Ch 7: Sex Determination

What is the XX/XY (Lygaeus-type) system of sex determination?

Females are XX (homogametic) and males XY (heterogametic); the Y chromosome determines maleness. Found in mammals and Drosophila.

What is the XX/XO (Protenor-type) system?

Females are XX and males XO (one X, no Y). Found in grasshoppers and some nematodes; presence or absence of the second X determines sex.

What is the ZZ/ZW system?

Males are ZZ (homogametic) and females ZW (heterogametic); W chromosome determines femaleness. Found in birds, butterflies, and some reptiles.

What is the X : A ratio system of sex determination?

Sex depends on the ratio of X chromosomes to sets of autosomes (A); in Drosophila, 1.0 (2X:2A) = female, 0.5 (1X:2A) = male.

What does “homogametic sex” mean?

The sex producing only one type of gamete regarding sex chromosomes; e.g., human females (XX).

What does “heterogametic sex” mean?

The sex producing two types of gametes (X and Y or X and O); e.g., human males (XY).

Which species have heterogametic females?

Birds, butterflies, and some reptiles, all using the ZZ/ZW system.

In XX/XY species, which parent determines the offspring’s sex?

The male; sperm carry either X or Y chromosomes, leading to XX (female) or XY (male) zygotes.

In ZZ/ZW species, which parent determines the offspring’s sex?

The female; eggs carry either Z or W chromosomes, producing ZZ (males) or ZW (females).

How is sex determined in X:A ratio systems?

By the relative number of X chromosomes to autosome sets, not by Y presence.

What is aneuploidy?

An abnormal chromosome number (2n ± x) caused by nondisjunction during meiosis or mitosis.

What is monosomy?

Missing one chromosome (2n – 1); example = Turner syndrome (45,X).

What is trisomy?

Having one extra chromosome (2n + 1); example = Klinefelter syndrome (47,XXY).

What is tetrasomy?

Presence of two additional homologs (2n + 2).

What is nullisomy?

Loss of both homologs of a chromosome pair (2n – 2); lethal in humans.

What is karyotyping?

Laboratory analysis where chromosomes from dividing cells are stained and arranged to identify number and structural abnormalities.

What causes aneuploidy?

Nondisjunction—failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

What condition results from the 45,X karyotype?

Turner syndrome—phenotypic female, short stature, rudimentary ovaries, infertility.

What condition results from 47,XXY?

Klinefelter syndrome—phenotypic male, tall, small testes, infertility, possible gynecomastia.

What 2 conditions result from nondisjunction?

Turner syndrome and Klinefelter syndrome

What condition results from 47,XXX?

Triple-X syndrome—phenotypic female, often normal but tall; extra X can cause mild learning differences.

What condition results from 47,XYY?

Jacob’s syndrome—phenotypic male, tall, sometimes mild learning differences.

What is the main biological function of the Y chromosome?

To initiate male sex determination and support spermatogenesis.

What is the SRY gene?

“Sex-determining Region Y”; encodes TDF (Testis-Determining Factor) that triggers testis development.

What are pseudoautosomal regions (PARs)?

Homologous regions on X and Y that recombine during meiosis, ensuring proper segregation.

What is the MSY region?

The Male-Specific Region of Y that does not recombine and contains SRY and fertility genes.

What are the subregions of the MSY?

X-transposed, X-degenerate, and ampliconic (repetitive, testis-specific) regions.

What genes on the Y are crucial for spermatogenesis?

DAZ genes within MSY; deletions cause azoospermia or infertility.

What are bipotential gonads?

Embryonic structures that can develop into either testes or ovaries depending on gene and hormone signals.

What directs the formation of testes in embryos?

Expression of SRY → production of TDF → testis development.

Which hormones are secreted by developing testes?

Anti-Müllerian Hormone (AMH) and testosterone.

What does AMH do?

Causes regression of the Müllerian ducts, preventing female internal structures.

What is the role of testosterone in development?

Promotes Wolffian duct differentiation into male internal ducts (epididymis, vas deferens, seminal vesicles).

How is DHT (dihydrotestosterone) involved in male development?

Converted from testosterone; required for development of external male genitalia.

What happens in the absence of SRY and androgens?

Gonads become ovaries; Müllerian ducts form uterus and oviducts; female phenotype develops.

What is the primary sex ratio?

The ratio of male to female zygotes at conception.

What is the secondary sex ratio?

The ratio of male to female births.

What did Orzack et al. (2015) conclude about the human primary sex ratio?

It is approximately 1 : 1, showing no male bias at conception.

Why is the secondary sex ratio slightly male-biased?

Because female embryos are lost slightly more often early in gestation, while male fetuses die more often later.

What provided the most direct evidence for the true sex ratio?

Genetic sexing of early human embryos (IVF and abortus studies) showing nearly equal XX and XY numbers.

Why is X-inactivation necessary in mammals?

To equalize expression of X-linked genes between XX females and XY males.

State the Lyon hypothesis.

One X chromosome in each female somatic cell is randomly inactivated early in embryogenesis and remains inactive (Barr body).

What is a Barr body?

The condensed, inactive X chromosome visible in interphase nuclei; number = N – 1.

What does “N – 1 rule” mean?

The number of inactive X chromosomes equals total X’s minus one.

What is the phenotypic effect of X-inactivation in heterozygous females?

Mosaic expression—some cells express one X allele, others the other; e.g., calico cats.

What provides direct evidence for X-inactivation?

Presence of Barr bodies and detection of XIST RNA coating the inactive X.

What provides indirect evidence for X-inactivation?

Patchy phenotypes in females heterozygous for X-linked traits (e.g., color blindness, anhidrotic ectodermal dysplasia).

What is the XIST gene?

“X-inactive specific transcript,” a long non-coding RNA that coats and silences the X to be inactivated.

What determines sex in Drosophila?

The ratio of X chromosomes to autosome sets (X : A), not the presence of Y.

What X : A ratio produces a female Drosophila?

1.0 (2X : 2A).

What X : A ratio produces a male Drosophila?

0.5 (1X : 2A).

What X : A ratio produces an intersex Drosophila?

0.67 (typically 3X : 4A or similar intermediate ratios).

What X : A ratio produces a metafemale Drosophila?

> 1.0; usually inviable.

What X : A ratio produces a metamale Drosophila?

< 0.5; typically sterile.

Does the Y chromosome determine sex in Drosophila?

No; it affects male fertility but not sex determination.

How can attached-X chromosomes or nondisjunction events affect Drosophila sex outcomes?

They alter the X number in gametes, creating XX or O eggs that yield intersex, metamale, or metafemale offspring depending on X : A ratios.

How is sex determined in C. elegans?

By X : A ratio; XX = hermaphrodite, XO = male.

What is the chromosomal composition of a C. elegans hermaphrodite?

XX

What is the chromosomal composition of a C. elegans male?

XO

How are C. elegans males produced in hermaphrodite populations?

By X nondisjunction during hermaphrodite gamete formation, generating XO males.

In humans, what determines maleness?

Presence of the Y chromosome and active SRY gene.

In Drosophila, what determines maleness?

X : A ratio of 0.5, regardless of Y presence.

What is the role of the Y chromosome in Drosophila?

Required for spermatogenesis and male fertility but not for primary sex determination.

How do mammals, Drosophila, and C. elegans achieve dosage compensation?

Mammals → inactivate one X in females.

Drosophila → hyper-transcribe male X.

C. elegans → reduce transcription of both X’s in hermaphrodites by half.