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BS3031 Human Genetics UOL > Genetics of Sex and the Sex Chromosomes > Flashcards

Genetics of Sex and the Sex Chromosomes Flashcards

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1
Q

How is sex determined in humans?

A

By the Y chromosome. It provides the initial genetic switch in the sex determination process

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2
Q

How did sex chromosomes evolve?

A

From autosomes

Selective pressure and accumulation of difference reinforced ability for the two proto-sex chromosomes to diverge

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3
Q

Which sex has more mutation?

A

Males

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4
Q

What is the parental age effect?

A

Older the parents, more likely for their child to carry a mutation

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5
Q

What is the dual role of meiotic recombination?

A
  1. Genetic diversity

2. Ensures accurate segregation of gametes

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6
Q

In which PAR region is recombination required for correct segregation of sex chromosomes?

A

PAR1

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7
Q

Which sex has more recombination?

A

Females

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8
Q

How does PRDM9 initiate recombination?

A

Acts on top of chromatic loops
Zinc finger domain interacts with specific DNA seqs
Set domain methylates lysine, opens chromatin
Allows SPO11 to induce double strand breaks and hence crossing over

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9
Q

What is non-disjunction?

A

Failure of meiotic

segregation

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10
Q

What is the consequence of non-disjunction?

A

Aneuploid gametes: games that don’t have the correct number of chromosomes

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11
Q

What is the only well-documented risk factor for Down syndrome?

A

Advanced maternal age

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12
Q

Two factors that are involved in non-disjunction?

A

Poorly placed recombination

Increased vulnerability with age

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13
Q

Features of meiosis I trisomy?

A

Usually one single exchange
Prone to loss of chiasmata prior to spindle attachment
50% risk non-disjunction

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14
Q

Features of meiosis II trisomy?

A

Increased exchange in centromere-proximal region

Entanglement of homologous until anaphase

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15
Q

What is the role of cohesins in chromosome disjunction?

A

Regulated separation of sister chromatids

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16
Q

Where are cohesin rings established?

A

During replication i.e. in vitro

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17
Q

What occurs in normal females in somatic cells?

A

One X chromosome is inactivated. Hypoactivation.

Seen as Barr body

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18
Q

What happens if there are extra X chromosomes?

A

All except one are inactivated. (n-1) rule

19
Q

How is X inactivation maintained?

A

It is clonally maintained. Inactivated X expressed relevant products to give rise to its own inactivation

20
Q

Is the same X inactivated in all cells?

A

No. Cells are somatic mosaics

21
Q

What are the characteristics of the inactive X?

A

Heterochromatinised
Late replicating
Associated with nuclear envelope
Promoters hypermethylated

22
Q

What is the X-inactivation centre?

A

Part of the counting mechanism

Two XICs are required, and must be on different copies of the X

23
Q

Which X is the XIST transcribed from and where does it remain?

A

The inactive X

Stays associated with the inactive X and remains in the nucleus

24
Q

Where do families with skewed X-inactivation have mutations?

A

In XIST promoter

25
Q

What happens when mouse XIST is knocked out?

A

X can’t be inactivated

26
Q

What happens in transgenic XIST mice?

A

Transgene can initiate inactivation

Inactivation is random

27
Q

What does the transgenic XIST mice experiment show?

A

Shows that XIST contains controlling centre

28
Q

What happens when transgene XIST is on the autosome?

A

Autosome starts showing some gene inactivation

See long range cis effects associated of XIST RNA and some gene inactivation

29
Q

What is Tsix?

A

XIST antisense gene, transcribed from opposite strand

30
Q

What does tsix knockout mouse show?

A

It shows preferential inactivation of mutant X

Indicates its a repressor

31
Q

Where is tsix expressed?

A

Initially from both X’s, then from active X after onset of X inactivation

32
Q

What does tsix do?

A

Block xist action so xist RNA only accumulates on future inactive X and not on the future active X

33
Q

How many hours does it take to “lock-in” X-inactivation?

A

72 hours

34
Q

Why is X-inactivation important?

A

For dosage compensation

35
Q

Which areas escape X-inactivation?

A

Genes within PAR1

36
Q

Why are autosomal aneuploidies poorly tolerated?

A

Because of gene dosage problems

37
Q

Why are sex chromosomal aneuploidies well tolerated?

A

Y is not essential for viability. At least one X chromosome is essential, but extra X’s are inactivated so doesn’t matter

38
Q

What is the general pattern of inheritance?

A

1 set of each autosome from each parent
Males - Y from father, X from mother
Females - X from father, X from mother
mtDNA from mother to all children

39
Q

Example of X-linked recessive trait

A

Colour-blindness

Duchenne muscular dystrophy

40
Q

Example of X-linked dominant trait

A

Hypophosphatemia

Incontinentia pigment

41
Q

What is the SRY?

A

Sex-determining region Y. It is the testis-determining factor

42
Q

Example of Y-linked inheritance

A

Hairy ears

43
Q

What problems can non-allelic homologous recombination between misaligned repeats cause?

A

Duplication / deletion of genes - lead to dosage problems

Inversions of gene segments - gene inactivation

44
Q

How are AZFa and AZFc infertility disorders caused?

A

Aberrant recombination between repeated sequences

BS3031 Human Genetics UOL (11 decks)

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