Chapter 5:
Sex Determination
and Sex
Chromosomes
Susan Chabot
Honors Genetics
2014-2015
Chapter Concepts
Sexual reproduction, which greatly enhances genetic
variation, requires a mechanism for sexual differentiation.
A variety of mutations can lead to sexual dimorphism
(differences).
Usually, specific genes on a single chromosome determine
male-ness or female-ness during development.
In humans, the presence of extra X or Y chromosomes
above the normal diploid number can be tolerated but will
lead to distinctive phenotypes.
Chapter Concepts
There is NOT a 1-to-1 ration of males to females at
conception; the ratio tends to favor males at conception.
Female mammals inherit 2 X-chromosomes at
conception; the additional information is inactivated in
early development.
Temperature of incubation for reptilian eggs will determine
the sex of offspring.
Introduction
•
Homomorphic Chromosomes:
•
Heteromorphic Chromosomes:
•
•
It is the GENES on the chromosomes that determine
sex of the organism.
MOST of these genes are on the sex chromosomes
but some can be found on the autosomes.
Introduction
•
•
•
Homomorphic Chromosomes: XX pair in
mammals.
Heteromorphic Chromosomes: XY pair in
mammals.
It is the GENES on the chromosomes that
determine the biological sex of the organism.
•
MOST of these genes are on the sex chromosomes but some can
be found on the autosomes.
Vocabulary
•
Primary Sexual Differentiation:
•
Secondary Sexual Differentiation:
Vocabulary
•
•
Primary Sexual Differentiation: involves
production and maturation of the gonads and
gametes.
Secondary Sexual Differentiation: involves
overall appearance of the organism; nonreproductive organs
Vocabulary
•
Unisexual/Dioecious/Gonohoric:
•
Bisexual/Monoecious/Hermaphroditic:
•
Intersex:
Vocabulary
•
•
•
Unisexual/Di-o-e-cious/Gono-choric: organism that
contains only male or only female reproductive
organs.
Bisexual/Mono-e-cious/Hermaphroditic: organism
that contains both male and female reproductive
organs. These organisms can produce both eggs
and sperm.
Intersex: organisms of intermediate sexual
condition; often sterile.
The Y Chromosome Determines
Maleness in Humans
•
•
Human DIPLOID number is 46; HAPLOID number is 23.
Remember, chromosomes are only visible during MITOSIS
and MEIOSIS. Visualization of chromosomes during cell
division will allow for the construction of a karyotype,
seen below.
Nondisjunction
Klinefelter Syndrome
•
•
•
•
•
•
•
•
47, XXY
1 in 600 male births!
Result of nondisjunction during Meiosis.
Tend to be tall, long arms, large hands
and feet
Outward appearance of male
Undeveloped testes that fail to
produce sperm.
Female secondary sex
characteristics are not
suppressed; rounded hips,
slight breast enlargement.
Below average intelligence.
Turner Syndrome
•
•
•
•
•
•
•
45, X
1 in 2000 female births!
Result of nondisjunction during Meiosis
Short stature, skin flaps on back of neck,
broad shield-like chest
Outward appearance of
female
Undeveloped ovaries, may
not produce eggs.
Normal intelligence
There are 3 levels to sexual
development:
• Chromosomal
• Gonadal
sex
sex
• Phenotypic sex
Levels of Sexual Development
• Chromosomal
sex: presence or
absence of the Y chromosome.
•Determined
at fertilization
•X or Y sperm that fertilizes egg.
Gonadal sex
• Phenotypic sex
•
Chromosomal Variants
Rates for chromosomal changes = about 1 in 1000 births
• Nondisjunction of sex chromosomes places
<haploid or >haploid number of sex chromosomes into
a sperm or egg cell.
•
•
•
•
Most common types involve 45 or 47 chromosomes
There are many other, rarer types, with 48 or even 49
chromosomes, such as 49,XXXXY.
•
•
Sex chromosomes are quite tolerant of variants.
Such conditions almost always lead to serious mental
deficiencies.
The general rule: if the Y is present, the person is
internally and externally male.
Nondisjunction
Levels of Sexual Development
•
Chromosomal sex
• Gonadal
sex (primary sex
determination): do the gonads
develop as testes or ovaries?
• Depends
on the presence or absence of the
SRY gene, usually found on the Y
chromosome
• Phenotypic sex
Early Gonad Development
• Gonadal
ridges are produced by the 5th
week of gestation.
• The gonad is indifferent: neither male nor
female.
•
• By
•
•
•
•
Bipotential Gonads – can become ovary or
testes
week 6-7, differentiation has begun
The CORTEX will produce OVARIAN tissue and
the medulla will disappear.
Mullerian ducts = fallopian tubes, uterus, and
vagina.
The MEDULLA will produce TESTICULAR tissue
and the cortex will disappear.
Wolffian ducts = vas deferens and urethra.
SRY
How the Y chromosome determines sex.
• The SRY is the primary determinant of sexual
development.
•
•
•
Although the SRY gene is usually on the Y
chromosome, it occasionally gets transferred to
the X.
•
•
If a developing embryo has a functional SRY gene in
its cells, it will develop as a male. And, if there is no
functional SRY, the embryo develops as female.
this leads to 46,XX males
Sometimes the SRY gene is inactivated by
mutation.
•
•
Leading to 46,XY females (Swyer syndrome)
it is also possible to have a partially inactive SRY gene,
leading to ambiguous genitalia
Gene Mutations
Rates for gene mutations on Autosomes, each
parent needs to contribute a mutated copy of
the gene, usually 1 in 10,000 births.
• Gene mutations can occur
•
•
•
On X or Y chromosome that impacts gonadal development
On Autosomes that produce enzymes responsible for converting
sex chromosomes during fetal differentiation.
5-alpha Reductase Deficiency (5-ARD)
•
•
•
Form of Intersex
Two copies of the gene must be defective.
5-alpha reductase is an enzyme that converts testosterone
into DHT.
•
•
At birth
•
•
•
•
Undescended testes and male ducts (with no female ducts)
Genitalia appear somewhere between female and ambiguous
Often raised as girls
At puberty
•
•
•
DHT is responsible for the development of external male genitals
Increase in testosterone is large enough that some DHT gets made
Develop a male appearance: the testes descend, the penis enlarges,
facial hair appears, the voice deepens, muscles develop.
Large group in the Dominican Republic: ~ 1 in 90 men
•
•
Guevedoces - Raised as girls, they easily switch to the male role.
Other groups found in Malta, Jordan, Pakistan, New Guinea
Androgen Insensitivity
•
Incidence about 1 in 20,000 births
•
Formerly “testicular feminization”.
•
46,XY with normal (undescended) testes.
•
During fetal development, testes secrete testosterone but cells lack
receptors for it.
•
•
Result - male ducts are not present. Testes secrete MIS, which causes
the female ducts to degenerate.
•
•
No receptor = no response to the hormone.
Typically external female genitals develop
At puberty, testes secrete excess testosterone and converts into a form
of estrogen (estradiol).
•
Female secondary sexual characteristics develop. No menstruation, no
ovaries, no uterus. Occasionally, the undescended testes can become
cancerous, so they are often surgically removed after puberty is complete
(so as to get normal female development).
Levels of Sexual Development
Chromosomal sex
• Gonadal sex
•
• Phenotypic
sex (secondary sex
determination): all internal and external
structures develop depending on
hormones secreted by the gonads.
• Phenotypic sex = internal ducts, and
external genitalia
• Two important times: fetal
development and puberty
Development of Phenotypic Sex
•
The cells of the newly formed testes start secreting
testosterone.
•
•
•
•
Testosterone secretion peaks about week 16 (levels near adult
males) then drops to about the same level as female fetuses.
The testes also secrete another hormone: Mullerian inhibiting
substance (MIS) to destroy developing ovarian tissue.
During the last trimester of pregnancy, the testes
descend into the scrotum.
The developing ovary secretes estrogen (important after
birth)
•
Mom’s estrogen overwhelms the female fetus during pregnancy.
Childhood and Puberty
•
During childhood, sex hormone levels are very low in both
sexes.
•
•
Puberty begins when brain and hypothalamus produce
the hormone gonadotropin releasing hormone.
•
•
Stimulates production of LH and FSH by the pituitary gland.
LH and FSH stimulate testes/ovaries to produce large
amounts of testosterone and estrogen
•
•
Surge of sex hormones in both sexes for a few weeks after birth.
In boys, some of the testosterone is converted to estrogen which
causes a growth spurt.
The adrenal glands also secrete male sex hormones in
both sexes starting in late childhood.
•
After puberty starts, the ovaries also produce androgens.