Sex Determination and Sex Chromosomes

Flashcards generated from lecture notes on sex determination and sex chromosomes, embryology of the reproductive system, X-inactivation, Barr bodies, and the Lyon hypothesis.

What are somatic cells?

All cells that contribute to one's body, excluding those that develop into gametes (the germline).

How many chromosomes are in the nucleus of human somatic cells?

46 chromosomes, arranged in 23 pairs.

What are autosomes?

22 pairs of chromosomes that are alike in males and females.

What comprises the remaining pair of chromosomes that are not autosomes?

Sex chromosomes: an X and a Y chromosome in males, and two X chromosomes in females.

Which parent does a male inherit his X chromosome from?

His mother.

Which parent does a male inherit his Y chromosome from?

His father.

What is G banding?

A technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes, useful for identifying genetic diseases.

How many genes are on the Y chromosome compared to the X chromosome?

At least 75 genes on the Y chromosome, compared to 900-1400 genes on the X chromosome.

What are Pseudoautosomal regions (PARs)?

Regions present on both ends of the Y chromosome that share homology with regions on the X chromosome and synapse and recombine with it during meiosis.

What is the Male-specific region of the Y (MSY)?

About 95 percent of the Y chromosome does not synapse or recombine with the X chromosome; contains euchromatic regions and heterochromatic regions.

What is Sex-determining region Y (SRY)?

A critical gene that controls male sexual development, located within euchromatin, adjacent to the PAR of the short arm of the Y chromosome.

What does the SRY gene do?

Encodes a protein called the testis-determining factor (TDF) that causes the undifferentiated gonadal tissue of the embryo to form testes.

When does the SRY gene become active in XY embryos?

At six to eight weeks of development.

What is the role of primordial germ cells?

Migrate from their earlier extraembryonic location to the paired genital ridges by the sixth week of development.

When is the developing gonad considered ambipotent?

Up to the sixth week of development, regardless of whether it is chromosomally XX or XY.

What determines development into an ovary or a testis?

The coordinated action of a sequence of genes in finely balanced pathways.

What is X chromosome inactivation?

The process by which most genes on one of the two X chromosomes in females are silenced epigenetically.

What is the principle of X inactivation?

In somatic cells in normal females, one X chromosome is inactivated early in development, equalizing the expression of X-linked genes in the two sexes.

What happens to extra X chromosomes in patients with extra X chromosomes?

Any X chromosome in excess of one is inactivated.

What is a Barr body?

An inactivated, condensed X chromosome found in female cells.

Why are Barr bodies essential?

To regulate the amount of X-linked gene product being transcribed.

What is XIST?

X Inactive Specific Transcript (Non-Protein Coding) - Long Intergenic Non-Protein Coding RNA, expressed on the inactive chromosome.

What is the Lyon Hypothesis?

Inactivation of X chromosomes occurs randomly in somatic cells early in embryonic development, and once inactivation has occurred, all descendant cells have the same X chromosome inactivated as their initial progenitor cell.

What is the result of heterozygous females displaying mosaic retinas?

Patches of defective color perception and surrounding areas with normal color perception.

What is the X inactivation center (XIC)?

A region on the X chromosome required for inactivation and contains the XIST gene.

What happens on structurally abnormal X chromosomes that lack the XIC?

X inactivation cannot occur and genes present on the abnormal X are expressed biallelically.

What is Klinefelter syndrome?

A sex chromosome abnormality (typically 47,XXY) where males are tall and thin, and develop hypogonadism at puberty.

What is Turner syndrome?

A sex chromosome abnormality (45,X) in females characterized by gonadal dysgenesis and amenorrhea.

What are disorders of sex development (DSD)?

Conditions ranging from gonadal abnormalities to complete incompatibility between chromosomal and phenotypic sex.

Define Gonadal dysgenesis.

A progressive loss of germ cells, typically leading to underdeveloped and dysfunctional (“streak”) gonads with consequent failure to develop mature secondary sex characteristics.

What is hypospadias?

A developmental anomaly in males in which the urethra opens on the underside of the penis or on the perineum.

What can imbalance in the expression of major genes in the sex development pathways lead to?

Testis formation, even in the absence of a Y chromosome, or to ovarian development, even in the presence of the Y.