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Flashcards in Sex Chromosomes Deck (37)
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Why do males have a Y chromosome?

Because the presence of the Y chromosome is decisive for initiating the developmental program that leads to a male phenotype.


Can persons have as many as 5 X chromosomes and just one Y chromosome and still be phenotypically male?



Where does synapsis occur in the X and Y chromosomes.

In their small region of shared homology.


Where does crossover occur?

In two regions of pairing called the pseudoautosomal regions.


What is the chromosome crossing over also known as?

Recombinant events.


Why are the psuedoautosomal regions called that?

Because any genes located in those regions (so far only 9 identified functions) are inherited just like any autosomal genes


How many copies of genes in the psuedoautosomal regions to males and females have?

2 copies as males have a copy in the PAR on their X and the corresponding copy in the Par of their Y, Females have 2 corresponding genes in their PARs on both their X chromosomes.


How can males inherit an allele originally on the X chromosome of their father?

Due to the crossing over of alleles in the PAR.


True or false: Although 95% of the Y chromosome lies between the pseudoautosomal regions, fewer than 80 genes have been found here.



What do the genes outside of the PAR on the Y chromosome code for?

Some encode for proteins used by all cells but some are for proteins that function only in the testes.


What is the SRY?

It is the sex determining region Y. It is located outside the PARs on the Y chromosome. It triggers the events that runs the embryo into a male.


What are some rarities associated with the SRY gene?

Humans with XX genotype have testicular tissue because translocation has placed the SRY gene on one of the X chromosomes.
There are also women with the XY genotype because the destruction or mutation of a SRY on the Y chromosome has caused them to be female.


True or false: The Y chromosome could be linked to cardiovascular disease.



What do the majority of genes in the X chromosome have anything to do with?

A persons sex/ sex characteristics.


What is an X-linked disorder that cannot be classified as either recessive or dominant?

Fragile X syndrome.


What is fragile X syndrome?

Where a person has a fragile X chromosome that displays a site susceptible to chromosome breakage. The secondary changes to structure in this gene cause it to be turned off.


How does fragile X syndrome affect males and females?

Affected males have autism or severe mental retardation and female carriers can also be affected.


How many active X chromosomes do females have?

Only one active X chromosome due to X inactivation.


When does X inactivation occur?

Early in embryonic development.


When a females X chromosome becomes inactivated what is it converted into?

A barr body. Which is an inactive X painted with Xist RNA.


How does X inactivation take place?

The chromosome that is inactive is random but after inactivation has taken place all the descendants of that cell will have the same X inactivated.


What is anhydrotic dysplasia?

An X-linked recessive disorder that results in sufferers not having sweat glands and female carriers do not have all their sweat glands.


What is required for the inactivation of a X chromosome?

A gene on that chromosome called X-inactivation specific transcript. (Xist)


What does the Xist do?

It encodes a large molecule of RNA. Xist RNA accumulates along the X chromosome containing the active XIST gene and proceeds to inactivate all (or almost all) of the other hundreds of genes on that chromosome.


What happens after one X chromosome has been inactivated?

Transcription of XIST ceases on the other X chromosome allowing all of its hundreds of other genes to be expressed. The shut-down of the XIST locus on the active X chromosome is done by methylating XIST regulatory sequences.


What are the only cells in the body where X-inactivation is not random?

The extra embryonic membranes where it is thought that the fathers X is inactivated.


What makes up the extra embryonic membranes?

The: 1. umbilical cord
2. amniotic fluid
3. amniochorionic membrane
4. placenta
5. uterine wall/ endometrium


What happens to the psuedoautosomal genes during X inactivation?

They escape inactivation so females will have 2 copies of those genes to mirror how males have 2 copies of those genes.


True or False: If a mutated gene in a X-linked condition is lethal it it unlikely to produce an affected female?



Give some examples of X-linked monogenic diseases>

Haemophilia A
Fragile X
Anhydrotic dysplasia
Colour blindness