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genetic inheritance > Genetic Inheritance > Flashcards

Genetic Inheritance Flashcards

1
Q

Genes

A

Segments of DNA in a chromosome, Each gene occupies a specific place, or locus (plural, loci)

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2
Q

Chromatid

A

One of two identical copies of a chromosome.

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3
Q

Centromere

A

Connects identical sister chromatids.

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4
Q

Telomere

A

Is a region at the end of a chromosome for stability.

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5
Q

Human somatic cells

A

Diploid cells that are differentiated. There are over 200 types of differentiated human somatic cells.

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6
Q

Gametes

A

Are haploid

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7
Q

Stem cells

A

Undifferentiated cells that can divide into two diploid cells..

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8
Q

Homologous chromosomes

A

Refer to pairs of chromosomes.

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9
Q

karyotype

A
  • Entire set of a patient’s chromosomes.
  • 46 chromosomes shown in the karyotype are in each of the patient’s diploid cells, unless mosaicism occurs.
  • Normal human karyotype is written 46XY.
  • Each chromosome in the karyotype is presented as a single condensed or as a duplicated chromosome.
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10
Q

Mosaicism

A 
Is a condition in which cells from a patient have different genotypes (& karyotypes):
- Downs Syndrome: some 
  46XX; some 47XX,+21   
- Klinefelter Syndrome: some 
  46XY; some 47XXY
- Turner Syndrome: some 
  46XX; some 45XO
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11
Q

Lyonization

A

Is called X- inactivation. The choice of which X chromosome to be inactivated is random.

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12
Q

Interphase

A

Chromosome duplication (E.C.C)

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13
Q

Cell division

A

One copy of each chromosome and 1⁄2 of the cytoplasm/organelles are distributed between the two daughter cells. (E.C.C)

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14
Q

Meiosis

A

Reduces the total number of chromosomes by half, producing four gametes.
Consists of: one round of DNA replication and two rounds of nuclear divisions.

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15
Q

Homologous recombination (meiosis)

A

Can produce new combinations of genes.

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16
Q

Non-disjunction

A

Is the failure of one or more pairs of homologous chromosomes, or sister chromatids, to separate normally during division.

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17
Q

Autosomes

A

Chromosomes common

in both genders, one from each parent.

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18
Q

Sex chromosomes

A
  • X, female

- Y, male

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19
Q

Meiosis creates genetic diversity in 2 ways:

A
  • Random segregation of
    homologs.
  • Cross-over exchange.
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20
Q

Nondisjunction

Meiotic Errors

A 
Homologs fail to separate properly.   
- Common and increase with 
  advancing maternal age.
- Cause of spontaneous 
  abortions and mental 
  retardation.
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21
Q

Aneuploid (Meiotic Errors)

A 
Cells with abnormal chromosome number.
- Trisomy 21 (Down 
  syndrome): most common 
  cause of mental retardation.   - In 90% of trisomy 21 
  patients, the additional 
  chromosome is maternal.
   - 70% occur during MI   
   - 30% in MII
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22
Q

Euploid (Meiotic Errors)

A

Cells with normal number of chromosomes.

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23
Q

Genomic Imprinting

A 
- Imprinted genes = 
  methylation = down 
  regulated.
- Two chromosomes from 
  same parent that have 
  parent-specific imprinting = 
  no gene product (Deletion of non-imprinted genes).
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24
Q

Two different outcomes of deletion on chromosome 15.

A 
- If paternal chromosome is 
  deleted = Prader Willi 
  syndrome.
- If Maternal chromosome 
  deleted = Angelman 
  syndrome.
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25
Q

Pedigree analysis

A

Mechanisms of inheritance from family history.

26
Q

Gene defect

A

Etiology of a disease.

27
Q

Molecular diagnosis

A

Disease from a patient’s tissue/fluid samples.

28
Q

Personalized medicine

A

Optimal course of treatment.

29
Q

Population genetics

A

Allele and disease frequency.

30
Q

Genotype

A
  • An individual’s genetic
    makeup.
    EX: Individuals with distinct
    genotypes can have a
    single phenotype
    (Cystic Fibrosis).
31
Q

Phenotype

A 
- What is actually observed.
  EX: Individuals with the 
        same genotype can 
        have multiple 
        phenotypes (PKU).
32
Q

Pedigrees

A 
- Proband (propositus): First 
  diagnosed person in the 
   pedigree.
- Arrow denotes the 
  proband.
- (Slide 22 & 23)
33
Q

Autosomal Dominant Inheritance

A 
- Only 1 allele of a gene is 
  needed for expression.
- Affected offspring has one 
  affected parent.
- Unaffected individuals do 
  not transmit trait.
- Males and females can 
  transmit trait to both males 
  and females – autosomal.
- Trait is expected in every 
  generation
- Recurrent risk is 50%
EX: Postaxial Polydactyly
34
Q

Punnet Square

for Autosomal Dominant Inheritance

A 
- Affected female mates with 
  unaffected male. 
- Combinations of alleles 
  from the fertilization of egg 
  by sperm.
  (Slide 25)
35
Q

Autosomal Dominant: Pedigree

A 
- Affected offspring have 
  one affected parent.
- Unaffected individuals do 
  not transmit trait.
- Both males and females 
  can transmit trait to both 
  males and females.
- Trait is expected in every 
  generation at 50%.
  (Slide 26)
36
Q

Autosomal Recessive Inheritance

A
  • 2 copies of a gene is
    needed to influence
    phenotype.
    EXAMPLE: TYROSINASE-NEGATIVE ALBINISM
37
Q

Punnet Square

for Autosomal Recessive Inheritance

A 
- Carrier female mates with 
  carrier male.
- Resulting combinations of 
  alleles from the fertilization 
  of egg by sperm.
  (Slide 28)
38
Q

Autosomal Recessive: Pedigree

A 
- Affected individuals have 
  normal parents
- Recurrent risk for 
  heterozygote parents is 
  25%.
- Both males and females 
  may be affected.
- Affected individuals who 
  mate with normal 
  individuals tend to have 
  normal children.
- Occurrence is more likely 
 among individuals who 
 share genes, as with 
 consanguinity (first cousin 
 mating).   (Slide 29)
39
Q

X-linked Recessive

males, one X females, two X

A 
- Disease allele on X in males 
  is termed “hemizygous.”
- Females can be 
  heterozygous or 
  homozygous.
- Unaffected males don’t 
  transmit the trait (no 
  carriers)
- Female carriers transmit the 
  disease allele to 50% of 
  sons and 50% of daughters
- All daughters of affected 
  males are heterozygous 
  carriers.
EXAMPLE: DUCHENNE MUSCULAR DYSTROPHY
40
Q

X-linked Dominant

A
  • Very rare; no carriers.
  • Males with the disease allele
    transmit the trait:
    • only to females
    • 100% transmission
  • Females with the disease
    allele transmit the trait:
    • To both males and females
    • 50% transmission to
      offspring
      EXAMPLE: HYPOPHOSPHATEMIA
  • Low phosphorus in blood due
    to defective reabsorption of
    phosphate in kidney Deficient
    absorption of calcium in
    intestines causes softening of
    bone (Rickets)
  • Vitamin D metabolism
    abnormal Short stature
  • Incidence: 1/60,000
    Treatment: oral phosphate &
    vitamin D
41
Q

Reduced Penetrance

A 
- The frequency a gene 
  manifests itself is called 
  penetrance
- In some cases, 100% of 
  individuals inheriting a 
  genetic defect show the 
  clinical presentation 
  (phenotype) of the disease 
  (100% penetrance)
- In other cases penetrance is 
  less than 100%
EXAMPLE: RETINOBLASTOMA:
autosomal dominant inheritance   Phenotype occurs in 90% of
individuals inheriting gene defect; so 90% penetrance.
42
Q

Variable Expressivity

A 
- Term used to describe the 
  range of phenotypes that vary 
  between individuals with a 
  specific genotype.
EXAMPLE: NEUROFIBROMATOSIS
 - Develop tumor-like growths 
   called neurofibromas
 - Patients have café-au-lait 
   spots – pigmented areas the 
   color of coffee with cream 
   (spots differ in number, 
   shape, size and position).
43
Q

Locus Heterogeneity

A 
- Single disorder, trait, or 
  pattern of traits caused by 
  mutations in genes at 
  different chromosomal loci
EXAMPLE:
OSTEOGENESIS IMPERFECTA
  - Brittle-bone disease.   
  - Mutations in
    collagen genes (two loci: 
    chromosome 7 and 17), 
    either mutation exhibits 
    the same phenotype.
44
Q

Basic Concepts of Probability

A 
Probability helps us:
 - Understand transmission 
   of genes thru generations.  
 - Analyze genetic variation 
   in populations.
 - Conduct risk assessment 
   (for genetic counseling)
45
Q

Probability

A 
- Is defined as the proportion 
  of times that a specific 
  outcome occurs in a series 
  of events.
- As proportions, 
  probabilities are between 0 
  and 1.
EXAMPLE: 
 - coin tossing: the probability 
   of ‘Heads’ is 1⁄2, of ‘Tails’ is 
   1⁄2
- meiosis: the probability that 
  a given member of a pair of 
  chromosomes will be 
  transmitted is 1⁄2, the other 
  is 1⁄2.
- Gender: the probability of 
  producing a girl is 1⁄2, a boy 
  is 1⁄2.
46
Q

INDEPENDENCE PRINCIPLE

A

Multiplication Rule & Addition Rule

47
Q

MULTIPLICATION RULE

A

Probability of a given outcome in multiple trials is the product of the probabilities of each trial outcome.
EX: What’s the probability of producing three girls? 1⁄2 x 1⁄2 x 1⁄2 = 1/8 What’s the probability of producing three boys? 1⁄2 x 1⁄2 x 1⁄2 = 1/8

48
Q

THE ADDITION RULE

A

Probability of either one outcome or another is the sum of the two probabilities.
EX: Probability of producing either three girls or three boys? 1/8 + 1/8 = 1⁄4

49
Q

Gene & Genotype Frequencies

A 
- Measure and understand 
  population variation in the 
  incidence of genetic 
  disease.
- Under simple conditions 
  these frequencies can be 
  estimated by direct 
  counting.
50
Q

Gene Frequencies

A

Specify the proportions of each allele in a population.

51
Q

Genotype Frequencies

A

Specify the proportions of each genotype in a population.

Slide 39

52
Q

Hardy- Weinberg Principle

A 
- Specifies the relationship 
  between Gene Frequencies 
  and Genotype Frequencies
- Useful in estimating Gene 
  Frequencies from Disease 
  Prevalence Data and in 
  estimating the incidence of 
  heterozygous carriers of 
  recessive disease genes
 - In the previous example 
  (MN blood group), due to 
  co-dominance, the three 
  genotypes can be easily 
  distinguished, blood-typed 
  and counted.  (Slide 40 & 41)
53
Q

Cystic Fibrosis

A 
- In recessive disease, only 
  the affected homzygotes, 
  with genotype aa, are 
  distinguishable
- H-W tells us that the 
  frequency of aa should be 
  q2.
- The incidence of CYSTIC 
  FIBROSIS (in European 
  population) is 1/2500 = q2.
- Therefore, q =  1/2500 = 1/50 
   = 0.02
- Because p + q = 1, then p = 
  0.98 
- 2pq equals about  1/25, 
  suggesting a lot of recessive 
  disease alleles are 
  effectively “hidden."
54
Q

Autosomal Dominant Inheritance

A

Is characterized by vertical transmission of the disease phenotype, a lack of skipped generations, and roughly equal numbers of affected males and females. Father-to- son transmission may be observed.

55
Q

Autosomal Recessive Inheritance

A

Is characterized by clustering of the disease phenotype among siblings, but the disease is not usually seen among parents or other ancestors. Equal numbers of affected males and females are usually seen, and consanguinity may be present.

56
Q

Consanguineous

A 
- Mating’s that are more 
  likely to produce offspring 
  affected by rare 
  Autosomal Recessive 
  Disorders.
- Studies show that 
  mortality rates among the 
  offspring of first-cousin 
  mating's are up to 9% 
  higher than those of the 
  general population.
57
Q

Multifactorial Inheritance: Basic Model

A 
- Traits in which variation is t 
  thought to be caused by the 
  combined effects of multiple 
  genes are called 
  POLYGENIC (“many genes”)
- When environmental factors 
  cause variation in the trait, 
  the term MULTIFACTORIAL 
  is used.
- Because these traits are 
  caused by the additive 
  effects of many genetic and 
  environmental factors, they 
  tend to follow a normal, or 
  bell-shaped, distribution in 
  populations.  (Slide 45)
58
Q

Multifactorial Inheritance: Threshold Model

A 
- For diseases that do not 
  follow the bell- curve 
  distribution there is an 
  underlying liability 
  distribution.
- For multifactorial diseases 
  that are either present or 
  absent, it is thought that a 
  threshold of liability must 
  be crossed before the 
  disease is expressed.
- Below the threshold, the 
  person appears normal
- Above the threshold, the 
  person is affected by the 
  disease.  (Slide 46)
59
Q

Pyloric Stenosis

A 
- Muscular hypertrophy 
  between stomach and 
  duodenum – leading to 
  vomiting and obstruction
- Five times more common in 
  males than females.
- Males need less risk genes 
  to show disease; females 
  need more risk
  genes.
- The least affected sex has a 
  higher risk threshold and 
  transmits the
  condition more often to the 
  most frequently affected 
  sex.
  - Children of women with 
    pyloric stenosis are more 
    likely to be born with
    condition (especially 
    males)
  - Children of affected males 
    with pyloric stenosis are 
    less likely to be born with
    condition.
60
Q

Recurrence Risks

And Transmission Patterns

A 
- In contrast to most single-
  gene diseases, recurrence 
  risks for multifactorial 
  diseases can change 
  substantially from one 
  population to another - This 
  is because gene 
  frequencies as well as 
  environmental factors can 
  differ among populations
  › The recurrence risk is 
    higher if more than one 
    family member is affected
 › If the expression of the 
   disease in the proband is 
   more severe, the 
   recurrence risk is higher
 › The recurrence risk is 
   higher if the proband is of 
   the less commonly 
   affected sex
 › The recurrence risk for the 
   disease usually decreases 
   rapidly in more remotely 
   related relatives
61
Q

Multifactorial vs Single- Gene Inheritance

A 
- Multifactorial Disease is 
  caused by the simultaneous 
  influence of multiple 
  genetic and environmental 
  factors.
- In some cases, a trait may 
  be influenced by the 
  combination of both a 
  single gene with large 
  effects and a multifactorial 
  background in which 
  additional genes and 
  environmental factors have 
  small individual effects.
  (Slide 48)

genetic inheritance (1 decks)

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