Exam 2: Sex Determination and Sex Chromosomes Flashcards
What is the heterogametic sex in humans?
The male (provides both an X and a Y)
Dioecious
Have only male or female reproductive organs
Monoecious
Have both male and female reproductive organs
– Common in plants and animals
– Can produce egg and sperm
What are the two sexes of C. elegans and the percentage of each after a hermaphroditic self-fertilization?
Hermaphroditic (XX)- 99%
Male (XO)- 1%
C. elegans, grasshopper, butterfly, etc. XO mode of sex determination
Males are XO. Only about half of the sperm produced by a male have an X chromosome. The other half of the sperm produced by a male lack a sex chromosome. Therefore, in a mating cross one would expect half of the progeny to be males and the other half hermaphrodites.
ZW Mode of sex determination
-Females are the heterogametic (ZW) sex
-Males are the homogametic (ZZ) sex
Ex: chickens, some amphibians and snakes, and fish
Homogametous
Producing gametes with like chromosomes
– Females with two X chromosomes can only produce gametes that contain an X
– Males with two Z chromosomes can only produce gametes that contain a Z
Heterogametous
Producing gametes with different chromosomes
– XY Males can produce produce gametes that contain a X or gametes that contain a Y
What is genic sex determination?
No obvious differences in the chromosomes of males and females.
– Genes that determine sex are present on
autosomes.
– Example: some plants, fungi, and protozoan
Explain sex determination in Drosophila
-Achieved by a balance of female determinants on the X and male determinants on the autosomes
– Ratio of X chromosomes to number of haploid sets of autosomes is predictive of sex
-Metafemale X>A
-Normal females (2X:2A)
–Triploid females (3X:3A)
-Intersex (3X: 4A)
-Male (X/XY:2A)
-Metamale (XY:3A)
Environmental sex determination examples
-Position on the stack of slipper limpets, the marine mollusk Crepidula fornicata (ex: females on bottom, males on top)
-Temperature in alligators, turtles, some lizards
True or False: X and Y are homologous chromosomes
False- X and Y are pseudo homologues/heteromorphic chromosomes because they pair during meiosis despite not being the same and will synapse and travel as a tetrad during meiosis 1 but have no crossing over.
Sex is determined by genes on chromosomes
PAR – Pseudoautosomal Region
Critical to segregation of X and Y chromosomes during male gametogenesis.
SRY: Sex-determining Region Y
Controls male development. (If found on X chromosome, would present as male.)
MSY: Male-Specific Y,
Non-recombining region of the Y
Non-disjunction in meiosis 1 vs meiosis 2 causes what kind of aneuploidy?
2 trisomic 2 monosomic
1 trisomic 1 monosomic 2 normal diploid
What is dosage compensation?
Dosage compensation is a mechanism to balance dose of X chromosome gene expression in males and females. This prevents excessive expression of X-linked genes in humans and other mammals.
Why is dosage compensation important?
A delicate balance of gene products is necessary to maintain normal development of an organism.
XO in humans presents as
Turner Syndrome
XXY, XXYY, XXXY, XXXXY
Klinefelter Syndrome
XXX
Triple X
XXXX+
Poly-X Female
If the ratio of X chromosomes to autosomal chromosomes on a a Drosophila melanogaster exceeds unity, what does it tell you?
A inviable female is produced (metafemale).
What is difference between heterochromatin and euchromatin?
Heterochromatin is defined as the area of the chromosome which is darkly stained with a DNA-specific stain and is in comparatively condensed form. Euchromatin is defined as the area of the chromosome which is lightly stained and rich in genes that actively participate in the transcription process.
Lyon Hypothesis
In 1961, Mary Lyon described the concept of X-chromosome inactivation, which is still referred to as ‘Lyonization,’ and made possible an understanding of inheritance of X-linked disorders such as hemophilia, Duchenne muscular dystrophy, and fragile X syndrome.
Why are all calico cats female?
The gene for coat color is carried on the X chromosome. To be calico, a cat must have two copies that are both activated for different parts of the coat.
True or False: A normal human female heterozygous for two X-linked alleles will express only one of those alleles in any given cell in her body.
True
Barr Bodies
Barr Bodies are condensed, randomly inactivated X chromosomes that are typically found exclusively in female mammals.
-They are a genetic mechanism that compensates for X dosage disparities.
– Darkly stained body toward the periphery of the nucleus
-Barr bodies occur early in embryonic development
Explain mammalian X-inactivation
– Explains dosage compensation
– Follows N - 1 rule (N = total number of X chromosomes)
Dosage Compensation in Drosophila
-Drosophila females have two copies of X-linked genes
-X-inactivation not observed in Drosophila
–Male X-linked genes transcribed at twice rate of
females
Why does X-inactivation not affect syndromes such as Turner or Kleinfelter?
Chromosome inactivation not in early stages of development for cells destined for gonadal tissue
– Not all X chromosomes forming Barr bodies are inactivated, around 15% escape inactivation
How many Barr bodies would you anticipate in a female with Turner syndrome?
0
How many Barr bodies would you anticipate in a male with Klinefelter syndrome?
1
SRY
SRY (which stands for sex-determining region Y gene) is found on the Y chromosome. In the cell, it binds to other DNA and in doing so distorts it dramatically out of shape. This alters the properties of the DNA and likely alters the expression of a number of genes, leading to testis formation. Most XX men who lack a Y chromosome do still have a copy of the SRY region on one of their X chromosomes. This copy accounts for their maleness. However, because the remainder of the Y chromosome is missing they frequently do not develop secondary sexual characteristics in the usual way.
Hemizygous
Hemizygous describes the condition of having only one copy of a gene or chromosome segment, often used to describe genes on the X chromosome in males due to their possession of only one X chromosome.
