Transcript document

Sex-Related Topics
A grab bag of subjects, vaguely
related to the typical eukaryotic
condition of having 2 sexes
Sex Determination
• Many groups use sex chromosomes to
determine sex. Mammals have the X and
Y chromosomes
• XX = female, XY = male
• All other chromosomes are called
“autosomes” Thus, humans have 46
chromosomes, 44 autosomes plus 2 sex
chromosomes.
X and Y Chromosomes
• The X has many genes on it, just like the autosomes.
Most of the genes on the X have nothing to do with sex.
• The Y has very few genes on it. It consists of mostly
inactive DNA.
• One gene on the Y is very important: SRY. The SRY
gene is the primary determinant of sex.
• If SRY is present, testes develop in the early embryo.
The testes secrete the hormone testosterone, which
causes development as a male.
• If SRY is absent (no Y chromosome), ovaries develop
instead of testes, and the embryo develops into a
female.
• The X and Y chromosomes share a common region at
their tips, the pseudoautosomal region. Crossing over in
meiosis occurs in this region.
Sex Determination in Birds
• Birds use a system of sex chromosomes very
similar to mammals. The bird sex chromosomes
are called Z and W.
• Big difference from mammals: in birds, a ZZ
individual is male, and a ZW individual is female.
• We can define some terms: “homogametic”
means having both sex chromosomes the same,
like female (XX) mammals and male (ZZ) birds.
“Heterogametic” means having different sex
chromosomes, like male (XY) mammals and
female (ZW) birds.
Sex Determination in Drosophila
• Drosophila also have X and Y chromosomes, with XX
female and XY male.
• However, Drosophila don’t use the SRY gene to
determine sex. Instead, they use the ratio of X’s to sets
of autosomes.
• 1 X plus 2 sets of autosomes is a normal diploid male.
• 2 X’s plus 2 sets of autosomes is a diploid female.
• The difference between sex determination mechanisms
comes in the odd cases:
--an XXY individual has a Y, so is a male mammal.
However, 2 X’s plus 2 sets of autosomes makes it a
female Drosophila.
---an XO individual (i.e. only 1 X, no other sex
chromosomes, but otherwise diploid) is a female
mammal (no Y) but a male Drosophila (1 X plus 2 sets of
autosomes).
Other Mechanisms
• Hymenopterans (wasps, bees, ants) are mostly
female. Females are diploid, and males are
haploid. Thus, a virgin female can lay
unfertilized eggs that will hatch into males that
can then fertilize her to produce more females.
• Nematodes (roundworms) have a single sex
chromosome, the X. An XX individual is female,
but an XO (only 1 X) is a hermaphrodite, an
individual with both male and female sex organs.
No true males exist.
More Mechanisms
• Some species have environmentally determined sex.
Among reptiles, the temperature at which the eggs
develop determines the sex. For example, in the turtles,
eggs incubated at 30oC become female, while those
incubated at lower temperatures become male.
• Some species have both sexes on the same individual:
this is very common among the angiosperms (flowering
plants), where 90% of the species have hermaphroditic
flowers, and many of the rest have separate male and
female flowers on the same plant. A few plants (e.g.
date palm and holly) have separate male and female
plants.
Sex Linkage
• Genes that are sex-linked are on the X chromosome.
Genes on the Y are NOT sex-linked; they are called
“holandric” instead.
• Because males (mammals, that is) have only 1 X, any
gene on the X in a male is expressed, whether dominant
or recessive. In contrast, females have 2 X’s, so
recessive traits are often covered up by the dominant
normal (wild type) allele. In most cases, genetic
diseases are recessive. Thus, most sex-linked genetic
diseases are much more common in males than in
females.
• having only 1 copy of a gene is called “hemizygous”;
sex-linked genes in male mammals are hemizygous.
That is, it is not possible for these genes to be either
homozygous or heterozygous, since those conditiosn
imply having 2 copies of the gene.
Common Sex-Linked Traits
• red/green colorblindness. The genes for
the red and green receptors are on the X.
The blue receptor is on an autosome.
• hemophilia. Blood doesn’t clot. Two of
the genes for proteins involved in clotting
are on the X.
• Duchenne muscular dystrophy. Muscles
degenerate, leading to death before age
20 in most cases.
Sex-linked Inheritance Patterns
• The father gives his X to his daughters only; sons get his
Y instead.
• Sons get their X from their mother.
• “Reciprocal crosses” are crosses with the same
phenotypes in the parents, but with reversed sexes.
Reciprocal crosses usually give different results with
sex-linked traits.
• For example, a colorblind male x normal female gives all
normal offspring. However, a normal male crossed with
a colorblind female gives colorblind male children and
normal female children.
• Colorblind females can occur as a result of a cross
between a colorblind male and a heterozygous (carrier)
female.
Dosage Compensation
• In mammals, males have 1 X while
females have 2. Having only 1 copy of
any other chromosome would be lethal.
How can the X be present in 1 copy or 2
copies and produce normal offspring in
either case?
• Basic answer: only 1 X is active in each
female cell.
Lyon Hypothesis
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It has long been known that
female cells contain “Barr bodies”,
blobs of chromatin located on the
inside of the nuclear membrane.
Each female cell has 1 Barr body;
male cells don’t have Barr bodies.
Mary Lyon proposed that Barr
bodies are inactive X
chromosomes, and that
mammalian cells inactivate all but
one of their X’s, converting the
extras into Barr bodies.
Proof: XXY individuals are male,
but have a Barr body; XO
individuals are female but have no
Barr bodies; XXX individuals are
female with 2 Barr bodies in each
cell.
Specifics of Inactivation
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When the embryo has about 200
cells, each cell randomly inactivates
one of its X’s, independently of the
other cells. The inactive X stays
inactive throughout the individual’s
life, through many cell generations.
A common example: tortoiseshell cats
have patches of black and orange fur.
Almost all tortoiseshells are female.
Heterozygous for the X-linked coat
color gene, one allele black and the
other allele orange. Only 1 allele is
expressed in each cell, and patches
on the fur result from cell division of
the original embryonic cells that
randomly chose an X to inactivate.
A similar human condition: anhidrotic
ectodermal dysplasia: absence of
sweat glands in the skin.
Sex-Influenced Traits
• A sex-influenced trait is an autosomal trait
that is dominant in one sex and recessive
in the other. Good examples: male pattern
baldness in humans and horns in sheep.
• Pattern baldness is found in both sexes,
but is rarer in females. Females usually
get very thin hair all over, instead of the
classic receding hairline and bald spot on
top that men get.
• Baldness is autosomal, but it is dominant
in males and recessive in females. Thus,
male heterozygotes are bald but female
heterozygotes have normal hair.
The Adams family
Sex-Limited Trait
• A sex-limited trait is expressed in one sex
but not the other. This is usually due to
anatomical or physiological limitations.
• An example: ability to produce milk is sexlimited, because only females have
breasts, the milk producing glands.
• Similarly, susceptibility of prostate cancer
is limited to men, because only males
have a prostate gland.
Mitochondrial Genes
• The mitochondria are organelles that produce most of the energy for
eukaryotic cells. Aerobic metabolism--the Krebs cycle and the
electron transport chain that produces ATP both occur in the
mitochondria.
• Mitochondria possess a small circle of DNA, like bacteria but unlike
the linear eukaryotic chromosomes. They also have other
characteristics similar to bacteria.
• The “endosymbiont hypothesis” put forth by Lynn Margulis states
that mitochondria (and chloroplasts in plants) are descended from
free-living bacteria, which developed an intracellular symbiosis with
primitive eukaryotic cells.
• Over time, most of the bacterial genes have moved into the nucleus,
but about 30 genes still remain in the mitochondrial genome.
• Analysis of the DNA sequences of the remaining genes has allowed
scientists to identify the bacterial groups that the mitochondria and
chloroplasts came from.
Endosymbiont Hypothesis
More Mitochondrial Genes
• Genes found in the mitochondria:
--ribosomal RNA and transfer RNA.
Mitochondrial ribosomes are of the prokaryotic
type, not eukaryotic.
--some electron transport chain proteins
• The genetic code is slightly altered in
mitochondria. For example, UGA is a stop
codon in the nucleus, but is codes for
Tryptophan in the mitochondria of humans and
yeast. Also, AUA codes for Isoleucine in the
nucleus, but it codes for Methionine in human
mitochondria (but not yeast mitochondria).
Mitochondrial Inheritance Pattern
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Mitochondria are inherited strictly
from the mother. The father’s
mitochondria are not passed to his
offspring.
Thus, any mitochondrial trait found
in the mother will be found in all of
her children.
This fact has allowed tracing of
mutations in mitochondrial DNA
through the human species. The
basic conclusions are that there is
more genetic diversity on Africa
than in the entire rest of the world
(implying that our species evolved
in Africa), and that the woman who
was the common ancestor of all
humans lived 100-200,000 years
ago.
Heteroplasmy
• Sometimes an individual has
more than one kind of
mitochondria. This is called
heteroplasmy. Since
mitochondria are divided
randomly during cell division,
different cells get different
proportions of the two types.
• If one mitochondrial type is
mutant and the other is normal,
severity of symptoms will vary
in different tissues depending
on the proportions of the two
types.
Maternal Effect Genes
• As we will discuss later, the egg cell in many
animals is haploid for only a very brief time, long
after it has been created.
• During production of the egg, the mother puts
many proteins and RNAs into the egg that are
produced by diploid maternal cells.
• Thus it is not surprising that some traits in an
offspring are determined by its mother’s
genotype, not the offspring’s genotype.
• The maternal effect rule: “Mother’s genotype
determines offspring’s phenotype.”
Shell Coiling in Lymnea
• The dominant D allele causes
coiling to the right, while the
recessive d allele causes coiling
to the left. Thus, all offspring of a
dd mother will coil to the left, and
all offspring of DD or Dd offspring
will coil to the right. The father’s
genotype and the offspring’s
genotype has no effect on the
offspring’s phenotype.
• “Mother’s genotype determines
offspring’s phenotype.”