Barr_body-karyotypes.html: 07_10-Barr_body-karyotypes.jpg
In various human karyotypes, all X chromosomes except one (N - 1) are inactivated.
For example, a 48,XXXX female would have 3 Barr bodies.
Barr_body.html: 07_09-Barr_body.jpg
An interphase epithelial cell from a human female shows a dark-staining Barr body
in the nucleus; representing an inactivated X chromosome.
A comparable cheek epithelial cell from a male does not show a Barr body.
Thus, dosage compensation prevents excessive expression of X-linked genes in mammals.
Calico_cat.html: 07_11a-Calico_cat.jpg
A female calico cat shows random distribution of orange and black patches of fur.
The gene for this locus is X-linked. Since the inactivation of X chromosomes occurs
randomly early in embryonic development,
all progeny cells from the embryonic precursor cell have the same X chromosome inactivated,
causing a patchy expression of the alleles.
The white patches are due to a separate epistatic "spotting gene"
that prevents pigments from migrating to the skin.
Chlamydomonas_life_cycle.html: ../ch05/05_18-isogametes.jpg
The green alga Chlamydomonas
spends most of the life cycle in the haploid vegetative phase, asexually producing daughter
cells
by mitosis.
Unfavorable nutrient conditions trigger the sexual phase, where some vegetative cells develop into isogametes, which can fuse to form a diploid resistant zygote adapted for surviving harsh conditions.
When conditions become suitable again, meiosis produces two plus types and two minus types;
mitosis of these zoospores returns the cells to vegetative colonies.
Chlamydomonas_mating_types.html: 07_02-Chlamydomonas_mating_types.jpg
Chlamydomonas isogametes are haploid
and consist of opposite mating types: plus (+) and minus (-).
The isogametes are morphologically and genetically indistinguishable, differing only in surface chemical makeup.
Mating (fertilization) only occurs between + and - cells, which can fuse to form a
diploid zygote.
Klinefelter_syndrome.html: 07_07a-Klinefelter_syndrome.jpg
Individuals with Klinefelter syndrome (47,XXY) are sterile males who often
exhibit ambiguous sexual development (intersexuality), such as enlarged breasts
(gynecomastia) and rounded hips. They are generally taller than average. Intelligence is often below normal.
Maize_life_cycle.html: 07_03-Maize_life_cycle.jpg
Anthers
on stamens -> (meiosis) 4 haploid microspores -> (mitosis) pollen grain
with 2 sperm + tube cell
Ovaries
in pistils -> (meiosis) 1 haploid megaspore -> (mitosis) embryo sac
with 8 nuclei
Pollination occurs when pollen grains make contact with the stigma (silks)
and develop pollen tubes that grow toward the embryo sac.
One sperm unites with the oocyte nucleus and the other unites with two endosperm nuclei.
This double fertilization produces a diploid zygote nucleus and a
triploid endosperm nucleus.
Tortoiseshell_cat.html: 07_11b-Tortoiseshell_cat.jpg
A female tortoiseshell cat has the same X-linked orange and black
fur color alleles as the calico cat, but lacks the "spotting" white fur allele.
Turner_syndrome.html: 07_07b-Turner_syndrome.jpg
Individuals with Turner syndrome (45,X) are sterile females.
They are usually short and have underdeveloped breasts. Intelligence is often normal.
XYY.html: 07_T01-XYY.jpg
47,XYY individuals are males who are often tall but otherwise show normal phenotypes.
Early studies suggested a possible correlation between 47,XYY
and antisocial behavior and subnormal intelligence, but most 47,XYY
individuals do not know their karyotype and lead normal lives.
Triple X (47,XXX) individuals are females who are often tall but otherwise show normal phenotypes,
though some may exhibit abnormal secondary sex characteristics or learning disabilities.
X_FISH.html: 05_F00-X_FISH.jpg
Human X chromosomes highlighted using fluorescence in situ hybridization (FISH),
where specific probes bind to specific sequences of DNA.
The green fluorescence is specific to the DNA of the X chromosome
centromeres.
The red fluorescence is specific to the DNA sequence of the Duchenne muscular dystrophy
(DMD) gene.
Y_chromosome.html: 07_08-Y_chromosome.jpg
Pseudoautosomal regions (PARs) on the Y chromosome
share homology with regions on the X chromosome
and synapse with it during spermatogenesis to allow segregation of the sex chromosomes.
The "sex-determining region" (SRY) encodes a testis-determining factor (TDF) that triggers differentiation of testes tissue in the embryo.
Most of the Y chromosome is a "male-specific" (MSY) area composed of euchromatic regions containing functional genes and heterochromatic regions lacking genes. Portions of MSY were originally derived from the X chromosome.
One gene in the euchromatic region is SRY, while other genes are
also associated with testes development.
anhidrotic_ectodermal_dysplasia.html: 07_12-anhidrotic_ectodermal_dysplasia.jpg
Human males hemizygous for anhidrotic ectodermal dysplasia lack sweat glands.
Heterozygous females exhibit random patterns of tissue with and without sweat glands,
due to random inactivation of one or the other X chromosome carrying the allele
early in development.
chromosome_compositions.html: 07_13-chromosome_compositions.jpg
In Drosophila, the ratio of X chromosomes to haploid sets of autosomes
(A) determines sex.
Normal males have a ratio of 0.5, normal females a ratio of 1, while
intermediate ratios produce sterile intersex flies that expressed both male and female morphology.
Ratios below and above the normal produce infertile and weak metamales or metafemales,
respectively.
gynandromorph.html: 07_14-gynandromorph.jpg
A bilateral gynandromorph of D. melanogaster (heterozygous for 2 loci) is
formed after one of its two X chromosomes is lost in one of the two cells during the first mitotic division.
The left side is composed of male cells containing a single X and expresses the mutant white-eye and miniature-wing alleles.
The right side is composed of female cells containing two X chromosomes heterozygous for both alleles.
human_karyotype.html: 07_06-human_karyotype.jpg
The human karyotype is composed of 46 chromosomes:
22 pairs of autosomes and two sex chromosomes.
Females have two X chromosomes; males have one X and one Y chromosome.
nematode_hermaphrodite.html: 07_04a-nematode_hermaphrodite.jpg
The roundworm C. elegans has two sexual phenotypes: males have only testes and
hermaphrodites have both
testes and ovaries. Adults consist of 959
cells,
and the precise lineage
of each cell can be traced to specific embryonic
origins.
nematode_mating.html: 07_04b-nematode_mating.jpg
Hermaphroditic nematodes can self-fertilize; the offspring
are mostly hermaphrodites; less than 1% are males.
But when hermaphrodites are mated with males,
the offspring are about 1:1 hermaphrodites:male.
Sex is determined by the ratio of X chromosomes to the number of sets of autosomes:
hermaphrodites have 2 X chromosomes and 2 copies of each autosome,
while males have only 1 X chromosome.
sex_determination-Lygaeus.html: 07_05b-sex_determination-Lygaeus.jpg
In the Lygaeus
(XX/XY) mode of sex determination the heterogametic sex (the male in this example)
is XY and produces different gametes with either an X or a Y chromosome
Females are homogametic (XX).
The offspring sex ratio is 1:1.
sex_determination-Protenor.html: 07_05a-sex_determination-Protenor.jpg
In the Protenor
(XX/XO) mode of sex determination the heterogametic sex (the male in this example)
is XO and produces different gametes with or without the X chromosome
Females are homogametic (XX).
The offspring sex ratio is 1:1.
temperature-dependent_sex_determinationI.html: 07_15-temperature-dependent_sex_determinationI.jpg
In this patterns of temperature-dependent sex determination (TSD),
a mix of male and female offspring result near a a narrow range of pivotal temperature (P).
Low temperatures yield 100 percent females, while high temperatures yield 100 percent males.
temperature-dependent_sex_determinationII.html: 07_15-temperature-dependent_sex_determinationII.jpg
In this patterns of temperature-dependent sex determination (TSD),
a mix of male and female offspring result near a a narrow range of pivotal temperature (P).
Low temperatures yield 100 percent males, while high temperatures yield 100 percent females.
temperature-dependent_sex_determinationIII.html: 07_15-temperature-dependent_sex_determinationIII.jpg
In this patterns of temperature-dependent sex determination (TSD),
low and high temperatures yield 100 percent females, while intermediate temperatures yield males.