Chapter 7 Sex Chromosomes and Sex Determination

Sex Chromosomes

refer to chromosomes that differ between males and females

Found in most species of animals and a few plant species

Designated X and Y

Sex Determination

process that determines the development of sexual characteristics that give rise to males and females

X-Y (Lygaeus) System in Mammals

Humans have 46 chromosomes

44 autosomes(non-sex)

2 sex chromosomes 

Males chromosomes 

one X and one Y

termed heterogametic 

Female chromosomes

termed homogametic

What does Y chromosomes

determines maleness

Y chromosome and male development

Y chromosome has at least 50 genes

Fewer genes than X chromosome (100 genes)

PARs: Pseudoautosomal regions

Present on both ends of Y chromosome

Share homology with regions on X chromosome

Synapse and recombine with X during meiosis

Pairing region critical to segregation of X and Y chromosomes during male gametogenesis

MSY: Male-specific region of the Y

Nonrecombining region of Y chromosome

23 million base pairs

Divided into three regions

MSY: Divided into three regions

X-transposed region (15 percent of MSY)

X-degenerative region (20 percent)

Ampliconic region (30 percent)

Ampliconic region (30 percent)

Encodes proteins specific to testis development and function

SRY: Sex-determining region Y

Located adjacent to PAR of the short arm of Y chromosome

Controls male development

Controls male development

TDF: Testis-determining factor

TDF: Testis-determining factor

At 6–8 weeks of development SRY gene becomes active in XY embryos

Encodes protein that triggers testes formation

Dosage compensation

Genetic mechanism

Balances dose of X chromosome gene expression in males and females

Prevents excessive expression of X-linked genes in humans and other mammals

Barr bodies (sex chromatin bodies)

Genetic mechanism compensates for X dosage disparities

Inactive X chromosome, highly condense

Darkly stained body in interphase nerve cells observed: Barr bodies Random inactivation

Occurs early in embryonic development

X-inactivation

Explains dosage compensation

Follows N − 1 rule (N = total number of X chromosomes)

Lyon hypothesis

Inactivation of X chromosome is random

Occurs in somatic cells at early stage of embryonic development

All descendant cells have same X-inactivation

Example: Calico cats and fur color/patterns

Xic: X inactivation center

Active only on inactive X

Has X-inactive specific transcript (XIST) gene critical for X inactivation

Two noncoding genes in Xic locus

Two noncoding genes in Xic locus

Tsix and Xite play important roles in X chromosome inactivation

Klinefelter and Turner Syndrome

Two human abnormalities

Characterized by aberrant sexual development

Both syndromes result from nondisjunction

Failure of X chromosomes to segregate during meiosis

Klinefelter Syndrome

Klinefelter syndrome (47,XXY)

Tall, long arms and legs

Large hands and feet

Internal ducts are male, rudimentary testes fail to produce sperm 

Feminine development not suppressed

Enlarged breasts common, rounded hips

Turner Syndrome

Turner syndrome (45,X)

Phenotypically female

Female external genitalia and internal ducts

Ovaries are rudimentary

Underdeveloped breasts

Short stature

Cognitive impairment

Triplo-X

47,XXX syndrome: Triplo-X

Three X chromosomes 

Normal set of autosomes

Results in female differentiation

Sometimes women are perfectly normal

Sometimes underdeveloped secondary sex characteristics (sterility and mental retardation) occur

47, XYY Condition

Only consistently shared characteristic – males are over 6 feet tall

Subnormal intelligence

Personality disorder

X-0 (Protenor) Sex Determination

In some insects and nematodes

Males are X0 and females (or hermaphrodites) are XX

X-0 (Protenor) Sex Determination

In other insects (fruit fly)

Males are XY and females are XX

X-0 (Protenor) Sex Determination

The Y chromosome does not determine maleness

Rather, it is the ratio between the X chromosomes and the number of sets of autosomes (X ∕ A) 

If X ∕ A = 0.5, the fly becomes a male

If X ∕ A = 1.0, the fly becomes a female

Z–W Sex Determination

Birds and some fish

The sex chromosomes are designated Z and W to distinguish them from the X and Y chromosomes of mammals

Z–W Sex Determination Males

Males contain two Z chromosomes

They are homogametic

Z–W Sex Determination

Females have one Z and one W chromosome

Females have one Z and one W chromosome

They are heterogametic

Haplodiploid Sex Determination in Bees

Males are known as the drones

Females include the worker bees and queen bees

Female Bees

They are diploid

Produced from fertilized eggs

Male Bees

They are haploid

Produced from unfertilized haploid eggs

Sex Differences May Depend on the Environment

Sex in some reptiles and fish is controlled by temperature

In alligators, environmental temperatures determine sex

Clownfish are protandrous hermaphrodites

the reproductive male can change sex to become a female when the female of a harem dies. The largest of the non-reproductive juveniles matures into a reproductive male.

Hormone levels control the expression of genes that regulate sex

The female of a harem prevents the formation of other females by exhibiting aggressive dominance; this behavior inhibits the production of hormones in other clownfish needed for female development.