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FBS1 - CGCC > Lecture 1 - Genetics And Inheritance > Flashcards

Lecture 1 - Genetics And Inheritance Flashcards

1
Q

Inherited traits are controlled by what?

A

Genes

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2
Q

What is genome?

A

This contains all the info needed to construct a living organism.

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3
Q

Mutations in genotype causes what type of diseases?

A
  1. Monogenic Disorders - Sickle cell disease, Cystic Fibrosis
  2. Polygenic Disorders - T2D, Schizophrenia and Rheumatoid Arthritis
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4
Q

Genotype also determines whether you are resistant to diseases and example of this is?

A

Individuals who lack CCR5 (chemokine receptor) are immune to HIV-1 as this is how HIV-1 usually acts to cause an infection.

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5
Q

Genotype also controls the response to drugs:

  1. Mutant type of CYP450 - slow metabolisers
  2. Polymorphism in CYP450 - fast metabolisers
  3. Breast Cancer (those who OE HER2)
A

CYP450 is used to metabolise 25% of drugs. This is important for converting codeine to morphine allowing to act as a pain killer.

  1. Individuals who have a mutant type of CYP450 are slow metabolisers so codeine can’t be metabolised to morphine. Hence, another drug has to be given —> PERSONALISED MEDICINE.
  2. However, individuals with a polymorphism in CYP450 are fast metabolisers so intake of codeine can lead to respiratory depression and death.
  3. Some cases of breast cancer (30%) are due to OE of HER2 —> this promotes high cell division. Herceptin can be given to these individuals as this blocks HER2 action. However, this is ineffective in the other 70% cases.
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6
Q

How does human genome fit inside nucleus?

How is accessed for transcription and translation?

A

Human Genome - 1.8m
Nucleus - 5µm

  1. DNA is wrapped around histone proteins (chromatin) and this is highly condensed and compact to form nucleosomes. This then condenses to form chromosomes which are highly compact allowing it to fit inside nucleus.
  2. In its compact shape, RNA polymerase cannot access DNA for transcription and translation. Hence, in transcription, the section of DNA required is unravelled and is made accessible for mRNA synthesis.
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7
Q

Features of Human Genome:

1) How many chromosomes are present and what type?
2) Nomenclature for females
3) Nomenclature for males
4) Are genes on Y chromosome essential?

A
  1. 46 Chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes)
  2. 46, XX - Female
  3. 46, XY - Male
  4. Genes on Y chromosomes is not essential as females don’t have a Y chromosome.
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8
Q

What is Cytogenetics and how is Cytogenetics performed?

A

Cytogenetics = is studying genetic component of cell by visualising and analysis chromosomes.

In Cytogenetics, you use chromosomes in metaphase stage as these are visible.
They are then stained with Giemsa (causes G-bands where chromosomes have unique light and dark bands).

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9
Q

How are chromosomes classified?

A

Chromosomes are characterised by:

  1. Size
  2. Patterns of G-bands
  3. Position of Centromere
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10
Q

What is a centromere and what are the different types of centromeres?

A

Centromere is a constriction in chromosomes. Kinetochore proteins binds to centromere. This is associated to mitotic spindle in cell division allowing separation of chromatids.

The three different types of centromeres:

  • Metacentric e.g. Chrom 3
  • Submetacentric e.g. Chrom 17
  • Acrocentric e.g. Chrom 21

Centromeres splits chromosomes into short (p) arm and long (q) arm.

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11
Q

Why do Chromosomes replicate?

A

1) To replace cells lost (with wound healing) and for cell growth and development.

Mitosis - allows production of 2 identical daughter cells which are identical to parent (diploid)

2) To pass genetic informations to offspring.

Meiosis - produces 4 daughter cells identical to each other (haploid) but not to parent (diploid). This allows zygote to be diploid when gamete fusion occurs. Also have genetic variation due to chromosome recombination.

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12
Q

Differences between Mitosis and Meiosis:

A

Mitosis:

  • 1 round of DNA replication
  • 1 round of chromosomal segregation
  • 1 cell (diploid) —> 2 cells (diploid)

Meiosis:

  • 1 round of DNA replication
  • 2 rounds of chromosomal segregation and recombination)
  • 1 cell (diploid) —> 4 cells (haploid)

Sperm cells make 4 daughter cells but oocytes don’t. They produce only 1 functional egg cell and the other three are lost.

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13
Q

Summary of Mitosis:

A

Diploid parent undergoes chromosomal replication in prophase. This causes cell to have sister chromatid pairs.

In metaphase, sister chromatid pairs align in equatorial plane.

In anaphase and telophase, sister chromatids separate and 2 identical daughter cells are generated.

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14
Q

Summary of Meiosis:

A

Diploid parent undergoes chromosome replication in prophase. This causes cells to have homologous chromosome pairs (1 mother and 1 father) which undergo recombination. This allows exchange of genetic information.

In metaphase, homologous pairs line up in equatorial region.

In anaphase, homologous pairs separate.

Another round of chromosome segregation occurs and then sister chromatids separate. This causes formation of 4 haploid identical daughter cells.

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15
Q

How much of Y chromosome confers maleness?

A

Chromosomal abnormalities show that a small region on P arm of Y chromosome confers maleness:

  • some males have 46, XX - found that one X chromosome has a small region of Y chromosome
  • some females are 46, XY - found that short arm of Y chromosome is missing a small region —> causes female phenotype.

This was found to be the SRY gene - this encodes for a protein which acts as a TESTES DETERMINING FACTOR which causes male development.

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16
Q

What is an aneuploidy?

A

Irregular no. of chromosomes. These individuals are said to have an abnormal karyotype.

17
Q

What are the consequences of an abnormal karyotype?

A

Can lead to spontaneous abortions (50% are due to trisomies). Some can lead to live births if trisomy occurs in short chromosomes.

Trisomies in long chromosomes (e.g. chromosome 1) is fatal and will impact development.

18
Q

What are the causes of aneuploidies:

A
  1. Non-disjunction (failure of homologous chromosomes to segregate properly in meiosis)
  2. Age - Ova are as old as mother as all females are born with all the oocytes they will need in their lifetime. Unlike males where spermatogenesis is continuous.

Ova at birth are found in the meiosis I stage. After ovulation, they arrest in meiosis II.

19
Q

Trisomy 21 or Down Syndrome - 47, XX or XY +21

A
  • Retarded Growth and Development
  • Mental Retardation (delay in mental and social skills)
  • Single Crease on hand (Simian crease)
  • Cardiac Abnormalities
  • Increased risk of Leukaemia (WBC cancer)
20
Q

Trisomy 13 or Patau Syndrome - 47, XX or XY +13

A
  • Heart Defects
  • Incomplete brain development
  • Infant only survives for 130 days
21
Q

Trisomy 18 or Edward’s Syndrome - 47, XX or XY +18

A
  • Heart defects
  • Intestines protrude outside body
  • 95% die before born
  • Rest usually die in 1yr of life
22
Q

Examples of abnormalities in Sex Chromosomes:

A
  • Klinefelter Syndrome
  • Turner Syndrome
  • Super-male Syndrome
23
Q

Klinefelter Syndrome - 47, XXY

A
  • appear normal
  • infertile
  • small testes
24
Q

Turner Syndrome - 45, X (monosomy)

A
  • Short stature and amenorrhoea
  • Infertile —> not born with egg cells
  • No adolescent growth spurt
25
Q

Super-male Syndrome - 47, XYY

A
  • Asymptomatic
  • Increased growth velocity (taller than average)
  • Inaccurately stereotypes as being violent criminals
26
Q

When is it necessary to determine karyotype?

When should amniocentesis be done?

A

If foetus is at risk of developing chromosomal abnormalities then karyotype has to be determined.

Amniocentesis is only done if offspring is at high risk of developing aneuploidies. This procedure is non-invasive and has a low risk of miscarriage.

27
Q

How to do amniocentesis?

A

1) Inject needle into amniotic sack to obtain foetal cells
2) Analyse chromosome 13, 18, 21 using qPCR (95% accuracy)
3) Karyotype is looked at for further accuracy (100%)

No. of amniocentesis tests increase with age as more likely to get trisomy if have baby later in life.

28
Q

What is Mendelian Inheritance?

A
  • simple genetic characters depend on genotype at a single locus
  • diseases caused by this can lead to monogenic disorders, such as SCA and CF
29
Q

What is Mendelian’s Law?

A
  • all individuals have pair of alleles for a given trait. One is passed to offspring.
  • Alleles of different traits segregate/assort independently
30
Q

What is non-Mendelian inheritance?

A

If genes are mutated on X chromosome then females show a different trait to males.

This is not autosomal but is X-linked.

31
Q

Modes of monogenic inheritance:

A
  1. Autosomal recessive - CF
  2. Autosomal dominant - HD
  3. X-linked recessive - Haemophilia A
  4. X-linked dominant
  5. Mitochondrial diseases
32
Q

Autosomal Recessive - CFTR:

Locus?
Alleles?
Most common mutant type?
Pedigree? (Autosomal/ X-linked/ recessive/ dominant)

A

Locus: Chromosome 7

Allele: C - wild type CFTR
c - mutant type

Most common form is missing a.a on 508th codon (∆F508).

CC - normal
Cc - Heterozygotes (normal/carriers)
cc - CF

Heterozygotes are normal as presence of dominant allele masks the phenotype of recessive allele. If both parents are heterozygotes, chances of child having CF is 1 in 4 chances.

This is an autosomal recessive disorder - autosomal as both males and females are equally affected. Recessive as trait skips generation.

33
Q

Pathophysiology of CFTR:

A

CFTR is a Cl- channel which is synthesised in ER and packaged into vesicles and taken to Golgi. This is then expressed on PM to allow Cl- export and H2O follows via osmosis. This keeps mucus mobile and fluid-like allowing it to trap bacteria in airways.

Mutation in CFTR e.g. CFTR∆F508. This is synthesises in ER but is misfolded so is degraded by the ER quality control systems. Hence, this is not expressed on PM —> [Cl] is high inside so H2O enters cell via osmosis. This causes mucus to be thick so can’t trap bacteria and leads to infections. This can also b locks ducts in pancreas and intestines.

34
Q

Autosomal Dominant: Huntington’s Disease

Type of disease?
Symptoms?
Causes?
Normal vs. Mutant?
Pedigree?
A
  • Neurodegenerative disease
  • Gradual cognitive decline
  • caused by mutation in HD locus
  • Normal HD has 28 repeats of CAG codon (Gln) —> forms a polyglutamine tract
  • mutant HD has more than 36 repeats of CAG codon; this causes:
    1. Expansion of polyglutamine tract
    2. Aggregation of protein as it is misfolded
    3. Precipitation in neural tissues - neuronal cell death (neurons don’t regenerate)

Autosomal dominant as presence of mutant allele will lead to symptoms (aggregation of proteins still occurs). However, this only appears in adulthood as it takes time for mutant HD to aggregate/ accumulate.
Trait is seen in every generation and affects both males and females equally.

35
Q

X-linked Recessive: Haemophilia A

Is it Mendelian?
Where is the mutation found?
What does the gene encode for?
Normal mother X Haemophiliac father?
Carrier mother X normal father?
Is it more frequent in males or females?
A
  • not Mendelian characteristics
  • gene is on X chromosome - son obtains X from mother and Y from father. If mother is affected, then all sons will be affected.
  • this is a mutation in gene encoding blood clotting factor VIII
  • If normal mother mates with a haemophiliac father:
    • all females are carriers
    • can’t be passed from father to son
  • if mother is carrier and father is normal:
    • half the sons will have disease
    • half the daughters will be carriers
  • more frequent in males than females
36
Q

What are polygenic disorders?

Does it follow mendelian’s law?
Two types of traits?
What is a multifactorial disease and give examples?

A
  • this is controlled by more than one locus (i.e. more than one gene)
  • not follow mendelian’s law
  • Traits are either continuous or discontinuous:
    • Discontinuous - T2D
    • Continous - Height

Multifactorial - if influenced by environ and genetics (i.e. more than one factor) e.g. T2D, CVD, obesity and mental illnesses.

37
Q

How do we assess the contribution made by genetic vs. environment for a given disorder:

A

Done through twin studies where monozygotic (identical) or dizygotic (non-identical) twins are used.

Monozygotic - same egg divides so genetically identical.
Dizygotic - 2 ovum are fertilised by 2 sperm at same time so not identical.

You can tell if a disease is inherited or environment by looking at prevalence of a disease in MZs and DZs (concordance).

If disease is more prevalent in MZs then disease is inherited i.e. if have high concordance in MZ than DZ then there is a genetic factor involved as MZs have same genome to each other.

FBS1 - CGCC (1 decks)

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