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Karan - MBBS Year 1: Principles Week 4 > Genetic Variation and Disease II > Flashcards

Genetic Variation and Disease II Flashcards

1
Q

What are the benefits of next generation DNA sequencing?

A
  • More sensitive to detect previously unknown SNPs
  • Has made DNA sequencing cheaper
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2
Q

Outline the process of next-generation DNA sequencing. PART 1

A
  • DNA chopped at random into small fragments
  • Adapters attach to ends of genomic DNA fragments
  • Double stranded fragments separated into single strands and attached to solid surface e.g glass slide
  • Individual fragments amplified by PCR - adapters act as primer sequences
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3
Q

Outline the process of next-generation DNA sequencing. PART 2

A
  • SEQUENCING REACTION - fragments act as templates for synthesis of complementary strands
  • New complementary bases to which base specific cluorescent label attached, added one at a time
  • Fluorescent signal read using camera to reveal base pair sequence of each fragment
  • Fragments put together by aligned against a reference genome
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4
Q

What are the advantages of next-generation DNA sequencing?

A
  • Entire genome can be sequenced (however requires bioinformaticians)
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5
Q

How would gene sequencing within a child with epilepsy work?

A
  • SNPs within genes of interest identified
  • Usually an SNP which has been reported to be associated with epilepsy
  • If not, labelled as a variant of uncertain significance - identified by genetic testing but may or may not be significantly associated with disease or risk of disease.
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6
Q

What are alleles?

A
  • Two or more alternative forms of a gene occupying the same genetic locus
  • Carry 2 alleles at each gene locus - 1 maternally, 1 paternally inherited
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7
Q

Describe the Mendelian law of segregation.

A
  • Sexually reproducing organisms possess genes that occur in pairs
  • Only one member of this pair transmitted to offspring
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8
Q

What does Mendel’s law of segregation say about chromosomes during meiosis? PART 1

A
  • Genes on chromosomes segregate during meiosis
  • Transmitted as distinct entities from one generation to next
  • Two alleles for each trait separate during meiosis
  • Each gamete contains a random collection of paternally and maternally inherited chromosomes
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9
Q

What does Mendel’s law of segregation say about chromosomes during meiosis? PART 2

A
  • During fertilisation, alleles pair again
  • 4 possible combinations of alleles
  • INDEPENDENT ASSORTMENT - genes on different loci transmitted independently
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10
Q

What is the main difference between dominant and recessive inheritance?

A
  • One allele at a locus can mask another allele at same locus
  • Dominant allele exerts effect in homozygous and heterozygous forms
  • Recessive allele only exerts effect in homozygous form
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11
Q

Describe autosomal dominant disease transmission. PART 1

A
  • Occurs when an unaffected parent mates with affected heterozygote
  • Affected parent can either pass a normal or diseased allele therefore chance of child with disease is 0.5
  • Affects autosomes - so males and females affected equally
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12
Q

Describe autosomal dominant disease transmission. PART 2

A
  • No skipping of generations - if individual has disease, one parent must have it. Causes vertical transmission patterns
  • Inheritance is independent - recurrence risk of another affected child is still 50%
  • FATHER SON TRANSMISSION IS POSSIBLE
  • EXAMPLE: Huntingdon’s
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13
Q

Describe autosomal recessive inheritance

A
  • Most commonly both parents are heterozygous carriers
  • So 25% chance of offspring having disease
  • Usually observed in siblings but not normally in ealier generations
  • Males and females affected equally. Consanguinity sometimes seen.
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14
Q

Give an example of an autosomal recessive condition.

A

CYSTIC FIBROSIS

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15
Q

What are most dominant diseases more severe in?

A
  • More severe in homozygotes than in heterozygotes e.g achondroplasia
  • Dominant alleles produce disease in heterozygotes, recessive alleles don’t (due to being masked)
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16
Q

What does it mean for a disease to be X-linked?

A
  • Characteristic or traits of disease are derived from mutations or variants of an X gene
17
Q

Describe X-linked recessive inheritance. PART 1

A
  • EXAMPLES OF CONDITIONS are haemophilia A and red-green colour blindness
  • Females inherit 2 copies of X gene - need to be homozygous for diseased allele, to be affected
  • Most X-linked loci, only one copy of allele in somatic cell due to X-inactivation.
  • Half of cells in heterozygote female express the diseased allele, half express normal allele.
17
Q

Describe X-linked recessive inheritance. PART 2

A
  • Males inheriting recessive X-linked gene are affected
  • No normal X-linked allele to compensate
  • X-linked recessive diseases more common in males
  • Most common form of inheritance: Normal male, carrier female
18
Q

Are X-linked genes transmitted from father to son?

A

NO
- Only transmit Y genes
- X-linked genes pass from heterozygous normal females
- If a father is affected, gene is passed to all daughters who act as carriers - transmit gene to half of their sons who are affected

19
Q

Describe X-linked dominant inheritance. PART 1

A
  • More common in females - two X chromosomes - twice as likely to inherit diseased gene
  • Lethal in hemizygous males
  • Heterozygous females - milder expression X-linked traits. 50% chance of passing diseased gene to offspring
  • CONDITION: Hypophosphataemic rickets
20
Q

Describe X-linked dominant inheritance. PART 2

A
  • Severity varies in affected females - random X inactivation
  • Uncommon skipped generations
  • Father to son transmission not seen - transmit the Y gene
  • Only one X-linked diseased gene required to cause disease
21
Q

Describe X inactivation. PART 1

A
  • X chromosome contain many protein coding genes
  • Females have 2 copies and males have only 1 yet males and females do not differ in X-chromosome gene expression
  • One X chromosome in each somatic cell randomly inactivated during female embryonic development
22
Q

Describe X inactivation. PART 2

A
  • Dosage compensation occurs - X linked gene products produced in similar amounts to males
  • Two populations - one active maternally derived and one active paternally derived X chromosome
  • X chromosome remains inactive in all descendants of that somatic cell
  • Reactivated in germline - so egg cells receive one copy of active X chromosome
23
Q

Why do some X chromosomes not undergo inactivation?

A
  • Not inactive
  • Remain homologous on Y chromosome
  • Preserving equal gene dosage across males and females
  • If these sections become missing, cause diseased phenotype

Karan - MBBS Year 1: Principles Week 4 (13 decks)

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