Sex Chromosome And Sex Determination Flashcards
What are heterotrophic chromosomes?
Dissimilar
Example: sex chromosomes X and Y
Why is differentiation of sexes evident?
Via phenotypic dimorphism
What determines sex?
By specific genes not entire chromosomes
Describe the mode of sex determination in butterflies
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
Describe the mode of sex determination of the milkweed bug
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
What is Homogametic sex ?
- Producing like chromosomes
- Zygotes with two X chromosomes
- Results in female offspring
What is heterogametic sex?
Producing unlike chromosomes
- Zygotes with one X and one Y chromosome
- results in male offspring
Describe females as heterogametic sex
- ZZ/ZW sex determination
- Females are the heterogametic (ZW) sex
- Males are the homogametic (ZZ) sex
Example: chickens
Explain how Y chromosome determines malenesss
Reveals one pair of chromosomes differs in males and females
- Karyotypes for females: 46, XX
- karyotypes for males: 46: XY
What is the human karyotypes?
Human karyotype
22 pairs of autosomal chromosomes
2 sex chromosomes
What are Klinefelter and Turner syndromes ?
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
What is the karyotype of Klinefelter?
47, XXY
What is the karyotype of turner?
45, X
Explain nondisjunctuon during meiosis 1 of sperm formation
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)
Explain non disjunction during meiosis 2 in sperm cell formation
- 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
- 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
How often does Klinefelter Syndrome, 47, XXY?
Can occur 1 in 660 births
What are the symptoms of Klinefelter syndrome?
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
What can reduced testosterone amount in Klinefelter lead to?
Reduced amount of testosterone leads to
- slight breast enlargement
- often have slightly rounded hips
- Reduced amount of facial and body hair
How can Klinefelter syndrome be treated?
Klinefelter, 47, XXY
Treatment includes testosterone replacement therapy
How often does Turner syndrome occur?
Can occur 1 in 2000 live births
What are the symptoms of Turner syndrome ?
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
How can Turner syndrome be treated?
Treatment includes growth hormone and estrogen therapy
What is 47, XXX syndrome ?
- 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
What is 47, XYY syndrome?
- 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
What diseases can result from nondisjunction in the first meiotic division?
Turner syndrome or Klinefelter syndrome
What happens during the 5th week of gestation?
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
Gonads are bipotential, what does this mean?
Can form either ovaries or Testes
What gender are the embryos before 5th week of gestation?
During early development, human embryos undergo a period when they are potentially hermaphroditic
How much protein coding genes are on the sex chromosomes?
Y chromosome has around 70 protein coding genes
X chromosome has around 800 protein coding genes
What are the pseudo autosomal regions(PARs)?
- 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
What is the MSY region?
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
What are the 3 regions of the MSY region?
X-transposed region
X-degenerative region
Ampliconic region
What is the X-transposed region ?
Region on Y chromosome MSY region
15% of MSY region derived from X chromosome, 99% identical to Xq21 region
What is the X-degenerative region?
Region on Y chromosome MSY region
20% of MSY - homology to X chromosome though enough change has rendered the genes inactive
What is the Ampliconic region?
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
What is the SRY region ?
SRY is the sexy determining region of the Y chromosome
Controls make development
Located adjacent to PAR of the short arm of Y chromosome
How does the SRY region control male develop?
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
How can the SRY gene be negatively affected?
- 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
What is the sex ratio?
- Refers to the actual proportion of male to female offspring
- World wide average 1.02(102 males to 100 females)
What is the primary sex ratio?
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
What is the secondary sex ratio?
Reflects proportion of each sex born
Thought that there is increased female mortality during development
What are Barr bodies?
Barr bodies are small, densely staining structure in the cell nucleus of female cells and consists of a condensed inactive X chromosome
How much more gene products do females make compared to males ?
Females have two X chromosomes and males have one
Hypothetically, females make twice as much gene products from X than males
What is Dosage compensation?
Dosage compensation that balances the dose of X chromosome gene expression in females
-Prevents excessive expression of X-linked genes in females
How are Barr bodies formed?
The formation of Barr Bodies is the genetic mechanism that compensates for X dosage disparities between males and females
How was X inactivation discovered?
X inactivation was discovered by geneticist Dr. Mary F. Lyon and this phenomena is sometimes called lyonization in her honor
Are all X chromosomes activated in a female somatic cell?
Each female somatic cell has one of the X chromosomes randomly inactivated and the other X is being used for gene products
What explains dosage compensation?
X-inactivation
What rule does X-inactivation follow?
Follows the N-1 (N= total number of X chromosomes)
How much Barr bodies are in each X-chromosome related syndrome?
- 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
Why do we have clinical features of Turner and Klinefelter despite dosage compensation of X-inactivation?
- 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
What is the Xic?
Xic is the X inactivation carrier
How much genetic info is in the Xic?
Xic is about 1Mb (10^6 base pairs) contains four noncoding genes (RNA transcripts with various activities but are never translated into proteins
What causes activation of Xic?
Active expression of the gene is only from the inactivated X chromosome and essential for the initiation and spread of X-inactivation
What is the role of Xist in X-inactivation?
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
What is Tsix?
Tsix is another RNA transcript from Xic which is antisense to Xist and will regulate what Xist does
In mammals, there is one chromosome from each parent, which one is inactivated?
Random
When does X-inactivation occur?
Occurs in somatic cells at early stage of embryonic development
One the inactivation occurs, all descendant cells have the same X-inactivation
Describe the Lyon hypothesis
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
Which chromosome encodes for Glucise 6-phosphate dehydrogenase?
The X chromosome
Who is affected by G6PD mutations? Why?
Coded on X-chromosome
GYPD deficiency is a recessive X-linked disorder affecting males
What was found from women who have G6PD deficiency ?
- 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
How can female carriers be affected by Glucose 6 phosphate deficiency?
Female carriers can get symptoms of the disease because of X-inactivation of the good X chromosome
Explain the the cause Glucose 6-phosphatase deficiency in detail
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
What are the symptoms of Glucose 6-phosphatase deficiency ?
- Paleness and yellowing of the skin
- whites of the eyes(jaundice)
- dark urine
- fatigue
- shortness of breath
What may trigger hemolytic anemia in G6PD deficiency ?
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)
