Bio3400
Chapter 7
Sex Determination and Sex Chromosomes
reproduction involving meiosis and
is important in many multicellular
organisms.
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.
Chlamydomonas
gametes are
and consist of two
types,
though there is no genetic difference between the
types.
The green alga Chlamydomonas spends most of the life cycle in the haploid vegetative phase, asexually producing daughter cells by mitosis. Unfavorable 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 of the zygote produces two plus mating types and two minus mating types. Mitosis of these zoospores returns the cells to vegetative colonies.
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.
Maize
is a
(sporophyte contains both male and female parts) plant that exhibits
alternation
of
, with a dominant
stage and a
stage.
Life cycle of an angiosperm (flowering plant). A plant alternates between a multicellular diploid (2n) sporophyte and a multicellular haploid (n) gametophyte generation.
A mature plant is a multicellular diploid sporophyte with reproductive structures.
Anthers contain microsporangia in which germ cells divide by meiosis to produce microspores.
Ovaries contain megasporangia in which germ cells divide by meiosis to yield 4 megaspores each.
Microspores divide by mitosis to form multicellular male gametophytes (pollen grains), which contain sperm cells.
One of the 4 megaspores divides by mitosis to form a multicellular female gametophyte (embryo sac), which contains an egg cell in an ovule.
Fertilization (pollination) occurs when a sperm fuses with an egg, producing a diploid single-celled zygote.
The zygote develops by mitosis to produce the sporophyte.
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.
Sexual development in the
nematode
Caenorhabditis elegans
is determined by the
ratio
of
chromosomes to sets of
.
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.
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.
In the
mode
and
mode
of sex determination, gametes from the
sex determine the sex of the progeny.
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.
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.
Humans possess
chromosomes:
22 pairs of
and two
chromosomes.
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.
of the X chromosomes (failure to segregate) during
meiosis
results in sex chromosome
such as
syndrome
and
syndrome.
Nondisjunction during meiosis can produce gametes with an extra or missing chromosome, or aneuploidy. Homologous chromosomes may fail to separate during meiosis I. Sister chromatids may fail to separate during meiosis II.
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.
Individuals with Turner syndrome (X0 or 45,X) are sterile females. They are usually short and have underdeveloped breasts. Intelligence is often normal.
Other sex chromosome abnormalities include
and
aneuploidies.
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.
In humans, the
region on the
chromosome
is necessary for male development.
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. Most of the Y chromosome is a "male-specific" (MSY) area composed of euchromatic regions containing functional genes and heterochromatic regions lacking genes. One gene in the euchromatic region is the "sex-determining region" (SRY) which encodes a testis-determining factor (TDF) that triggers differentiation of testes tissue in the embryo.
In mammalian females, one of the two X chromosomes in each cell is randomly
during embryonic development and can be seen as a compact
body.
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.
In various human karyotypes, all X chromosomes except one (N - 1) are inactivated. For example, a 48,XXXX female would have 3 Barr bodies.
A female
heterozygous
for an X-resident gene may
exhibit
a
phenotype.
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.
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.
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.
Sex determination in
Drosophila
depends on the
of
chromosomes
to autosomes; loss of an X during development can result in a bisexual
fly
called a
.
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.
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.
Many reptiles exhibit
-dependent sex determination (TSD),
which
can be
one
of three
modes.
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.
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.
In this patterns of temperature-dependent sex determination (TSD), low and high temperatures yield 100 percent females, while intermediate temperatures yield males.