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Genetics PMY-3 > Sex Chromosome And Sex Determination > Flashcards

Sex Chromosome And Sex Determination Flashcards

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1
Q

What are heterotrophic chromosomes?

A

Dissimilar

Example: sex chromosomes X and Y

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2
Q

Why is differentiation of sexes evident?

A

Via phenotypic dimorphism

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3
Q

What determines sex?

A

By specific genes not entire chromosomes

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4
Q

Describe the mode of sex determination in butterflies

A

Mode of sex determination: Protenor

  • XX/XO mode of sex determination
  • depends on random distribution of X chromosome into half of male gametes
  • presence of two X chromosomes in zygote results in female offspring
  • Presence of one X chromosome results in male offspring

Comeback

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5
Q

Describe the mode of sex determination of the milkweed bug

A

Lygaeus mode of sex determination
-XX/XY mode of sex determination

  • Female gametes have one X chromosome
  • Male gametes have either an X or Y chromosome
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6
Q

What is Homogametic sex ?

A
  • Producing like chromosomes
  • Zygotes with two X chromosomes
  • Results in female offspring
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7
Q

What is heterogametic sex?

A

Producing unlike chromosomes

  • Zygotes with one X and one Y chromosome
  • results in male offspring
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8
Q

Describe females as heterogametic sex

A
  • ZZ/ZW sex determination
  • Females are the heterogametic (ZW) sex
  • Males are the homogametic (ZZ) sex

Example: chickens

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9
Q

Explain how Y chromosome determines malenesss

A

Reveals one pair of chromosomes differs in males and females

  • Karyotypes for females: 46, XX
  • karyotypes for males: 46: XY
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10
Q

What is the human karyotypes?

A

Human karyotype

22 pairs of autosomal chromosomes

2 sex chromosomes

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11
Q

What are Klinefelter and Turner syndromes ?

A

Two human syndromes that result from variations in the numbers of sex

  • Characterized by altered sexual development and other phenotypical changes
  • Both result from the failure of the X chromosome to segregate during meiosis
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12
Q

What is the karyotype of Klinefelter?

A

47, XXY

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13
Q

What is the karyotype of turner?

A

45, X

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14
Q

Explain nondisjunctuon during meiosis 1 of sperm formation

A

Meiosis 1 nondisjunction—> homologous chromosomes do NOT separate

2 sperm cells have no sex chromosomes but have all autonomies

Other 2 spermatogenesis cells have a X and Y sister chromatid in each cell(sister chromatid separates)

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15
Q

Explain non disjunction during meiosis 2 in sperm cell formation

A
  1. X chromosome doesn’t separate. This results sister X chromatids do not separate and are in the same sperm cell

No sex chromosome but all the autosome (Y chromosomes separate correctly and one Y chromosome in each of the remaining two spermatozoa cells

  1. The Y chromosome doesn’t separate. Y sister chromatids do not separate. This results in one sperm cell with no. sex chromosomes but all autosomes and another has 2 Y chromatids. Other 2 sperm cells have an X chromosome each
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16
Q

How often does Klinefelter Syndrome, 47, XXY?

A

Can occur 1 in 660 births

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17
Q

What are the symptoms of Klinefelter syndrome?

A

Males with Klinefelter Syndrome have subtle symptoms

  • tall with long arms and legs
  • Large hands and feet
  • Male external genitalia, internal ducts are Testes produce less amounts of Testosterone and no sperm
  • reduced amount of testosterone
  • Affected males may have learning disabilities but tend to have better receptive language skills
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18
Q

What can reduced testosterone amount in Klinefelter lead to?

A

Reduced amount of testosterone leads to

  • slight breast enlargement
  • often have slightly rounded hips
  • Reduced amount of facial and body hair
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19
Q

How can Klinefelter syndrome be treated?

A

Klinefelter, 47, XXY

Treatment includes testosterone replacement therapy

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20
Q

How often does Turner syndrome occur?

A

Can occur 1 in 2000 live births

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21
Q

What are the symptoms of Turner syndrome ?

A

Females with Turner syndrome have subtle symptoms

  • short stature
  • Female external genetalia and internal ducts
  • Ovaries are rudimentary
  • underdeveloped breasts
  • Webbed neck often seen
  • Affected females May have cognitive impairment
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22
Q

How can Turner syndrome be treated?

A

Treatment includes growth hormone and estrogen therapy

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23
Q

What is 47, XXX syndrome ?

A
  • Can occur 1 in 1000 live births
  • Females with three X chromosomes have normal set of autosomes
  • Some females have underdeveloped secondary sex characteristics and are unable to conceive
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24
Q

What is 47, XYY syndrome?

A
  • frequency unknown as these males can father children and often do not come to the attention of geneticists
  • Affected makes often over 6 feet tall
  • Some studies have shown a correlation to exhibit behavioral problems though there is a great debate as to the validity of these studies
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25
Q

What diseases can result from nondisjunction in the first meiotic division?

A

Turner syndrome or Klinefelter syndrome

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26
Q

What happens during the 5th week of gestation?

A

Gonadal primordia (the tissue that will arise from the gonad) arise as a pair of gonadal (Genital)ridges which are associated with each embryonic kidney

Gonadal phenotype is still sexually indifferent

Development progresses and primordial germ cells migrate to the ridges

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27
Q

Gonads are bipotential, what does this mean?

A

Can form either ovaries or Testes

28
Q

What gender are the embryos before 5th week of gestation?

A

During early development, human embryos undergo a period when they are potentially hermaphroditic

29
Q

How much protein coding genes are on the sex chromosomes?

A

Y chromosome has around 70 protein coding genes

X chromosome has around 800 protein coding genes

30
Q

What are the pseudo autosomal regions(PARs)?

A
  • Present at both ends of Y chromosome which shares homology with regions on the X chromosome
  • Y will synapse and recombine with X during meiosis.
  • This pairing region is critical to segregation of X and Y during male gametogenesis
31
Q

What is the MSY region?

A

This is the male specific region on the Y chromosome

Nonrecombining region of the Y chromosome flanked by the PARS regions

23 million base pairs

32
Q

What are the 3 regions of the MSY region?

A

X-transposed region

X-degenerative region

Ampliconic region

33
Q

What is the X-transposed region ?

A

Region on Y chromosome MSY region

15% of MSY region derived from X chromosome, 99% identical to Xq21 region

34
Q

What is the X-degenerative region?

A

Region on Y chromosome MSY region

20% of MSY - homology to X chromosome though enough change has rendered the genes inactive

35
Q

What is the Ampliconic region?

A

Region on Y chromosome in MSY region

30% of MSY- which encodes proteins for development and function of testis and unique to the Y chromosome
-60 genes, 9 families of genes with multiple copies, each repeated unit is called an Amplicon

36
Q

What is the SRY region ?

A

SRY is the sexy determining region of the Y chromosome

Controls make development

Located adjacent to PAR of the short arm of Y chromosome

37
Q

How does the SRY region control male develop?

A

Encodes the protein Testis-determining factor (TDF)
-At 6-8 weeks of the development, SRY gene becomes active in XY embryos

  • Encodes a protein that triggers testes formation
38
Q

How can the SRY gene be negatively affected?

A
  • Human males with two X and no Y chromosomes have an altered X chromosome with the SRY region
  • Human females with XY but the Y is missing the SRY region
39
Q

What is the sex ratio?

A
  • Refers to the actual proportion of male to female offspring
  • World wide average 1.02(102 males to 100 females)
40
Q

What is the primary sex ratio?

A

Reflects proportion of males to females conceived in population.

This should be 1 as males produce equal numbers of X and Y sperm

- each type of sperm has equivalent viability and motility
- Egg surface equally receptive to both X and Y bearing sperm. 
- PSR equals 1 with assisted reproductive technology
41
Q

What is the secondary sex ratio?

A

Reflects proportion of each sex born

Thought that there is increased female mortality during development

42
Q

What are Barr bodies?

A

Barr bodies are small, densely staining structure in the cell nucleus of female cells and consists of a condensed inactive X chromosome

43
Q

How much more gene products do females make compared to males ?

A

Females have two X chromosomes and males have one

Hypothetically, females make twice as much gene products from X than males

44
Q

What is Dosage compensation?

A

Dosage compensation that balances the dose of X chromosome gene expression in females

-Prevents excessive expression of X-linked genes in females

45
Q

How are Barr bodies formed?

A

The formation of Barr Bodies is the genetic mechanism that compensates for X dosage disparities between males and females

46
Q

How was X inactivation discovered?

A

X inactivation was discovered by geneticist Dr. Mary F. Lyon and this phenomena is sometimes called lyonization in her honor

47
Q

Are all X chromosomes activated in a female somatic cell?

A

Each female somatic cell has one of the X chromosomes randomly inactivated and the other X is being used for gene products

48
Q

What explains dosage compensation?

A

X-inactivation

49
Q

What rule does X-inactivation follow?

A

Follows the N-1 (N= total number of X chromosomes)

50
Q

How much Barr bodies are in each X-chromosome related syndrome?

A
  • No Barr bodies are seen in Turner, 45, X
  • One Barr body seen in somatic cells of 46, XX
  • One seen In Klinefelter 47, XXY males
  • Two are seen in 47, XXX females and 48, XXXY males
  • Three are seen in 48, XXXX Females and 49, XXXXY males
51
Q

Why do we have clinical features of Turner and Klinefelter despite dosage compensation of X-inactivation?

A
  • There is no a activation in the early stages of development for cells destined for gonadal tissue
  • 85% of the genes on the X-chromosome are monoallelic (monogenic)
  • 15% of the genes on the X-chromosome are biallelic
52
Q

What is the Xic?

A

Xic is the X inactivation carrier

53
Q

How much genetic info is in the Xic?

A

Xic is about 1Mb (10^6 base pairs) contains four noncoding genes (RNA transcripts with various activities but are never translated into proteins

54
Q

What causes activation of Xic?

A

Active expression of the gene is only from the inactivated X chromosome and essential for the initiation and spread of X-inactivation

55
Q

What is the role of Xist in X-inactivation?

A

One of the genes X-inactive specific transcript (Xist) which recruits a protein complex to silence transcription of genes on the same chromosome it was made from

  • Xist transcript coat the inactive X chromosome
  • Xist recruits a protein complex to coat the inactive X chromosome
  • Inactive X chromosome condenses
56
Q

What is Tsix?

A

Tsix is another RNA transcript from Xic which is antisense to Xist and will regulate what Xist does

57
Q

In mammals, there is one chromosome from each parent, which one is inactivated?

A

Random

58
Q

When does X-inactivation occur?

A

Occurs in somatic cells at early stage of embryonic development

One the inactivation occurs, all descendant cells have the same X-inactivation

59
Q

Describe the Lyon hypothesis

A

Example is Calico and Tortoise shell cats and fur color/pattern

Genes for orange or black coat color is in the X chromosome

All calico and tortoise shell cats are female

Orange vs. black coat color depends on X-inactivation. White comes from lack of pigment controlled by another gene on an autosomal chromosome

60
Q

Which chromosome encodes for Glucise 6-phosphate dehydrogenase?

A

The X chromosome

61
Q

Who is affected by G6PD mutations? Why?

A

Coded on X-chromosome

GYPD deficiency is a recessive X-linked disorder affecting males

62
Q

What was found from women who have G6PD deficiency ?

A
  • Women who carry the disorder have given skin biopsies to scientists and fibroblasts cells are cultured in the laboratory from the biopsy
  • Clones of individual cells have been studied and scientists have confirmed that random permanent inactivation of the X chromosome occurs
63
Q

How can female carriers be affected by Glucose 6 phosphate deficiency?

A

Female carriers can get symptoms of the disease because of X-inactivation of the good X chromosome

64
Q

Explain the the cause Glucose 6-phosphatase deficiency in detail

A

Cause is a defect in glucose-6-phosphatase enzyme which leads to increased levels of reactive oxygen species which breaks down the membrane of red blood cells (RBCs)

  • This destruction of RBCs is called hemolysis
  • Also called hemolytic anemia which occurs when red blood cells are destroyed faster than the body can replace them
65
Q

What are the symptoms of Glucose 6-phosphatase deficiency ?

A
  • Paleness and yellowing of the skin
  • whites of the eyes(jaundice)
  • dark urine
  • fatigue
  • shortness of breath
66
Q

What may trigger hemolytic anemia in G6PD deficiency ?

A

People with G6PD deficiency may have hemolytic anemia triggered by infections or drugs or after eating fava beans (all increase levels of reactive oxygen species which cause RBCs to be destroyed)

Genetics PMY-3 (27 decks)

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