Modes of Inheritance Flashcards

1
Q

Describe DNA in bacteria

A
  • Bacteria usually have one DNA chromosome (unlike human cells which generally have 23 pairs of chromosomes)
  • Asexual reproduction – one bacteria divides into two
  • Their genomes encodes ~ 4000-6000 genes
  • Each gene encodes for an mRNA
  • Each mRNA encodes one protein
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2
Q

What is homozygous and heterozygous?

A
  • Someone with two identical copes of an allele is homozygous for that gene
  • Someone with two different alleles for a particular gene is heterozygous
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3
Q

What is a single position?

A

A locus

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4
Q

What doe each gene have?

A

alleles

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5
Q

How many genes, chromosomes, autosomal pairs do we each have?

A
  • Human genetic material encoded on chromosomes
  • 25000 genes
  • 46 chromosomes (23 pairs)
  • 22 autosomal pairs
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6
Q

What are sex chromosomes?

A
  • Sex chromosomes XX and XY

* DNA obtained from the father and mother

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7
Q

What are human genetic disorders ?

A
  • Sometimes a human has genes that are mutated or missing from their make-up altogether. In such cases, the missing or abnormal genes can cause a human genetic disorder
  • Single gene Disorders (Focus of this lecture)
  • 4000 different types
  • Most common approximately 1/500 to very rare per disease
  • BUT they affect a large number of people and affect all body systems and hence all specialties
  • Chromosomal Disorders
  • 1/150 babes
  • Complex Disorders
  • Inherited and environmental factors
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8
Q

What is dominant autosomal?

A

•A characteristic is dominant if it manifests in a heterozygote (ie two different alleles at a locus)

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9
Q

Describe dominate autosomal disorders

A
  • Single gene/allele disease or trait
  • Disease passed down to offspring with multiple generations affected
  • Each affected person normally has one affected parent
  • Each child of an affected person has a 1 in 2 chance of being affected
  • Males and females are equally affected and equally likely to pass on the condition
  • Vertical pedigree pattern
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10
Q

What is Huntington’s disease?

A

Huntington’s disease background: 1/20000 in UK
•Symptoms usually start 30-50 years of age
•Difficulty concentrating, depression, stumbling, involuntary jerking, problems swallowing
Mutation
•Results from a DNA expansion of a CAG repeat on chromosome 4
Result
•Abnormal intracellular Huntington protein aggregate gains a pathological function and is toxic to neurons resulting in cell death

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11
Q

What is Osteogenesis imperfecta?

A

Osteogenesis imperfecta – brittle bone disease: 1/15000
•Group of genetic disorders mainly affecting bones
•Bones break easily
•Mild to severe
•Hearing loss, breathing problems, short height, blue tinge to whites of eye
Mutations
•Type I – insufficient quantities of Collagen
•Type II, III and IV mutation of gene results in an abnormal protein has an altered structure and interferes with the function of the normal protein (expressed from the normal gene)
Result
•Weakening connective tissue particularly bone

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12
Q

What do dominant autosomal disorders tend to be?

A
  • Gain-of-function- gene now makes a protein with a new function eg longer lifespan/new location thus increasing their effect
  • Dominant negative effect- the mutated form interferes with the activity of proteins it binds eg dimers or multimers which reduces activity
  • Insufficient- mutant in one gene results in ½ the amount of a protein that is not enough for normal function
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13
Q

What re carriers of autosomal recessive disorders?

A

•Carriers of recessive diseases have lost a single copy of a gene but the normal one is sufficient to maintain normal function

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14
Q

What are autosomal recessive disorders?

A
  • Recessive means that two copies of the abnormal (non-working) gene must be present in order for the disease or trait to develop
  • Tend to be ‘loss of function’ mutations (eg deletions)
  • Parents and children of affected people are normally unaffected
  • One of more siblings affected
  • Each subsequent sibling of an affected child has a 1 in 4 chance of being affected
  • Males and females equally affected
  • A horizontal pedigree pattern
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15
Q

What is cystic fibrosis?

A
Cystic fibrosis 1/3000 newborns
•Failure to thrive
•Defective chloride ion channel
•Results in impaired airway defense
•Prone to respiratory infections
•Digestive issues eg meconium ileus
•Largest cohort of chronically infected patients 
Mutations
•Various mutation in gene encoding chloride ion channel (CFTR gene on chromosome 7)
Result
•Defective chloride ion channel
•Loss of function, work less well, degraded faster or present in inadequate amounts
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16
Q

What does recessive mean?

A

Recessive means that two copies of the abnormal (non-working) gene must be present in order for the disease or trait to develop

17
Q

What are consanguineous marriages?

A
  • One fifth of the worlds population live in communities with preference for consanguineous marriages
  • Consanguineous marriages elevate the risk of autosomal recessive diseases
  • If the family has multiple consanguineous marriages affected individuals may be seen in several generations
18
Q

What are X and Y chromosomes?

A

•Consist of an X and a Y chromosome
•Determine the sex
Female: XX and Male: XY
•Sperm determine the sex of the progeny
•Y chromosomes only come from the father
•Any alternation on a gene on the X chromosome can have a more severe impact on a boy (as only one copy of X)

19
Q

What are X linked recessive disorders?

A
  • Affects mainly males
  • Females can be carriers and affected males are linked through females
  • Affected boys may have affected uncles
  • Females who are homozygous for the mutation (two copies) have the disorder or condition
  • Parents and children of affected people are most commonly unaffected
  • X-linked recessive
  • Brothers of affected son have a 1 in 2 risk of having disorder
  • Sisters 1 in 2 chance of being a carrier (see example)
  • All daughters of a man with an x-linked disorder will be carriers as men only have one X chromosome
  • All sons will be healthy as inherit the Y from father
20
Q

What is an example of X linked disorder recessive disease?

A
  • Haemophilia – More frequent or severe bleeds, different levels of severity, factors VIII or IX
  • In some cases female carriers exhibit subtle signs of the disease eg Fabry’s Disease
21
Q

What are X linked dominant disorders?

A
  • X-linked dominant
  • Similar to autosomal dominant pattern (seen in both sexes)
  • BUT all daughters and no sons of an affected father are affected
  • Condition often milder and more variable in females than in males
22
Q

What is an example of X linked dominant disorders?

A
  • Example: X-linked hypophosphatemia
  • Kidneys can not retain phosphate
  • Results in vitamin D-resistance rickets
  • XLH gene mutation resulting in inactivity of PHEX protein
23
Q

What are Y linked disorders?

A
  • Affects only males
  • All sons of an affected father
  • Vertical pedigree pattern
24
Q

What are an example of Y linked disorders?

A

Example: Retinitis Pigmentosa
Mutation in RPY (retinitis pigmentosa, Y-linked) gene
Cells of retina produce a defective protein

25
Q

What are mitochondria?

A
  • Specalised organelle of eukaryotic cells
  • The ‘power house of the cell’ – primary function production of adenosine triphosphate (ATP)
  • Also store calcium and involved with cell signaling and cell death
  • Share a evolutionary past with bacteria
  • Have their own DNA but also use proteins encoded by nuclear genes
  • Many mitochondria in the majority of human cells (not present in red blood cells)
26
Q

What are mitochondria disorders?

A
  • Are maternally inherited
  • Mutations in mitochondrial DNA can affect function
  • Children of affected men are never affected
  • All children of an affected woman may be affected
  • However mitochondrial conditions are typically extremely variable even within a family
  • A vertical pedigree pattern
27
Q

Why can mitochondrial conditions be variable within a family?

A
  • Many mitochondria in each cell
  • All mitochondria are inherited from the mother
  • Random segregation
  • Severity of symptoms vary with:
  • amount of WT to mutated mtDNA
  • Severity of mutation
28
Q

What are examples of mitochondrial disease?

A

•Can present as unrelated multi-system symptoms
•Motor and nerve function common
•Example:
•Leber’s hereditary optic neuropathy (LHON)
•Visual loss in young adulthood, degeneration of optic nerve and retina
•Typically in males, occasionally in females
Mutations generally disrupt function of mitochondria (eg oxidative phosphorylation