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FLS > Inheritance > Flashcards

Inheritance Flashcards

1
Q

What can mutations involve?

A

Can involve:
A single gene
A chromosomal segment which affects thousands of genes.
Several genes acting with environmental influences.

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2
Q

What are single gene disorders?

A

Mutations in single genes by:
Autosomal dominance
Autosomal recessive
X-linked

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3
Q

What is a genotype?

A

A pair of alleles at a locus
e.g. AA, Aa, aA, aa, which then gives the phenotype.

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4
Q

What is a dominant allele?

A

A dominant alelle will determine a phenotype when only one copy is present in the genome of the individual.

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5
Q

What are autosomal chromosomes?

A

Chromosomes number 1-22, not the sex chromosomes.

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6
Q

What are examples of autosomal dominant disorders?

A

Achondroplasia - dwarfism, FGFR3 gene.
Marfan Syndrome - FBN1
Neurofibromatosis

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7
Q

What are features of autosomal dominant inheritance?

A

Affects every generation.
Male and female equally likely to be affected.
Inherited from one or other affected parent.
Can arise from a new mutation.

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8
Q

Why are heterozygous individuals seen only in autosomal dominant families?

A

The homozygotes are usually very severe phenotypes and are lethal.

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9
Q

What are autosomal recessive disorders?

A

A recessive allele has no effect on the organism’s phenotype if only one copy of that allele is present in the genome.
So there are carriers of recessive diseases.

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10
Q

What are examples of autosomal recessive disorders?

A

Primary haemochromatosis
Cystic fibrosis
Sickle cell disease
Phenylketonuria
Ataxia telangiectasia

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11
Q

What is cystic fibrosis?

A

Encodes transmembrane protein CFTR that transports Chloride ions.
Mutations disrupt chloride conductance.

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12
Q

How is the difference between a dominant and recessive mutant allele identified?

A

If the residual amount of normal gene product is sufficient to perform its function then the mutant allele and its disorder is recessive.
If insufficient, then it is dominant.

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13
Q

How does dominance of a mutation occur?

A

Reduced or increased gene dosage
Altered expression of mRNA
Increased protein activity
Dominant negative effects
Altered structural proteins
Gain of a new function.

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14
Q

What is reduced gene dosage?

A

This is where expression from one normal allele is not sufficient for function.

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15
Q

What is increased gene dosage?

A

Due to the presence of 3 copies of mutant gene.
E.g. Trisomy 21 - down syndrome
Marie-Tooth Type 1
Gene amplification

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16
Q

What is altered expression of mRNA?

A

Due to loss of control of regulation of mRNA expression.
Hereditary persistence of fetal haemoglobin.

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17
Q

What is increased protein activity?

A

Increased half life or loss of normal inhibitory regulation

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18
Q

What are dominant negative effects?

A

In the heterozygous state these mutants antagonise the activity of the remaining normal allele.
They are often multimeric proteins.

19
Q

What are altered structural proteins?

A

A mixture of normal and abnormal structural alterations will disrupt the whole structure.
E.g. Fibrillin in Marfans affects myosin heavy chains causing cardiomyopathy.

20
Q

What is gain of new protein function?

A

Chromosome translocations giving rise to new genes/proteins.
e.g. fusion proteins - in chronic myeloid leukaemia.

21
Q

What are the characteristics of X linked inheritance?

A

Higher incidence in males than females.
The gene responsible for the condition is transmitted from affected man to all daughters.
Never transmitted from father to son - sons always inherit X chromosome from mother.

22
Q

What is an example of X-linked inheritance diseases?

A

Duchenne Muscular Dystrophy

23
Q

What are multifactorial diseases?

A

Variants in genes causing alteration in function, often acting with environmental factors.
Behaviours are also mutlifactorial - involving mutliple genes affected by many factors.

24
Q

What are examples of multifactorial inheritance?

A

Heart disease, diabetes and most cancers.
There is genetic contribution to behavioural disorders such as alcoholism, obesity, mental illness and Alzheimers.

25
Q

What are chromosome disorders?

A

Chromosomal imbalance causes alteration in gene dosage.

26
Q

What are examples of chromosome disorders?

A

Down syndrome - caused by extra chromosome 21.
Prader-Willi syndrome - caused by absence of a group of genes on chromosome 15.
Chronic Myeloid Leukemia

27
Q

What is Chronic Myeloid Leukemia?

A

Caused by chromosomal translocation, where portion of chromosome 9 and 22 are exchanged.
No material is gained or lost, but new abormal gene is formed which causes cancer.

28
Q

What are mitochondrial disorders?

A

Lead to primary defect in oxidative phosphorylation.
Affect organ systems with high energy requirement.
Neuromuscular disorders are predominant.

29
Q

What is the frequency of mitochondrial disorders?

A

High frequency because: mtDNA has a higher proportion of coding DNA (93%), than nuclear DNA (1.5%).
So higher mutation rate in mtDNA.

30
Q

What is mitochondrial DNA?

A

Small - 15.4kb circular and numerous copies in each cell.
Inherited from maternal oocyte - approx 100,000.

31
Q

How are new mitochondrial mutations fixed in the genome?

A

In the inherited mitochondria, 99.9% of mtDNA are identical - homoplasmy, which is good.
If a new mutation arises in the mt population and spreads there will be two populations - heteroplasmy which is bad.

32
Q

What are the characteristics of mitochondrial disorders?

A

Reduced penetrance
Variable expression
Pleiotropy - affects multiple traits

33
Q

How are mutations corrected in mitochondrial DNA?

A

Mitochondria are deficient in recombination, so an mt molecule has to replace all the others to return to a state of homoplasmy.
It does this quickly by mtDNA bottleneck.

34
Q

What is mtDNA bottleneck?

A

The number of mtDNA molecules within each oocyte is reduced before being greatly amplified in the mature oocyte.
Only a small number of mtDNA molecules are selected for further amplification which explains the variability of mutant mtDNA.

35
Q

How can transmission of mitochondrial diseases be prevented?

A

For those with pathogenic mt mutations:
Mitochondrial donation using IVF.
This involves transfer of nucleus into a donor oocyte that has normal mitochondria.

36
Q

What does pathogenic mean?

A

Capable of causing disease.
Deletions, nonsense mutations and frameshifts are true pathogenic mutations.

37
Q

When is a DNA sequence change pathogenic?

A

A missense mutation is likely to be pathogenic if:
It affects a functionally important part of the protein.
The amino acid is conserved over evolution.
Amino acid substitutions are non-conservative.

38
Q

What is the analysis of the mutational nature of a sequence change?

A

Evolutionary conservation
Splice-site changes
Loss of protein features
Changes that might affect the amount of mRNA.
Test results are evaluated by a naive Bayes classifier, which predicts disease potential.

39
Q

What is chromosomal non-disjunction?

A

Results from a chromosome pair failing to separate.

40
Q

What are examples of non-disjunction of sex chromosomes?

A

Klinefelter 47XXY
Turner Syndrome
47XXX
47XYY

41
Q

What are examples of non-disjunction of an autosome?

A

Trisomy 21-Down Syndrome.
Extra chromsome 21 results from non-disjunction at meiosis.

42
Q

What is aneuploidy?

A

Abnormalities of chromosome number
Mainly: Loss of one autosome - monosomy.
Or Gain of one chromosome - trisomy.

43
Q

What are less common chromosome abnormalities?

A

Deletion of a small segment of chromosome leading to monosomy of the segment. e.g. Prader-Willi syndrome.
Chromosome translocations - breakage of two chromsomes and exchange between them, can occur in somatic cells and linked to cancer.

FLS (14 decks)

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