Sexual reproduction allows offspring that vary from parents
Fertilization: mechanism by which new organisms are produced via sexual reproduction
In animals, including humans, sexual differentiation is evident as phenotypic dimorphism of males and females
Phenotypic sexual dimorphism example: presence of an Adam’s apple in male humans
Sex chromosomes: characterize one sex or the other in a wide range of species
Heteromorphic chromosomes: dissimilar chromosomes (example is sex chromosomes X and Y)
Sex determination is done by specific genes instead of an entire chromosome
Homogametic sex:
producing like/same chromosomes
zygotes with two X chromosomes
results in female offspring
Heterogametic sex:
producing unlike/different chromosomes
zygotes with one X and one Y chromosome
results in male offspring
XX/XO mode of sex determination:
Protenor (butterfly)
depends on random distribution of X chromosome into half of male gametes
presence of two X chromosomes in zygote results in female offspring
presence of one X chromosome results in male offspring
A hermaphrodite is also considered a homogametic sex. They also have XX. this can be seen in C. Elegans. (worm) these hermaphrodites are capable of making eggs and sperm at the same time and are thus capable of self mating.
ZZ/ZW sex determination:
Xenopus laevis (frog)
females are actually the heterogametic sex (ZW)
males are the homogametic sex (ZZ)
Danio rerio (fish) have no determinate sex chromosomes despite possessing male and female counterparts of the species
Y chromosome determines maleness
Human karyotype:
22 pairs of autosomal chromosomes
2 sex chromosomes
reveals on pair of chromosomes differs in males and females
(Human) Female: XX
(Human) Male: XY
Klinefelter and Turner syndrome:
two human abnormalities
characterized by aberrant sexual development
both syndromes result from nondisjunction which is the failure of the X chromosome to segregate during meiosis
Normal meiotic division will result in 4 gametes with a single “n”
Nondisjunction in meiosis 1 will result in four gametes that are n+1, n+1, n-1, and n-1
Nondisjunction in meiosis will result in four gametes that are n+1, n-1, n, and n
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 is not suppressed so enlarged breasts are common as are rounded hips
occurs in about 1 of every 660 male births
Turner syndrome (45, X):
Phenotypically female
female external genitalia and internal ducts
ovaries are rudimentary
underdeveloped breasts
short stature
occurs in about 1 in 2,000 female births
47, XXX syndrome: Triplo-X
three x Chromosomes
normal set of autosomes
results in female differentiation
sometimes women are perfectly normal
sometimes underdeveloped secondary sec characteristics occur and sterility and mental retardation presents
47, XYY condition:
the only consistently shared characteristic is that males are over 6 feet tall.
subnormal intelligence
personality disorders
Sexual differentiation in humans:
Early development
Fifth Week of development
further into development
similar time point as ovary and testis development
completion of development
(Sex determination) Early development: embryo is potentially hermaphroditic
(Sex determination) Fifth week of development: gonadal (genital) ridges form but the individual is still potentially hermaphroditic
(Sex determination) further into development: germ cells move to the gonadal ridges and begin developing ovaries or testis. if cells have a y, testis form. If cells have no y, ovaries form.
(Sex determintaion) Similar time point as ovary and testis development: reproductive tracts develop from undifferentiated ducts. if cells have a y, male tract forms. If cells have no y, female tract develops.
(Sex determintaion) Completion of development: the development of male or female organs continues to drive sexual differentiation throughout the rest of the developmental process
SRY gene produces the testis determine SRY protein which then initiates the production of multiple proteins that cause gonad medulla to differentiate into a testis
Testis have leydig cells which secrete testosterone which controls the development of wolffian duct into accessory structures and development of male external genitalia
Testis have Sertoli cells which secrete anti-müllerian hormone which causes the reduction of the müllerian duct
Müllerian duct is the fallopian tubes and uterine tubes
Y chromosom:
has at least 75 genes
fewer genes than the X chromosome
some of these genes are homologous to genes on the x chromosome and some are not
PARs: Pseudoautosomal regions
present on both ends of Y Chromosome
share Homology with regions on X chromosome
MSY: Male specific region of the Y chromosome
non-recombining region of Y chromosome
contains euchromatin (SRY: Sex-determining region of Y chromosome, located adjacent to PAR of the short arm of Y chromosome. Controls male development. Encodes protein: testis-determining factor (TDF)
heterochromatin
Dosage compensations:
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 condensed
darkly stained body in interphase nerve cells observed: Barr bodies