Transcript sex chromosomes

Sex and Development
Chapter 1
Central Points
 Fetus’ chromosomal sex determined at
fertilization
 Fetal development has many stages
 During development, a fetus becomes male or
female
1.1 How Is the Sex Determined?
 Complex interactions between the genes and
environment early in fetal development
 Secondary sex characteristics are not directly
related to reproduction
Human Chromosomes
 22 pairs of autosomes and 1 pair of sex
chromosomes (=46 total chromosomes)
 The chromosomal make of an individual can be
determined by a karyotype
 Sex chromosomes determine sex in humans
Females XX
Males XY
The Human Chromosome Set
Giemsa – binds to where the DNA is rich in adenine and thymine
Sex Ratios
 ~Equal numbers of X- and Y-bearing sperm are
produced
 Sex ratio: proportion of males to females, expect
1:1
 At birth, about 1:1.05
 Between ages 20 and 25, ratio is close to 1:1
 Ratio decreases with age
X and Y Chromosomes in Sex Determination
Procedures to Select Sex of a Baby
Scientific procedures for
sex selection:
 Sperm sorting
 Preimplantation
genetic diagnosis
(PGD)
Sperm Sorting
Y
Layer
with
sperm
1) Sperm are
separated from
surrounding
fluids, using a
centrifuge.
2) Sperm are then
placed in a saline
solution containing
a fluorescent dye,
which binds to the
DNA in the sperm.
X
3) As the sperm
pass one by
one through a
detector, a laser
bounces light off
the dyed DNA.
4) Because X-bearing
sperm have more
DNA and reflect more
light than Y-bearing
sperm, each can
be separated into
different test tubes.
Sperm Sorting
 Sperm sorted by laser based on how brightly they
fluoresce
 Sperm placed in uterus or eggs fertilized in a
dish: $4,000–$6,000 (more if eggs must be
collected)
 Success rate: ~90% for female births, ~73% for
male births
 Acceptable method for some situations
Preimplantation Genetic Diagnosis (PGD)
Eight-cell embryos
Single cells
Embryos
Transfer of selected
embryos to patient
Removal of
single cell
for sex
chromosome
analysis
1) Eggs are removed
2) Single cells
3) Embryos
from the ovary,
fertilized, and grown
to the eight-cell stage.
are identified
as either male
or female.
of the
desired sex
are selected.
4) The selected
embryos are
transferred to the
uterus for
development.
Preparation for Preimplantation Genetic
Diagnosis (PGD)
 Woman given hormone treatments
 Multiple eggs surgically removed
 Eggs placed in a dish with sperm until the eggs
are fertilized by in vitro fertilization (IVF)
 Embryos develop to the eight-cell stage, then
PGD is performed
Preimplantation Genetic Diagnosis (1)
 Process of PGD
• Single cell removed from each embryo
• Chromosomes analyzed to determine the sex
• Embryos of the desired sex placed into the
mother’s uterus
• Unused embryos discarded or frozen for later use
or donation to others
Preimplantation Genetic Diagnosis (2)
 More invasive procedure increased risk for mother
 $12,000–$15,000
 Success rate: ~100%
Stages in Human Fertilization
1.2 Development: Fertilization to Birth
 Fusion of sperm and egg, generally occurs in
fallopian tube (oviduct)
 Sperm deposited in vagina move through the
cervix, uterus, and oviduct
 Usually one sperm enters the egg (chemical
changes prevent > one sperm)
 Nuclei fuse to form a zygote with 46
chromosomes
Development: Fertilization through
Implantation
Development: Embryo to Blastocyst
 Embryo moves to the uterus (3–4 days), cell
division continues
 Forms a large hollow ball of cells or blastocyst
 It contains an inner cell mass, an internal
cavity, and an outer layer of cells
 Implantation in uterine wall is complete by ~12
days
Development: The Chorion Forms
 Produces human chorionic gonadotropin
(hCG), which prevents loss of uterine lining
 Chorion forms villi
 Maternal tissues and villi form the placenta
In weeks 5 and 6, the embryo grows dramatically to a length of
about 11 inches (28 cm). Most of the major organ systems,
including the heart, are formed. Limb buds develop into arms
and legs, complete with fingers and toes. The head is very large
relative to the rest of the body because of the rapid
development of the brain.
By about 8 weeks, the embryo is large enough to
be called a fetus. Although chromosomal sex (XX
in females and XY in males) is determined at the
time of fertilization, the fetus appears to be neither
male nor female at the beginning of the third
month. The sex organs cannot be seen in
ultrasound scans until the 12th to 15th week. All
the major organs have formed and are functional.
By 16 weeks, major changes include an increase
in size and further development of organ
systems. Bony parts of the skeleton begin to
form, and the heartbeat can be heard with a
stethoscope. Fetal movements begin in the third
month, and by the fourth month the mother can
feel movements of the fetus’s arms and legs.
It has a well-formed face, its eyes can open, and it
has fingernails and toenails.
Stepped Art
p. 13
Third Trimester
 Rapid growth
 Circulatory system and respiratory system
mature
 Fetus doubles in size during the last eight weeks
 Chances for survival outside the uterus increase
rapidly
1.3 Stages of Sexual Development
 Formation of reproductive organs controlled by
interaction between:
• How genes function
• Hormones
• The maternal environment
 Sex can be defined at several levels:
• Chromosomes
• Internal organs
• External appearance
Stages of Sexual Development
Stages of Sex Development (1)
 Chromosomal sex begins at fertilization
 Zygote has either an XX or XY
 Sex organs do not develop until week 7 or 8
 Expression of a gene on the Y chromosome
causes the gonads to become testes
 Sex is only one of thousands of traits
Stages of Sex Development (2)
 Testes secrete testosterone, controls
development of the male reproductive organs
 If Y chromosome is not present, the gonad
develops as an ovary
• Female duct systems develop to form
reproductive structures
• Male duct systems degenerate
 Phenotypic sex develops
SRY Gene
 The “sex-determining gene”
 A gene on the Y chromosome that confers the
male sex
 Sex-determining Region Y
Complete Androgen Insensitivity (CAI) (1)
 Mutation of androgen receptor (AR) gene on
the X chromosome
 Lack receptors for testosterone or hormones
derived from testosterone
 Cells in the gonad cannot respond to hormone
Complete Androgen Insensitivity (CAI) (2)
 Female duct system and external genitals develop
 Individuals are chromosomal males (XY)
 Physically appear to be females with welldeveloped breasts but limited pubic hair
 Do not menstruate, infertile, and have
undescended testes in their abdomens not ovaries
Individual with AIS
Barr Bodies
 In females, one X chromosome is inactivated,
forms Barr Body
 Some cells express genes from father’s X
chromosome, some express mother’s X
chromosome
 Males and XO individuals do not form Barr
Bodies
Barr Bodies
Intersexuality
 Chromosomal sex of a person is not consistent
with his or her phenotypic sex
 Or the phenotype is not classifiable as either
male or female
 Other conditions in this category: CAI, Klinefelter
syndrome (47,XXY), and Turner syndrome
(45,X)