Chapter 7 | Sex Determination and Sex Chromosomes

Y Chromosome

Smaller than the X chromosome; contains fewer genes

Hemizygous

Males have only one X and one Y chromosome, so they have a single copy of genes located on the X (and Y)

Undifferentiated gonad

The embryonic structure that can develop into either testes or ovaries

TDF (Testis Determining Factor)

A transcription factor that initiates male development by triggering testes formation

Default embryonic state

Female; male development requires active suppression of female pathways by TDF

Role of TDF in development

Promotes regression of Müllerian ducts and development of male reproductive structures like vas deferens

Transcription factor

A protein that binds to DNA promoters to regulate gene expression by recruiting or blocking RNA polymerase

SRY gene

Located on the Y chromosome; codes for TDF

Promoter

DNA sequence upstream of a gene that transcription factors bind to in order to initiate transcription

Epigenetics

Study of heritable changes in gene expression that do not involve changes in DNA sequence

Euchromatin

Loosely packed chromatin; accessible to transcription machinery

Heterochromatin

Tightly packed chromatin; inaccessible to transcription machinery

Reversibility of epigenetics

Chromatin can switch between euchromatic and heterochromatic states

Epigenetic inheritance

Some chromatin states are passed from cell to cell and parent to offspring

Dosage compensation

Mechanism to balance X-linked gene expression between males (1 X) and females (2 Xs)

X-inactivation

Process where one X chromosome in female cells becomes inactivated and forms heterochromatin

Lyon Hypothesis

Random inactivation of one X chromosome in early female development; all daughter cells inherit this pattern

Female genetic mosaic

A female has patches of cells expressing genes from different X chromosomes due to random X-inactivation

Xist RNA

Non-coding RNA that coats the X chromosome to be inactivated, initiating heterochromatin formation

Barr Body

The inactivated X chromosome in a female cell.

G6PDH gene

X-linked gene used as an example of X-inactivation; involved in glycolysis

Pseudoautosomal regions (PARs)

Homologous regions on X and Y chromosomes allowing them to pair during meiosis

MSY (Male-specific region of the Y)

Part of the Y chromosome that does not recombine with the X

Chromatin types on Y chromosome

Contains both euchromatin (active) and heterochromatin (inactive)

Primary sex ratio

1 male:1 female at fertilization

Secondary sex ratio

Approximately 1.06 males per female at birth; due to higher female embryonic mortality

Nondisjunction

Failure of chromosomes or chromatids to separate properly during meiosis or mitosis

Meiosis I nondisjunction

Homologous chromosomes fail to separate; results in gametes with extra or missing chromosomes

Meiosis II nondisjunction

Sister chromatids fail to separate; also results in abnormal gametes

Monosomy

Having only one copy of a chromosome; often lethal

Trisomy

Having three copies of a chromosome; can cause syndromes like Down syndrome

Mitotic nondisjunction

Occurs during early embryo development; can result in mosaic conditions

Aneuploidy

Abnormal number of chromosomes (46 ± 1)