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Neurospora - a haploid fungus; products of meiosis occur in a
linear array; arrangment can be used to calculate the map
distance between a single gene and the centromere.
Distance between gene and centromere =
(0.5 x (# of second division segregants))/total asci scored) x 100
Sex Determination
and Sex
Chromosomes
Cpt. 7
Asexual Reproduction
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Parthenogenesis: development of a new
individual from an unfertilized egg
(Cnemidophorus uniparens whiptail lizard)
Adventitious embryony: development of a
new individual from a single somatic cell
(e.g. millet Pennisetum squamulatum)
Vegetative reproduction: development of a
new individual from a group of somatic
cells (e.g. Iris bulbs)
Chlamydomonas - the “cockroach” of the algae world
most of life spent in haploid
vegetative phase
gametes formed under
unfavorable nutrient
conditions
there are two types of
isogametes (+ and -). the only
types of matings involve
opposite types of gametes:
+ and - or - and +
Chlamydomonas - the “cockroach” of the algae world
most of life spent in haploid
vegetative phase
gametes formed under
unfavorable nutrient
conditions
there are two types of
isogametes (+ and -). the only
types of matings involve
opposite types of gametes:
+ and - or - and +
Zea mays - corn; cultivated from a wild grass in Mexico
monoecious (both anisogametes in a single plant)
Caenorhabditis elegans - roundworm; popular in genetic
studies because it is known that the hermaphroditic adult
has exactly 959 cells; genome has been sequenced
X and Y chromosomes were first linked to sex
determination early in the 20th century
XX/XO sex determination
In 1906, Edmund B. Wilson demonstrated
that females of the hemipteran insect
Protenor contain 14 chromosomes,
including 2 X chromosomes. Oogenesis
produces gametes with 7 chromosomes.
Males contain 13 chromosomes, including
a single X. Spermatogenesis produces
gametes with either 6 (without a X) or 7
(with a X) chromosomes.
X and Y chromosomes were first linked to sex
determination early in the 20th century
XX/XY sex determination
Wilson also experimented with
Lygaeus turicus. The species has
12 autosomes. In addition, females
have 2 X chromosomes, while the
males have a single X and a
smaller heterochromosome labeled
the Y chromosome. Oogenesis
produces (6A + X) gametes.
Spermatogenesis produces
(6A + X) and (6A + Y) gametes.
Other sex chromosome configurations
Homogametic sex: sex that produces uniform gametes with regard
to chromosome numbers and types (female humans, female
Protenor, female Lygaeus)
Heterogametic sex: sex that produces unlike gametes with regard
to chromosome numbers and types (male humans, male
Protenor, male Lygaeus)
In some organisms, the heterogametic sex is the female instead of the
male. To designate this, we use the notation ZZ/ZW for the
sex chromosomes instead of XX/XY.
Examples where the female is the heterogametic sex include birds,
butterflies, some reptiles, and at least one species of plant.
How was it determined that the Y chromosome plays a
role in sex determination?
By studying the genetics of mutant phenotypes
Klinefelter syndrome (47, XXY): 2 per 1000
male births; male genitalia and ducts, but the
testes are rudimentary and fail to produce sperm.
They are generally tall and have long arms and
legs and large hands and feet. Also have some
feminine characteristics (slight enlargement of
breasts, rounded hips).
Turner syndrome (45, X): 1 per 2000 female births; female
genitalia and ducts, but the ovaries are rudimentary. They are
generally short in stature (< 5 ft), have skin flaps on the back of
their neck, and underdeveloped breasts. A broad, shieldlike
chest is sometimes noted. Intelligence is usually normal.
Other sex chromosome abnormalities
47, XXX syndrome: 1 per 1200 female births; results in a female;
the expression of this chromosomal abnormality is highly variable;
some women with it are perfectly normal. In other cases,
underdeveloped sex characteristics, sterility, and mental retardation
may occur
47, XXY syndrome: A 1965 study of a male prison population in
Scotland found that 9 of 315 males had this karyotype. All 9 were
above average in height and all had been incarcerated because of
antisocial criminal acts. 7 of the 9 were of subnormal intelligence
and all had personality disorders.
Initiated a large study in 1974 to identify to identify XXY
individuals at birth and track their behavioral patterns. Study
abandoned in 1975.
The Y chromosome
pseudoautosomal regions (PAR) homologous with regions on X; synapsis and
recombination occurs during meiosis.
nonrecombining region (NRY) - everything else.
euchromatin - region that contains functional genes
heterochromatin - region that lacks genes
sex-determining region Y (SRY) - gene that controls
male sexual development; produces testisdetermining factor (TDF), a product that triggers the
formation of testes from undifferentiated embryonic
gonadal tissue
Dosage compensation prevents excessive expression of
X-linked genes in humans and other mammals
Since females have two copies of
the X chromosome and males only
one, there is the potential for females
to produce twice as much of each
gene product for all X-linked genes.
In mammals, one of the X
chromosomes in females is inactivated
in cells early in the embryonic cycle
These inactivated chromosomes are
called Barr bodies, and are observable
as darkly staining bodies in a Feulgen
reaction for DNA.
Dosage compensation prevents excessive expression of
X-linked genes in humans and other mammals
In females, one X is from the father and one is from the mother.
So, which one is inactivated?
The Lyon Hypothesis postulates that inactivation occurs randomly in
somatic cells at a point early in embryonic development. Thus females
are mosaics for all heterozygous X-linked alleles. Depending on the
embryonic origin of a cell, the adult may express one or the other allele.
The Lyon Hypothesis postulates that inactivation occurs randomly in
somatic cells at a point early in embryonic development. Thus
females are mosaics for all heterozygous X-linked alleles. Depending
on the embryonic origin of a cell, the adult may express one or the
other allele.
Black and yellow-orange patches of female tortoise-shell
and calico cats due to Lyon Hypothesis. Neither occur in
males because males are hemizygous (only have one X).
Sex Determination in Drosophila
Drosophila has a haploid number of 4, composed of 3 autosomes
and 1 sex chromosome. They have XX/XY inheritance.
In 1916, Calvin Bridges performed a series of experiments that
elucidated sex determination in Drosophila
Nondisjunction during meiosis produced abnormal gametes, some
with n+1 chromosomes, some with n-1 chromosomes. Bridges
crossed these with normal haploid gametes and found that the
resulting XXY flies were normal females and the XO flies were
sterile males. Concluded that Drosophila Y lacks male determining
factors, but does contain genetic information related to male fertility.