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Genetics & Evolutionary Biology > X-linked > Flashcards

X-linked Flashcards

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1
Q

What is a X:A ratio?

A

The ratio of the number of X chromosomes to the number of autosomal sets

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2
Q

How do X:A ratios affect sex determination?

A

A diploid organism with two X chromosomes has an X:A ratio of 1:1 or just 1, meaning that they have one set sex determining x chromosomes (XX) and one autosomal set of chromosomes (AA), this is what females have. Individuals with one X chromosome, aka males (XY), have the ratio 1:2, or 0.50. This is due to their singular x chromosome (XY) and their pair of autosomal chromosomes(AA).

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3
Q

What would genotypes look like as X:A ratios?

A
  • XY : AA = Male

- XX : AA = Female

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4
Q

What would the X:A ratio of a XY : AA be?

A

0.5 or 1:2 (because there is only one X chromosome and two A chromosomes)

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5
Q

What would the X:A ratio of a XX : AA be?

A

1 or 1:1 (because there are two X chromosomes and two A chromosomes)

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6
Q

What is the Sxl gene?

A

Sex-lethal (Sxl) is the master switch gene for somatic sex determination in Drosophila melanogaster.

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7
Q

How does the Sxl gene affect humans?

A

In XX animals, Sxl becomes activated and imposes female development; in X(Y) animals, Sxl remains inactive and male development ensues.

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8
Q

How do X:A ratios and the Sxl gene correlate?

A

If the X:A ratio is 0.5 (1:2) then the Sxl gene is turned off causing the embryo to become male. If the X:A ratio is 1 (1:1) then the Sxl gene is turned on and the embryo becomes female.

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9
Q

What happens to the X:A ratio and Sxl gene in cases of non-disjunction?

A

In cases of non-disjunction, where the number of chromosomes is abnormal, eg. XXY, the X:A ratio would depend on if there is more than one x chromosome. For example, for XXY the X:A ratio would be 1:1. This is due to there still being two x chromosomes and there is always at least two A chromosomes, hence the XXY would be female.

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10
Q

Draw a diagram of how X:A ratios and the Sxl gene works

A

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

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11
Q

What happens to the Drosophila when there are more than two autosomal sets of chromosomes?

A

The influence on the number of autosomal chromosomes on sex is indirect, however, they could be affecting the timing of developmental events and therefore how long sex-determining genes on the X chromosome are active. For example, XX flies with three autosomal sets (XX, AAA) have an X : A ratio of 0.67 and develop an intersex phenotype. In these flies, the presence of three autosomal sets causes a critical developmental stage to shorten, not allowing female factors encoded on the X chromosomes enough time to accumulate, with the result that the flies end up with an intersex phenotype.

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12
Q

How does intersex occur in humans?

A
  • SRY mutations, mixed chromosomal abnormalities or hormone deficiency/excess disorders, and various degrees of mosaicism of these and a variety of others.
  • Intersex in humans: 47XXY, 46XX/46XY, or 46XX/47XXY or XX & XY with
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13
Q

What is mosaicism?

A

The property or state of being composed of cells of two genetically different types

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14
Q

Why do we need Gene Dosage Compensation of X-Linked Genes?

A
  • X chromosome harbours 100s of genes necessary for normal development of both sexes
  • The evolution of X/Y and X/O sex determination has necessitated the co-evolution of gene dosage compensation mechanisms for X-linked genes
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15
Q

How do x-linked genes differ for males and females

A
  • Different mechanisms adjust for the unequal dosage of X-linked genes in male and female animals
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16
Q

What is an example of a Gene Dosage Compensation mechanism?

A

Hyperactivation of X-linked genes in males is when the SXL gene has been turned off, this then causes a protein complex to bind to the x chromosome and stimulate gene expression.

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17
Q

Why do males have to have mechanisms that increase the amount of x-linked genes that are expressed?

A

This is because females have two x chromosomes and therefore have more x-linked genes expressed. However, males only have one x chromosome and therefore need to have more of the genes on their only x-chromosome expressed so that they have the same or a similar amount as females. This is because the x-chromosome has many genes that make up a normal human.

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18
Q

What happens when there is too much or too little of SXL expression?

A

Inappropriate levels of SXL expression in either males or females will lead to lethality arising from inappropriate X chromosome dosage compensation.

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19
Q

Draw a diagram of Hyperactivation of X-linked genes

A

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

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20
Q

What is the difference between gene dosage compensation in Drosophila and in mammals (humans specifically)

A
  • Unlike the situation in Drosophila where the single male X chromosome is hyper-activated, in mammals one of the X chromosomes is inactivated.
  • In this way the females don’t get a double dose of X related genes
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21
Q

What is the first step in the mechanism for X In-activation in female mammals?

A
  • In every x chromosome there is a section called the x-inactivation centre (XIC) where a repressor of inactivation binds to one x chromosome.
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22
Q

What is the second step in the mechanism for X In-activation in female mammals?

A
  • Inactivation spreads out from the XIC of the opposite x chromosome of which the repressors have not bound to
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23
Q

What is the third step in the mechanism for X In-activation in female mammals?

A
  • The inactivation continues to spread until one of the chromosomes are completely inactivated.
24
Q

Draw an label a diagram of the mechanism for x inactivation in female mammals

A

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

25
Q

When does x-inactivation occur?

A
  • Inactivation events occurs when embryo consists of a few thousand cells
  • Each cell makes an independent decision to inactivate one of its X chromosomes
26
Q

How are the inactivated x chromosomes chosen and do they stay inactivated?

A

The chromosome to be inactivated is chosen at random and remains inactivated in all the descendent cells, except in female germ line cells where the inactivated X chromosome is reactivated

27
Q

How are female mammals able to show two different phenotypes?

A

Female mammals are genetic mosacis

- A female, heterozygous for an X-linked gene, is able to show two different phenotypes

28
Q

Do the inactive x chromosomes have any affect on the body?

A

The in-active X chromosome forms Barr bodies in the somatic cells of mammals

29
Q

What is a Barr body?

A
  • A small, densely staining structure in the cell nuclei of females, consisting of a condensed, inactive X chromosome
  • It replicates out of step with other chromosomes in the cell
30
Q

What is a well known example of x-inactivation?

A

In cats where a coat colour located on the X chromosome is dependant on the active x chromosome

  • The X chromosome is randomly inactivated in somatic cells early in development
  • Coat colour will depend on which X chromosome remains active
31
Q

Draw a diagram of the inactivated x chromosome example for cats

A

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

32
Q

Since a female only needs 1 X chromosome can you survive if you are born with only 1 copy?

A

Turner syndrome (XO)

  • 1:3000 female births
  • underdeveloped secondary sex characteristics
  • short, low hairline, broad chest
  • folds of skin on neck, normal intelligence
33
Q

What is Cytological Analysis

A
  • Cytogenetic analysis usually focuses on chromosomes in dividing cells. It is the analysis of cells from the body under a microscope. This is done to determine what the cells look like, and how they form and function.
34
Q

What is Karyotyping?

A

Karyotyping is the process of pairing and ordering all the chromosomes of an organism, thus providing a genome-wide snapshot of an individual’s chromosomes.

35
Q

How is a Cytological Analysis carried out?

A
  • Dyes such as quinacrine and Giemsa create banding patterns that are useful in identifying individual chromosomes within a cell
  • A karyotype shows the duplicated chromosomes of a cell arranged for cytogenetic analysis
36
Q

Draw and label a diagram on the process of a Cytological Analysis

A

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

37
Q

How are human chromosomes arranged into groups?

A

Human chromosomes are divided into 7 groups & sex chromosomes based on size & centromere location

38
Q

What are the groups that human chromosomes are divided into?

A
  • A 1-3: Large metacentric
  • B 4,5: Large submetacentric
  • C 6-12 & X: Medium sized, submetacentric
  • D 13-15: Medium-sized acrocentric
  • E 16-18: Short submetacentric
  • F 19-20: Short metacentrics
  • G 21,22 & Y: Short acrocentrics
39
Q

Study the example of a Karyotype provided

A

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

40
Q

What is chromosome painting?

A

Chromosome painting is a term used to describe the direct visualisation using in situ hybridisation of specific chromosomes in metaphase spreads and in interphase nuclei.

41
Q

What is a Euploid?

A
  • Euploid organisms have complete sets of chromosomes (eg. Diploid = 2n)
42
Q

What is a Polyploid?

A
  • Polyploid organisms contain extra set of chromosomes (eg. Triploid = 3n; Tetraploid = 4n)
  • Common in plants but rare in animals
43
Q

What is a Aneuploid?

A
  • Aneuploid organisms have particular chromosomes or parts of chromosomes under- or over-represented
  • Aneuploidy—a numerical change in part of the genome
44
Q

What is the difference between Aneuploidy and Polyploidy?

A
  • Aneuploidy implies a genetic imbalance; polyploidy does not
45
Q

What are “rearrangements” in genetics?

A
  • Rearrangements are changes in chromosome structure.
46
Q

What is Trisomy?

A
  • Triplication of one chromosome
47
Q

What is a Hypoploid?

A
  • Hypoploid—an organism in which a chromosome or chromosome segment is underrepresented
48
Q

What is a Hyperploid?

A
  • Hyperploid—an organism in which a chromosome or chromosome segment is overrepresented
49
Q

What is a Monosomy?

A
  • Monosomy—the absence of one chromosome in an otherwise diploid individual
50
Q

What is a common example of Trisomy?

A
  • Down Syndrome, the 21st chromosome has a third copy
51
Q

Other than Downs Syndrome, what other examples of Trisomy are there?

A
  • Trisomies for chromosome 13 (Patau Syndrome) & 18 (Edward Syndrome) reported, but individuals usually die a few weeks after birth.
52
Q

Why is it not recommended for older women to have children?

A

The frequency of nondisjunction increases with maternal age:
@ 25 years: 1 in 1500
@ 40 years: 1 in 100

53
Q

What is the first step in the origin of Down Syndrome?

A
  • In human females, meiosis begins in the fetus, but is arrested in prophase of meiosis I.
54
Q

What is the second step in the origin of Down Syndrome?

A
  • Prior to ovulation meiosis I is completed (and again arrested). The 2nd meiotic division only takes place after the oocyte is penetrated by the sperm.
55
Q

What is the third step in the origin of Down Syndrome?

A
  • In the first suspended state prior to ovulation, the chromosomes may become unpaired
  • The longer the time in prophase, the greater the chance for unpairing and subsequent chromosome non disjunction
56
Q

What causes Turner Syndrome to occur?

A

Turner syndrome (XO or 45, X) individuals can originate:

(a) from eggs or sperm that lack a sex chromosome
(b) from the loss of a sex chromosome in mitosis after fertilization —> Somatic Mosaics

57
Q

What are one of the Affects of Turner Syndrome that has scientists baffled?

A
  • 45, X individuals have no Barr bodies in their cells

- This means that they have the same number of active X chromosomes as normal XX females. So why aren’t they normal?

Genetics & Evolutionary Biology (16 decks)

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