Transcript Sex-Related Topics

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. Instead of the SRY gene, bird sex is determined by
the DMRT1 gene.
• 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. There are about 20 known holandric conditions in
humans
• 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; sexlinked genes in male mammals are hemizygous. That is,
it is not possible for these genes to be either
homozygous or heterozygous, since those conditions
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.
Answers!
• Normal color vision. A: 29; B: 45; C: --; D:
26
• Red-Green colorblindness. A: 70; B: --; C:
5; D: -• Red colorblind. A: 70; B: --; C: 5; D: 6
• Green colorblind. A: 70; B: --; C: 5; D: 2
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.
Barr bodies: condensed, inactive X
chromosomes found in the nucleus
of individuals with more than one X
chromosome.
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
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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. (but, let me warn
you that there is also considerable evidence
showing that male pattern baldness is NOT sexinfluenced!)
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.
Singing voice is also sex influenced: the highest
and lowest notes you can reach is affected very
much by your gender. The genotype which
causes males to have deep bass voices is the
same genotype that causes females to have
high soprano voices. The genotype that causes
males to have high tenor voices causes females
to have deep contralto voices.
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
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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.”