Sex Determination + Sex Linked Inheritance Flashcards
Hermaphrodite
Organisms with BOTH female and male gametes
Sex determination in most mammals and humans
Female = XX
Male = XY
Y is the determining sex chromosome
The y chromosome is the…
sex determining chromosome
Other Binary Sex Determination Systems: (3.5)
1) X-O System
2) Z-W System
3) Haplo-Diploid System
3.5) Environmental Factor Dependence
X-O System
Determine by number of X chromosomes:
Male = X
Female = XX
Z-W System
Opposite of our system:
Male = ZZ
Female = ZW
Haplo-Diploid System
Sex determined by ploidy:
Male = Haploid –> Develops from unfertilized egg
Female = Diploid –> Develops from fertilized egg
Environmental Dependent Sex Determination
Some animals depend on environmental conditions to determine sex
–> Ex: Like some turtles depend on temperature to determine sex
In humans, what is the chance of an offspring being female or male?
50%
SRY Gene
A gene found ONLY on the Y chromosome (NOT found on the X chromosome)
= “Sex determining region of the Y chromosome”
Responsible for turning a mammalian embryo to a male
SRY Gene encodes for
a protein (transcription factor) that triggers/regulates the expression of many other genes that leads to a cascade of male development
Individuals w/ SRY gene vs w/o
With SRY Gene = Testes develop
Without SRY Gene = Ovaries develop
“femaleness” is the…
default pathway
–> SRY is needed to trigger a cascade, but without it, the cell would undergo the female development
When SRY gene was injected into a female zygote, what happened?
The mouse that developed had male reproductive organs
What did the experiment with the SRY transgenic mouse demonstrate?
SRY gene was not only necessary but also SUFFICIENT to change the path of development from female to male
X Chromosome
Contains ~1,000 genes –> Many of which are ESSENTIAL to life
What type of abnormality in sex chromosomes is inviable?
No X chromosome
Y chromosome
Gene poor: contains ~78 genes
–> None are essential to life
–> Paternally inherited
Why does the Y chromosome stay constant through generations?
Because X + Y are not homologs so the Y chromosome escape recombination
== Unless mutations occur, a father will pass down almost an exact copy of his own Y chromosome to his son
X and Y are NOT _____________ but they behave like __________ for _____________
1) Homologs
2) Homologs
3) Meiosis I
Why are X and Y not homologs?
They are quite difference in sequence and the genes they carry
How do X and Y “act” like homologs in meiosis?
Through the pseudo-autosomal regions
Pseudo-Autosomal Regions
Small sections at the ends of the X and Y chromosomes that have homologous sequences
–> Allows for “homologous” pairing during meiosis but NOT crossing over
How does a female having 2 X chromosomes affect gene dosage compared to males (with only 1X)?
There is no effect due to inactivation of an X chromosome in females
Barr Body
Compacted/Condensed X chromosome = inactive
X Inactivation
During female development, one X chromosome per cell condenses into a compact + INACTIVE barr body
The selection of which X chromosome to inactivate is…
random
Mosaic of X Chromosomes
The X that is inactivated in each cell is not consistent: some cells may express the maternal X and others may express the paternal X
–> ~50% of cells will have the maternal X activated and the other 50% will have the paternal X activated
Calico Cats
Get their patchy coloration due to different patches of cells with different active X chromosomes that encode for different hair color trait
Sex-Linked Gene
A gene located on either sex chromosome (don’t solely determine sex)
Fathers vs Mothers X Allele Passage
Fathers –> Pass their X allele only to daughters
Mothers –> Pass an X allele to sons and daughters
To get a homozygous recessive daughter in an X linked trait you must cross…
A heterozygous female (carrier) with an affected male (hemizygous recessive)
Aneuploidies of sex chromosomes are more _____________ than in autosomes
–> Because…(2)
Tolerated
Because:
1) Y chromosome carries very few genes, none of which are essential
2) Extra X chromosome gets inactivated as barr bodies
More males have _________________ than do females
X-Linked Recessive Diseases
Trisomy X
XXX
–> Female with extra X
Klinefelter
XXY
–> Male with extra X
Turner Syndrome
X
–> Female with missing X
