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BS3031 Human Genetics > Genetics of sex and the sex chromosome > Flashcards

Genetics of sex and the sex chromosome Flashcards

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Biology 101 flashcards
1
Q

Why does sex matter?

A

Because it is the fundamental distinction between individuals within a species

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2
Q

Give examples of some of the many aspects of genetics that sex affects

A

Mutation
Recombination
Gene expression
Disease manifestation through sex chromosomes

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3
Q

Which sex chromosome is dominantly sex-determining in humans?

A

Y

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4
Q

Which sex chromosome in humans provides the initial genetic switch in the sex determination process?

A

Y, thereafter sex is determined hormonally

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5
Q

What possible rare genotypes could a phenotypic female have?

A 
46, XX (normal)
45, X
47, XXX
48, XXXX
49, XXXXX
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6
Q

What possible rare genotypes could a phenotypic male have?

A 
46, XY (normal)
47, XXY
47, XYY
48, XXYY
49, XYYYY
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7
Q

The X and Y chromosomes are heteromorphic. True or false?

A

True

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8
Q

Describe the gene content of X

A

Average (~1000)

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9
Q

Describe the gene content of Y

A

Low (78)

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10
Q

Why is the phenotype of a liveborn with an absence of Y (45, X) not as severe when compared with other monosomies?

A

Because Y bears few genes which are not involved in male-specific development

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11
Q

What is the probable mechanism for the expansion of NRY?

A

Segmental inversion

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12
Q

Do the bird Z and W chromosomes have independent origin or not?

A

Yes, they have independent origin

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13
Q

Are there more mutations in males or females?

A

Males

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14
Q

How have disease studies deduced that most mutations come from fathers?

A

Identified parental origin of new base mutations using linkage

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15
Q

Give examples of highly-penetrant dominant diseases passed on from fathers

A

Achondroplasia, Apert, Pfeiffer or Crouzon syndromes (= craniosynostoses)
MEN2A and -2B (= multiple endocrine neoplasias type 2)

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16
Q

Give an example of an extreme disease case in which all mutations occur in males

A

Achondroplasia (FGFR3 gene)

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17
Q

What is the average male to female mutation ratio in disease studies?

A

~10:1

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18
Q

In males there are more cell divisions of the gametes, so this means more of what?

A

More DNA replications

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19
Q

What might explain the male mutation bias and paternal age effect?

A

If DNA is mutagenic

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20
Q

An 80-year-old father has undergone 40x as many replications as a person at what age?

A

15 years old

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21
Q

Describe the recent genome-wide approach to identify base substitutions

A 
Whole genome sequencing
78 parent-offspring trios
Mutations increase with father's age
~2.01 mutations/year
=> paternal mutations double every 16.5 years!
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22
Q

Describe the recent genome-wide approach to identify microsatellite mutations

A 
2477 loci in 24832 trios
2058 inherited mutations
3:1 male bias
Rate in fathers doubles from 20-58 years!
Longer alleles are more mutagenic
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23
Q

What is deCODE genetics?

A

Icelanders
= 1st population Biobank (1996)
Health records and DNA of ~95% present day population
Genealogical records back to 1000 years ago

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24
Q

What complex in PAR1 helps to form the XY bivalent in pachytene?

A

The synaptonemal complex

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25
Q

What does PAR1 stand for?

A

Pseudoautosomal region 1

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26
Q

If there is an ancestral relationship between X and Y, you would expect regions of ancestral what?

A

XY-homology

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27
Q

What did Koller and Darlington discover about the sex chromosomes in 1934?

A

They pair in a specialised region

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28
Q

What term did Burgoyne coin in 1982 regarding the sex chromosomes?

A

‘Pseudoautosomal’ - partially sex-linked behaviour of loci

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29
Q

What gave rise to the strictly homologous PAR1 sequences?

A

A single obligate recombination event (male meiosis)

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30
Q

What no longer commonly occurs in most 47, XXY individuals?

A

There is no recombination in PAR1

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31
Q

What is recombination in PAR1 important for?

A

The correct segregation of sex chromosomes

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32
Q

What is the size of PAR1?

A

2.7Mb

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33
Q

What percentage recombination of PAR1 is there in female meiosis?

A

7%

~3-fold over the genome average

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34
Q

What percentage recombination of PAR1 is there in male meiosis?

A

50%

~20-fold over the genome average

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35
Q

What does PAR2 stand for?

A

Pseudoautosomal region 2

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36
Q

What regions of PAR2 sometimes associate to form synaptonemal complexes?

A

Yqter and Xqter

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37
Q

In what year was PAR2 shown molecularly?

A

1992

38
Q

PAR2 sequences are X-specific in other primates, true or false?

A

True, e.g. of acquired XY-homology

39
Q

How many genes does the PAR2 region contain?

A

4

40
Q

What is the size of the PAR2 region?

A

320kb

41
Q

Is recombination of PAR2 obligate or not?

A

Not obligate, neither necessary nor sufficient for proper sex chromosome segregation

42
Q

What is the percentage recombination of PAR2?

A

2%

= 6-fold elevation over the genome average

43
Q

What is the genome average percentage recombination?

A

1% per Mb
OR
1cM per Mb

44
Q

In terms of recombination, which sex chromosome resembles an autosome?

A

The X in female meiosis

45
Q

What is the genome-wide sex averaged rate of recombination?

A

1cM per Mb

46
Q

How many times higher is the female recombination rate compared to the male recombination rate?

A

~1.65 times

47
Q

How many recombination events per meiosis are there in females?

A

~80

48
Q

How many recombination events per meiosis are there in males?

A

~50

49
Q

Is the activity of recombination hotspots, i.e. the distribution of recombination, the same or different between males and females?

A

Different

50
Q

How can you identify recombination ‘jungles’ and ‘deserts’?

A

By high-density SNP genotyping of ~400,000 SNPS (AffySNP 5.0) of over 2300 meioses

51
Q

What percentage of male and female ‘jungles’ exist in the 5% most telomeric portion of each chromosome arm?

A

70% male ‘jungles’

18% female ‘jungles’

52
Q

Give a reason why there is a sex difference in recombination on the autosomes

A

Because females form longer axial elements and have more chromatin loops

53
Q

Describe what happens after a double strand break (DSB) forms at the axis

A

Homology search and pairing

Crossover, i.e. repair of DSB using homologue

54
Q

The interference distance is the same if measured in microns of synaptonemal complex (SC) but is not the same in..

A

Megabases

55
Q

Define non-disjunction

A

The failure of meiotic segregation

56
Q

What is the percentage of aneuploidy in human fertilised eggs when compared with a mouse?

A

10-30% human

1-2% mouse

57
Q

What is the leading cause of foetal death in humans, causing ~50% of spontaneous abortions to 15 weeks gestation age?

A

Aneuploidy

58
Q

When does most gametic aneuploidy originate?

A

During oogenesis

59
Q

The frequency of non-disjunction errors increases with what?

A

Female age

60
Q

Trisomy 21 is more commonly known as what disease?

A

Down syndrome

61
Q

What is the most common autosomal trisomy in liveborns?

A

Trisomy 21, 1 out of 600

62
Q

Trisomy 21 is usually the result of what type of meiotic error?

A

Non-disjunction

63
Q

What is the only well documented risk factor for having a child with trisomy 21?

A

Advanced maternal age

64
Q

What is the most common single cause of mental retardation?

A

Trisomy 21

65
Q

In human oogenesis, what happens during the 2nd-7th month of gestation?

A

~1000 oogonia dvide to form ~7E6 germ cells

Then most die - ~2E6

66
Q

In human oogenesis, when does most of meiosis I occur?

A

During foetal development: synapsis and crossing-over

67
Q

How many oocytes maintained in prolonged diplotene are females born with?

A

1-2E6

68
Q

When do groups of oocytes periodically resume meiosis?

A

At the onset of adolescence

69
Q

Segregation during meiosis I and II is delayed until when?

A

Ovulation and fertilisation

70
Q

How is segregation during meiosis I and II delayed?

A

Meiosis-specific cohesion complexes released sequentially

=> Orderly segregation of homologues and sister chromatids at meiosis I and II respectively

71
Q

What is the theory called (postulated by Henderson and Edwards in 1968) that was originally thought to explain why aging human oocytes are prone to aneuploidy?

A

‘Production line’ theory

72
Q

Describe the ‘production line’ theory

A

Occytes that are first produced by the foetus are more fit and the first to be ovulated
As a woman approaches menopause, she has more ‘bad’ eggs

73
Q

Why was the ‘production line’ theory disproven?

A

Realised that less fit eggs had poorly placed/absent chiasmata

74
Q

What is the theory called (postulated by Hawley et al. in 1996) that is now thought to explain why aging human oocytes are prone to aneuploidy?

A

‘Two-hit’ theory

75
Q

Describe the ‘two-hit’ theory

A

Hit 1 = diminished recombination (caused by lack of or misplaced chiasma) produces ‘susceptible’ chromosomes
Hit 2 = eg. reduction in ability of meiosis molecular machinery with increasing age

76
Q

How can meiosis I trisomy be distinguished from meiosis II trisomy?

A

Using centromere markers

77
Q

Give a candidate gene from a mouse model for the age-related ‘hit 2’

A

SMC1beta

78
Q

Give a candidate gene from a yeast model for the age-related ‘hit 2’

A

Shugoshin

79
Q

What are the SMC1beta and shugoshin genes members of and how does this make them candidates for the age-related ‘hit 2’?

A

Members of cohesion complex

Show age-dependent degradation and subsequent premature sister chromatid separation prior to anaphase II

80
Q

Why are cohesions important during development?

A

Cohesions are loaded onto chromosomes during foetal development
Necessary and sufficient to mediate cohesion in the mature oocyte

81
Q

What are the human MAD2 and BUB1 proteins and what is their function?

A

Conserved kinetochore-associated proteins
RT-PCR found that the concentration of transcripts decreases as human oocytes age
Involved in the regulation of progression from metaphase to anaphase

82
Q

What must occur in females due to the presence of two X chromosomes?

A

Dosage compensation

83
Q

At 7 weeks gestation, are gonadal cells different or indifferent?

A

Indifferent

84
Q

What type of chromatin allows gene expression?

A

Euchromatin

85
Q

What are the non-recombining parts of the sex chromosomes known as?

A

Protosex chromosomes

86
Q

Males have a higher mutation rate which means that the child is more likely to inherit..

A

A mutation from dad

87
Q

Females have a higher recombination rate which means that the child is more likely to inherit..

A

Recombination from mum

88
Q

When does female meiosis occur?

A

All occurs in utero

89
Q

When does male meiosis occur?

A

From puberty and every year thereafter

90
Q

Do recombination ‘jungles’ and ‘deserts’ differ between males and females?

A

Yes, but not exclusively

Some are hared and some are favoured more than others

91
Q

Where does PRDM9 appear to act?

A

On chromatin loops

BS3031 Human Genetics (2 decks)

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