Discovering Behavioral Neuroscience: An Introduction to Biological Psychology
Detailed textbook notes for Chapter 10 Sexual Behaviour.
Freberg, L. (2018). Discovering Behavioral Neuroscience: An introduction to Biological Psychology. Cengage Learning.
W9 CH 10 - Sexual Behaviour
Sexual Development
The Genetics of Sex
- X chromosome - One of two types of sex chromosomes; individuals with two X
chromosomes will usually develop into females.
- Y chromosome - One of two types of sex chromosomes; individuals with a Y
chromosome will usually develop into males.
- Karyotype - A profile of chromosome number and appearance in the nucleus of a cell as
seen under a light microscope.
TURNER SYNDROME
- When a child receives only a single X chromosome (45, X) instead of the usual pair (46,
XX or 46, XY).
- Individuals with Turner syndrome have normal female external appearance and genitalia
but are usually infertile due to abnormally developing ovaries. This condition is also
associated with short stature, skin folds in the neck, and difficulty with spatial relations
tasks.
KLINEFELTER SYNDROME
- Caused by an XXY genotype, and characterized by frequent problems with fertility,
secondary sex characteristics, and verbal skills.
- These individuals are phenotypically male, but they usually experience reduced fertility
and require hormone treatment at puberty to promote the development of secondary
male sex characteristics (facial hair, deeper voice, development of external genitalia) and
to inhibit female characteristics such as breast development.
47,XYY SYNDROME
- Jacob’s syndrome; occurs in about 1 out of 1 ,000 male births. The physical and
behavioral correlates of the condition are typically subtle and generally do not prompt
the parents to seek a postnatal genetic analysis (Abramsky & Chapple, 1997). The boys
, appear to be physically within typical limits, although they tend to be somewhat taller
and leaner, suffer from acne, and have a higher risk for
minor physical abnormalities of the eye, elbow, and chest.
Average IQ scores are slightly below the average of males
with typical 46,XY genotypes (Linden, Bender, &
Robinson, 1996). Language difficulties and autism
spectrum disorder (ASD; see Chapter 16) are common in
cases of 47,XYY (Lepage et al., 2014). Men with the 47,XYY
genotype are fertile, but they are slightly more likely than
typical men to produce sperm with sex chromosome
abnormalities (Rives, Simeon, Milazzo, Barthelemy, &
Mace, 2003).
LESSONS LEARNED FROM INDIVIDUALS WITH SEX CHROMOSOME ABNORMALITIES
- Consider the fact that each gene produces a protein, which means that we can see
dose-dependent effects of a genotype on the resulting nervous system
- Regions on the X chromosome that are under-expressed are correlated with visuospatial
deficits, while over-expression is correlated with verbal deficits. These observations are
consistent with the cognitive challenges faced by individuals with Turner syndrome and
Klinefelter syndrome, respectively.
- In comparisons between individuals with Turner syndrome versus typically developing
females and Klinefelter syndrome versus typically developing males, the member of each
pair with the greater number of sex chromosomes demonstrated increased gray matter
volume in the parieto-occipital region of the cortex accompanied by decreases in the
temporo-insular area.
- Again, this pattern of cortical thickness is quite consistent with the characteristic
cognitive challenges seen in Turner and Klinefelter syndromes. Individuals with 47,XYY
showed patterns of brain structure that were more similar to males with Klinefelter
syndrome than to typically developing males, again supporting a dose-dependent effect
of extra sex chromosomes.
Three Stages of Prenatal Development
Development of male and female reproductive structures involves three distinct processes: the
development of gonads, of internal organs, and of external genitalia.
- Gonads - The internal organs, ovaries in females and testes in males, that produce
reproductive cells (eggs and sperm) and secrete sex hormones
- External genitalia - The external sexual organs, including the penis and scrotum in
males and the labia, clitoris, and lower third of the vagina in females.
In rare conditions known as intersex, elements of both male and female development occur in
the same fetus.
THE DEVELOPMENT OF THE GONADS
- Up until the sixth gestational week, both male and female embryos have identical
primordial gonads that have the capacity to develop into either ovaries [the female
, gonads; the source of ova and sex hormones], or testes [the male gonads; the source of
sperm and sex hormones].
- At about six weeks gestation, a gene on the short arm of the Y chromosome, known as
the sex-determining region of the Y chromosome, or SRY, is expressed in male
embryos.
- Testis-determining factor, the protein encoded by the SRY gene, switches on
additional genes that cause the primordial gonads to develop into testes.
- In female embryos, which lack the SRY gene and its ability to produce testis-determining
factor, alternate genes, including Wnt4 (located on Chromosome 1) and DAXl (located
on the X chromosome) guide the development of the primordial gonad into ovaries.
DIFFERENTIATION OF THE INTERNAL ORGANS
- Until about the third gestational month in humans, both male and female embryos
possess a male Wolffian system and a female Müllerian system.
- Wolffian system - The internal system that develops into seminal vesicles, vas
deferens, and the prostate gland in males.
- Müllerian system - The internal system that develops into a uterus, fallopian tubes,
and the upper two thirds of the vagina in the absence of anti-Müllerian hormone.
- During the third gestational month, the male's relatively new testes begin to secrete two
hormones, testosterone and anti-Milllerian hormone.
- Testosterone, one of several types of male hormone or androgen, promotes the
development of the Wolffian system. Anti-Miillerian hormone initiates the degeneration
of the Miillerian system.
- In the absence of any androgens or anti-Miillerian hormone, the Miillerian system will
develop in the typical female direction. In the absence of androgens, the Wolffian system
regresses, although nonfunctional remnants can persist in adult females.
- Androgen insensitivity syndrome (AIS) - A condition in which a genetic male fetus
lacks functional androgen receptors, which leads to the development of female external
genitalia and typically female gender identity and sexual behavior.
- Disrupts the normal development of the Wolffian system in males. In AIS, a
defective gene produces abnormal androgen receptors. As a result, the fetus's
tissues are blind to the presence of androgens. Fetuses with AIS have an XY
genotype and normal testes that remain undescended within the abdomen. The
testes release androgens and anti-Miillerian hormone in a typical manner, but the
lack of functional androgen receptors prevents the development of the Wolffian
system.
- However, anti-Miillerian hormone still works normally, so the female Miillerian
system also fails to develop. Because the Miillerian system is responsible for the
upper two thirds of the vagina and the female internal organs, the result is a
shallow vagina and no ovaries, fallopian tubes, or uterus. Although adult
individuals with AIS are infertile, their external appearance is quite typically
female.
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