Download Ch.6: Sexual Identity

Ch.6: Sexual Identity
Genetics
Males (XY) are the heterogametic sex.
Females (XX) are homogametic sex.
X chromosome has over 1,000 genes.
Y chromosome is much smaller and are not sure
how many genes it carries.
Primary sex ratio (fertilization)-160 males for every
100 females
Secondary sex ratio (at birth)-105 males for every
100 females
Tertiary sex ratio- (20-25 yrs of age) 1:1
Thereafter, females outnumber males.
All embryos contain the same 2-sided
unspecialized gonads until the 6th week of
pregnancy. (See Fig.6.1, p.114)
Mullerian ducts-potential female structures
Wolffian ducts-potential male structures
On the Y chromosome is the male determining
gene called the SRY gene which stands for the
sex-determining region of the Y.
If the SRY gene is not expressed, the gonad
develops into an ovary. If the SRY gene is
expressed, the gonads specializes as testes.
Once the SRY gene has directed the testes
to form:
1) Sustentacular cells (Sertoli cells) in the
testes secrete anti-Mullerian hormone
that stops those ducts from forming
female structures.
2) Interstitial cells (Leydig cells) in testes
secrete testosterone which stimulates
development of internal male structures.
3) Some testosterone is converted to DHT
(dihydrotestosterone) which directs the
development of external male structures.
Genetic Abnormalities
1. Testicular feminization- (X-linked trait)chromosomal males develop as females. Testes
formation is normal & testosterone & MIH
production begins as expected. MIH (mullerian inhibiting hormone) brings about degeneration of
mullerian duct so no internal female traits are
produced. Mutation in X chromosome gene
blocks ability of cells to respond to DHT or
testosterone. Individuals develop female
structures. They are often attractive females,
but with little pubic hair and no menstrual cycle.
• Male Pseudohermaphroditism(autosomal recessive gene) testes are
present (indicate that SRY gene is
functioning) and Anti-mullerian hormone is
produced b/c the female set of tubes has
degenerated, no male external structures
till age 12.
Homosexuality
Researchers believe that homosexuality is 50%
genetically controlled and 50% environmentally
controlled.
Evidence presented
1. Feelings at a young age before knowing meaning of
term.
2. Timing of hormonal surges during prenatal
development show impaired mating ability.
3. Twin studies suggest genetic influence.
4. Two brain areas are of different sizes in homosexual
men vs. heterosexual men.
5. Studies by Hamer suggest that genes causing
homosexuality reside on X chromosome.
6. Studies have shown that if 2 brothers are homosexual
and 1 brother is heterosexual that the heterosexual
brother does not share the X chromosome markers.
7. Alterations in male fruit flies’ genes have led to
homosexual behavior.
• Sex-linked traits in humans are determined by genes
that are carried only on the X chromosome with no
apparent alleles on the Y chromosome.
• Males are considered hemizygous for sex-linked traits
because they have half the number of sex-linked genes
that the female has.
• If a trait is sex-linked, a male will pass it on to all his
daughters who may be heterozygous or homozygous for
the condition. A female heterozygote will pass a
recessive trait to 50% of her sons, each of whom will
express the recessive phenotype.
Lyon Hypothesis
Simply stated: compensation is accomplished by inactivating or turning
off ALL the genes on ONE OF THE X chromosomes in females.
1. In the somatic cells of the female mammals, one X chromosome is
active and the second X chromosome is inactive and tightly coiled
to form the Barr Body.
A normal male has NO barr bodies b/c his one X chromosome
remains active.
2. The inactive chromosome can be from the father, or from the
mother. In different cells of the same individual, a female
expresses the X chromosome genes inherited from the father in
some cells and those from her mother in other cells.
3. Inactivation takes place early in development. After 8-10 rounds of
mitosis following fertilization, each cell of the embryo randomly
inactivates one X chromosome. This inactivation is permanent in
somatic cells but not in the germ line cells which will become the
oocytes.
Lyons hypothesis also offers the idea that
females are mosaics, constructed of two
different cell types: one with the maternal
X chromosome active and one with the
paternal X chromosome active.
Ex: tortoise-shell cat and calico cats; Almost
always female, the only way a male cat
can have the coat color pattern is if he
inherits an extra X chromosome.
X inactivation has no effect on females
homozygous for sex-linked genes. However, for
heterozygous sex-linked genes X inactivation
leads to the expression of one allele or the other.
Manifesting heterozygote-a female carrier of a
sex-linked recessive gene who expresses the
phenotype because the wild type allele (normal)
is inactivated in some affected tissues.
X inactivation also has valuable medical
application in detecting carriers of some sexlinked disorders.
• Sex-limited Traits-affects a structure or
function of the body that is present in only
males or females.
Ex: breast development, facial hair growth
and heavy beard
• Sex-influenced Traits-traits that are
expressed in both sexes but the results of
the expression from allele is dominant in
one sex but recessive in the other.
Ex: Male pattern baldness
Genes on the X chromosome are classified as
dominant or recessive based on the patterns of
inheritance in the females.
X-linked Recessive traits: hemophilia,
Duchennes muscular dystrophy, color
blindness.
1. Affected males pass to all daughters.
2. Never transmitted from father to son but from
grandfathers to grandsons.
3. Phenotypic expression more frequent in males
than females.
X-linked Dominant Inheritance-a female who
inherits a dominant sex-linked allele has the
illness or trait. A male who inherits the
dominant allele is usually more severely
affected because he has no other
corresponding allele.
1. Affected males produce all affected daughters
and no affected sons.
2. A heterozygous affected female will transmit
the trait to ½ her children, with males and
females equally affected.
3. A homozygous female will transmit the trait to
all her offspring.