tests

Chromosome analysis takes 2-3 weeks

Low yield of fetal DNA, 1% miscarriage risk

17
Q

Preimplantation Genetic Diagnosis (PGD)

A

testing embryos produced by IVF for inherited disorders- embryos that are free of the disorder to be replaced.

Suitable for couples at substantial risk of transmitting a serious genetic condition to their children.

  • chromosome translocations
  • x-linked disorders
  • some single gene disorders

Not available for all conditions.

Requires considerable preparation – clinical, counselling, laboratory

18
Q

Polygenic inheritance

A
  • Polygenic inheritance: inheritance controlled by many genes with small additive effects
  • Genes at many different loci contribute to the phenotype
  • No one gene is dominant or recessive to another – there is a cumulative effect
  • Remember, in locus heterogeneity, more than one gene leads to the same phenotype
19
Q

Multifactorial inheritance

A

• is where a condition is cause by the interaction of multiple genes and the environment

20
Q

liability-threshold model

A
  • The sum of environmental influences and genetic predisposition gives a persons liability to be affected
  • If the threshold is exceeded the condition results
21
Q

Recurrence risks:

A

•empiric figures from population studies

Golden rules:

  1. relatives of an affected person have a greater genetic tendency to that condition, so their curve is shifted and more individuals cross the threshold
  2. The risks are greatest for close relatives
  3. The risk is greater for relatives when the person is more severely affected – eg severe cleft lip and palate or long segment Hirschsprung disease
  4. The risk is higher if more than one relative is affected eg 3 relatives with coronary artery disease rather than 1
  5. The risk is higher if a child of the lower risk gender is affected eg a girl with autism as more boys are affected by this condition or pyloric stenosis - the risk for the brother of an affected girl is 9.2%, but for the brother of an affected boy is 3.8%
  • Even someone with a high genetic liability will not be affected if the environment is favourable
  • Recurrence risk data collected in one population may not apply to another as the risks are dependent on the incidence of the condition in that population
22
Q

mendelian inheritance

A

controlled by a single locus in an inheritance pattern.

(a disease controlled by a single gene)

23
Q

Neural tube defects

genetic and environment

A

Celtic origin

Specific variant in MTHFR gene
(methylene-tetrahydrofolate reductase)

Poor socioeconomic status

Multiparity

Valproic acid embryopathy

Periconceptual folic acid reduces recurrence risk

24
Q

How is trisomy 21 screened for

A

routine prenatal screening

ultrasound scan

nuchal translucency scan (increased)

-Thickness of NT measured by ultrasound scan in relation to crown-rump length

Biochemical serum screening in second trimester (not for diagnosis)

•Reduced MSAFP levels alpha feto protein

25
Q

Indications for targeted prenatal tests

A
  1. personal and family history

targeted and diagnostic screening tests

  1. abnormalities identified in pregnancy

abnormal screening test result/ scan

Medicine 1 (53 decks)

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