Sex Determination and Sex Chromosomes

Sex Chromosomes

The last pair of human chromosomes, are known as?

Ploidy

refers to the number of sets of chromosomes in a cell.

X & Y chromosomes.

Within the normal 23 pairs of human chromosomes, one pair was shown to vary in males & females.

Y chromosomes

Between the two sex chromosomes, which influences the immature gonads to become testis?

Genetic sex

is established at the time of fertilization and is dependent on whether an X- or Y-bearing sperm fertilizes the X-bearing egg. 

Gonadal sex

the type of gonads that develop, is determined by the sex chromosome complement (XX or XY) and sex-determining genes

Before the seventh week

Role in Sexual Differentiation when does of embryonic life, the gonads of both sexes appear identical.

Pseudoautosomal Regions

Distal region of the p arms of the X and Y chromosomes contains highly similar DNA sequences.

PAR2

a region of homology is at the distal ends of Xq and Yq, which has been observed to associate during male meiosis, with proven recombination events. and also has 320 kb and has 4 genes only.

PAR1

is 2.6 Mb and contains at least 24 genes

Pseudoautosomal Region 2

What is the region located at the distal ends of the long arms of X & Y chromosomes ?

Dr. Mary Lyon

Who developed the Lyon hypothesis?

Lyon hypothesis

The X inactivation is also termed, the

Barr body.

when a female’s X chromosome becomes inactivated, it is converted into a

Susumo Ohno

was the first to suggest that the Barr body arises from one of the two X chromosomes

XIST (X-inactive-specific transcript) 

A gene that controls X inactivation is

X-inactivation center (XIC) at band Xq13

Where is XIST (X-inactive-specific transcript)  located?

XIST RNA

it accumulates along the X chromosome containing the active XIST gene and proceeds to inactivate all (or almost all) of other genes on that chromosome

Male Specific Region of the Y (MSY)

located between PAR1 and PAR2 

Azoospermia factor ( AZF )

A gene controlling spermatogenesis

Tiepolo and Zuffardi

proposed that Azoospermia factor is a gene controlling spermatogenesis and mapped to the distal part of the euchromatic Yq11 region.

Testis-determining factor (TDF)

leads to differentiation of the indifferent gonads into testes, located on the short arm of the Y chromosome

Gene SRY  (sex-determining region Y)

It is located on the short arm of the Y at band p11.3 in the MSY region

Mosaicism

is more common for sex chromosome abnormalities than autosomal abnormalities

Turner Syndrome (45,X)

•  occurs in approximately 1 in 2,500 live-born females 

• One of the most common chromosome abnormalities in spontaneous abortions

• 75% of patients have this, and the X chromosome is maternal in origin.

Trisomy X (47,XXX)

• is the most frequent sex chromosome disorder present at birth in females, occurring in 1 in approximately 1,000 live female birth

superfemale

Trisomy X was termed _________ but gives a misconception of the syndrome and is no longer in use

Klinefelter Syndrome (47,XXY)

• First sex chromosome disorder to be described and its cytogenetic cause identified

• Most common cause of hypogonadism and infertility in males

47,XYY

One in 800–1,000 males has an extra Y chromosome.

This arises through nondisjunction at paternal meiosis II.

 Isochromosome X

consisting of two copies of the long arm (missing all or most of the short arm), as a single cell line or, more commonly, in mosaicism with a 45,X cell line.

Y chromosome

• Lead to deletion of the proximal long arm might be associated with azoospermia, infertility, and short stature. 

• Marker chromosomes are important to detect due to the risk of gonadoblastoma in females with Turner syndrome. 

• FISH probes and chromosome microarray identify marker