sex determination and differentiation

sexual dimorphism

the observable differences in physical appearance, behavior, and other traits between males and females of the same species; males and females exhibit distinct characteristics that set them apart. 

experiments

this must be done to look at the underlying cause (biological and/or environmental factors) of sex differences in behavior

strategies used to understand human behavioral sex differences

1. different cultures with different child rearing practices to identify commonalties in the behavior of all children

2. fluid samples obtained through amniocentesis testing

3. people with differences in sexual development

4. animal models

although these are indirect, converging evidence from several approaches can help us understand behavioral sex differences

parthenogenesis

asexual reproduction in insects and some vertebrates

• only one sex (female)

• produce genertically identical eggs that develop into female offspring that are genetically identical to their mother

reproductive success

from evolutionary standpoint, this reflects the amount of genetic material an individual passes on to their offspring

asexual reproduction

100% reproductive success; however there is no genetic variation, which creates a risk for extinction when environmental conditions change

sexual reproduction

50% reproductive success but produces more genetic variation and evolutionary flexible

why do sexual dimorphisms arise?

Sexual selection favors sexual dimorphisms - males compete and females choose the males with the best genes; so males have evolved to be “better”

Different cultures with different child-rearing practices to identify commonalities in the behavior of all children

behaviors that emerge consistently regardless of different cultural practices suggest that those differences may be mediated by biological factors

Fluid samples obtained through amniocentesis testing

hormone concentrations correlated with future behavioral patters

People with differences in sexual development

this can help separate environmental versus hormonal influences

Animal models

can experimentally control environmental conditions and manipulate hormonal conditions 

Sexual differentiation

development process of becoming a male or female

Sex determination

sperm that fertilizes the egg carries an X or a Y chromosome; determines chromosomal sex

Chromosomal Sex:

Female: XX (homogametic)

Male: XY (heterogametic)

Gonadal Sex:

Female: Ovaries (eggs)

Male: Testes (sperm)

Hormonal Sex:

Female: high estrogen, low androgen

Male: low estrogen, high androgens

Morphological Sex

differences in body type (i.e. size/shape, external genitalia)

Behavioral Sex:

discriminated on the basis of male and female typical behaviors

fertilization

when chromosomal sex is determined

1. Development of Gonads

2. Development of Accessory Sex Organs

3. Development of External Genitalia

Three stages of sexual differentiation after chromosomal sex

Gonads

internal sex organs; females have ovaries, while males have testes.

germinal ridge

bipotential primordial gonad; every individual (females and males) possesses this gonad; later develops into testes OR ovaries

testes development

Presence of SRY gene (gene located on Y chromosome) = leads to the secretion of testis-determining factor = which causes the germinal ridge to develop into testes

ovary development 

No Y chromosome = No SRY gene = No secretion of testis-determining factor = germinal ridge develop into ovaries 

Accessory sex organs

 organs that connect the gonads to the outside environment.

Wolffian System

Male accessory sex organs precursor; develops into seminal vesicles and vas deferens 

Mullerian System

Female accessory sex organs precursor; develops into fallopian tubes, uterus and cervix 

Testosterone and Mullerian Inhibiting Hormone

hormones involved in the differentiation of the accessory sex organs 

Testosterone:

promotes the development of Wolffian System (masculinizing effect)

Mullerian Inhibiting Hormone:

prevents Mullerian system from developing (defeminizing effect)

female accessory sex organ development

DOES NOT REQUIRE HORMONES! 

Presence of Ovaries OR Absence of Gonads - the Mullerian system develops (feminizing effect) and the Wolffian system regresses (demasculinizing effect)

Masculinzation

induction of male traits

demasculinzation

removal of the potential of male traits

feminization

induction of female traits

defeminization

removal of potential of female traits

Penis and Scrotum

Male external genitalia 

Labia, Clitoris and Outer Vagina

Female external genitalia

genital tubercle

genital folds

early on in the development, both sexes have these structures that develop into external genitalia 

5(alpha)-DHT

androgen that propels the genital tubercle develops into a penis and the gential folds fuse into a scrotum

5(alpha)-reductase

converts testosterone into 5(alpha)-DHT

Female external genitalia development

No androgens = a clitoris develops from the genital tubercle and the vagina labia develops from the genital folds; no hormonal activity required for the development of female genitalia

evidence that chromosomes may directly influence sex difference in the brain and behavior.

1. SRY mRNA has been detected in the brain during development 

2. Many genes are differentially expressed between male and female brains prior to gonadal formation 

3. Four core genotypes model

Four core genotypes model

1. mice are genetically modified so that the sex chromosomes are unrelated to the animal’s gonadal sex (females with testes, females with ovaries, males with testes, males with ovaries)

   1. Used to show chromosomal sex differences in behaviors and disease 

differences of sexual development (DSD)

intersex; individuals who are born with reproductive or sexual anatomy that does not seem to fit the traditional definitions of male or female bodies 

 DSDs that result from sex chromosome abnormalities

Turner Syndrome (X)

Klinefelter Syndrome (XXY)

XYY

Turner Syndrome (X)

1. Congenital lack or damage to the second chromosome 

2. Features: female typical external appearance and genitalia, underdeveloped ovaries, require hormone treatment to induce puberty, infertile, short stature due to other hormonal abnormalities that slow growth

Klinefelter Syndrome (XXY)

1. Presence of Y chromosome is sufficient for the SRY gene to be activated and masculinization to occur 

2. Features: appear male but testes are small; sterile; reduced muscle mass, less body hair, disproportionally long limbs, poor-coordination; normal intelligence, but may have reading or speech problems

XYY

1. Due to random event during sperm formation

2. Features: typical male appearance but above average height; learning disabilities

Congenital adrenal hyperplasia causes:

1. Adrenal dysfunction resulting from lack of an enzyme which is needed to make adrenal hormones leading to under-production of cortisol and aldosterone as well as over-production of androgens

   1. Treatment: cortisol and aldosterone 

2. Prenatal exposure to exogenous androgens

   1. Treatment - treatment or raise the XY female as a boy 

Congenital adrenal hyperplasia features

1. External genitalia in XY males is unaffected

2. XX females - females exposed to exogenous androgens have moderate-severe masculinized genitalia + behavioral masculinization

5(alpha)-reductase deficiency

caused by the lack of 5(alpha)-reductase enzyme (which converts testosterone to 5(alpha)-DHT, which leads to the formation of external male genitalia such as the penis and scrotum). 

5(alpha)-reductase deficiency features

1. XY males with female appearing or ambiguous external genitalia; development of testes and accessory sex organs is unaffected 

2. Hypospadias - urethra opens at base of penis; reduces fertility

Finasteride

drug that is used to treat enlarged prostate and male pattern baldness 

• Blocks 5(alpha)-reductase

testicular feminization mutation 

Androgen Insensitivity Syndrome in rodents 

Androgen Insensitivity Syndrome (XY genotype)

gonads develop as testes, which release testosterone and MIH 

• Genetic mutation prevents the formation of androgen receptors → testosterone does not have a masculinizing effect and Wolffian system does not develop → external genitalia develop in the female direction 

• MIH has a defeminizing effect - so there is somewhat development of male accessory sex organs 

treatment: testes surgically removed, vaginoplasty performed and estrogen treatment

Androgen Insensitivity Syndrome (XY genotype) features

• Genetic males with normal-appearing female external genitalia; female appearance and unambiguous female gender identity  

• Discovered at puberty when individual males to menstruate and there is sparse/absent pubic hair