Download Unit 2 PPT 6 (Sex determination)

Sex determination
CfE Advanced Higher Biology
Unit 2: Organisms and Evolution
2biii – Sex determination
Contents and number of slides for each topic
1. Gender (2)
4. Dosage
compensation (2)
2. Chromosomal sex
determination (2) 5. Environmental sex
determination (3)
3. Sex linkage (5)
6. Changing sex (2)
SQA mandatory key information
• Sex determination in mammals and Drosophila, sex
ratio and resource availability.
• Environmental factors can affect sex determination.
Change of sex through size, competition or parasitic
infection.
• Hermaphrodites.
• Sex linked patterns of inheritance in carrier females
and affected males in terms of gene products.
• Random inactivation on X chromosomes in females
prevents a double dose of gene products. Half of the
cells in any tissue will have a working copy of the gene
in question therefore carriers remain unaffected by
any deleterious mutations.
Key concepts 1
• Many species are hermaphroditic. For some species
environmental rather than genetic factors determine
sex. Environmental sex determination in reptiles
controlled by environmental temperature of egg
incubation.
• Sex chromosomes, such as XY in live-bearing
mammals and some insects including Drosophila. In
many of the mammals a gene on the Y chromosome
determines development of maleness. In some
species the sex ratio of offspring can be adjusted in
response to resource availability.
Key concepts 2
In live-bearing mammals, the heterogametic (XY) male
lacks homologous alleles on the smaller (Y) chromosome.
This can result in sex-linked patterns of inheritance as seen
with carrier females (XBXb) and affected males (XbY). In
the females, the portions of the X chromosome that are
lacking on the Y chromosome are randomly inactivated in
one of the homologous X chromosomes in each cell. This
effect prevents a double-dose of gene products. Carriers
remain unaffected by any deleterious mutations on these X
chromosomes as the X-chromosome inactivation is
random, half of the cells in any tissue will have a working
copy of the gene in question.
Gender
Males and females
Male holly
flower
Female
holly flower
•
•
•
•
Many animals have separate genders.
In some animal species the males and females are very similar.
Other animal species show sexual dimorphism.
Some plants are unisexual and have male and female plants.
For example, only female hollies can produce berries.
Swans: Ralfie / Wikimedia, Ducks: Keven Law / Wikimedia, Holly flowers: Tigerente / Wikimedia
Gender
Hermaphrodites
• In Greek mythology, Hermaphroditus
was the child of Hermes and
Aphrodite and had both male and
female characteristics.
• Some animals and plants have male
and female organs in one individual.
• Reproductive structures in many
flowers mature at different times to
prevent self-pollination.
• Earthworms and most land molluscs
are hermaphroditic but must meet
another individual to reproduce.
Chromosomal sex determination
X and Y in humans
• Karyotypes developed in early 20th Century and
it took until 1956 to agree on 46 chromosomes
for humans.
• By 1959, agreed on 44 autosomes and two sex
chromosomes in humans:
– Females have two large sex chromosomes,
both called X
– Males have an X and a small Y chromosome.
• Live-bearing mammals have this system - all
mammals except Platypus and Echidna.
• Drosophila melanogaster also has this system with 6 autosomes
and two sex chromosomes, X and Y (this is unusual for insects).
Chromosomal sex determination
SRY gene
• Default setting for human embryonic development
is to become female (hence male nipples!).
• Y-chromosome has a gene called SRY
(Sex determining Region of Y-chromosome).
• SRY controls the development of male genitalia and
male characteristics.
• SRY is a ‘master switch’ which produces a
transcription factor to switch on other genes.
• Many of these genes are on the autosomes but are
only transcribed in males due to SRY control.
• XX male due to SRY translocated to X-chromosome.
• XY female due to SRY deleted from Y-chromosome.
Sex linkage
X and Y are homologous chromosomes
• X and Y are an
exception to the
homology definition.
• Human X-chromosome
is larger than Y and has
about 2000 genes
compared to about 450.
• X and Y are homologous
because they pair up
during meiosis I.
• Pairing is due to a small
area of homology
around the centromere.
• The Y-chromosome lacks
many genes found on its
homologous X-chromosome.
• This leads to a pattern of
inheritance called sex
linkage.
• In XX females, a recessive
allele on one X can be
masked by a dominant allele
on the other X.
• In XY males, a recessive allele
on the X has no second copy
to mask its effects.
Sex linkage
Homogametic and heterogametic
• Because they are homologous, the sex chromosomes are
separated during meiosis into different gametes.
44 + XX
22 + X
44 + XY
22 + X
22 + X
22 + Y
• Human females produce all gametes with the same
combination of chromosomes = homogametic.
• Human males produce gametes with two possible combinations
of chromosomes = heterogametic.
Sex linkage
White-eyed males
• In the early 1900s, T. H. Morgan studied
inheritance in Drosophila melanogaster to try
to disprove Mendel’s theory.
• It took two years to find any variation in his vast
fly breeding programme.
• Eventually a male fly was found which had
white eyes. (His wife found it.)
• This white-eyed male was crossed with a
normal red-eyed female.
• We will use the notation XR to show the red eye
allele is on the X-chromosome and is dominant
to white eye, Xr.
XRXR
XrY
XR
Xr
XRXr
Y
XRY
Sex linkage
Carrier females
• All the flies in the F1 had
red eyes.
• Flies with genotype XRXr
are carrier females.
• Morgan crossed the F1
with each other.
• F2 came out with a 3:1
ratio of red-eyed flies to
white-eyed flies.
• But all the white-eyed flies
were males.
• Which flies could be
crossed to get white-eyed
females?
XRY
XRXr
XR
Xr
XR
XRXR
Red-eyed
female
XRXr
Red-eyed
carrier female
Y
XRY
Red-eyed
male
XrY
White-eyed
male
Sex linkage
Human examples
• Even though the mechanism of inheritance
was not understood, examples of sex
linkage had already been noticed in humans
before Morgan found the white-eyed fly in
1910.
• Red-green colour blindness, 1798
• Haemophilia, 1803
• Duchenne muscular dystrophy, 1834
• Other examples have been noticed since
then, such as Vitamin D resistant rickets
(1937), which is unusual in being a
dominant allele.
Dosage compensation
X-chromosome inactivation
• Males are heterogametic (with only one X-chromosome) while
females are homogametic (two X-chromosomes).
• Despite this, female cells do not have a double-dose of gene
products from their X-chromosomes.
• In females, one X-chromosome is partially inactivated early in
embryonic development.
• The regions inactivated are those
that are lacking on Y-chromosome.
(Deacetylation of histones, methylation of bases.)
• The inactivated X-chromosome
shows up as a highly condensed
region on the inside of the nuclear
membrane – a Barr body.
Xi = Barr body (inactivated X)
Steffen Dietzel / Wikimedia
Dosage compensation
Mosaics
• Because of random X-inactivation, an XX female is a mosaic of
cells with different X-chromosomes active and inactive.
• A carrier of a deleterious alleles (e.g. XDXd) remains unaffected
because she still has a working copy of the gene in half of the
cells in every tissue.
• Cats show this mosaic in their
fur as a coat colour gene is
found on the X-chromosome.
• The two alleles are black and
ginger and tortoiseshell cats
are females, XBXG.
• If you see a ginger cat, why is it more likely that it is male than
female?
Suziecat / Wikimedia
Environmental sex determination
Temperature affects sex ratio in reptiles
• In turtles and crocodiles, the level of expression
of some genes is affected the temperature
during a sensitive period in development.
• For some reptiles, there is trend in the effect of
temperature on the sex ratio.
– In Hermann’s Tortoises, below 31C is all
males and above 32C is all females.
• In other reptiles, the extremes cause the
production of one sex while the intermediate
temperature produces the other sex.
– In Mississippi Alligators, males only develop
at 32–34C while females are produced at
temperatures below 32C and above 34C.
Environmental sex determination
Parasites can affect sex ratio
Transmission electron microscope image
of Wolbachia bacteria inside an insect cell
• In some arthropods, particularly
insects, sex ratio is affected by
infection by bacteria of the genus
Wolbachia.
• Some species having chromosomal sex
determination but the bacteria kill the
males or feminize them.
• Other insects rely on infection by
Wolbachia to cause their sex
determination!
• Research reported in 2007 showed that human mothers infected
by the protist, Toxoplasma gondii, have a higher proportion of
male offspring. This parasite may also affect human behaviour.
Environmental sex determination
Competition affects sex ratio
• Some species can actively change the sex ratio of
their offspring.
• When a solitary female Lesser Mouse Lemur
detects the urine of other females in her habitat
she switches from producing one-third males to
two-thirds males.
• Male offspring disperse away from the home range
sooner than female offspring so this reduces
competition.
• Female Seychelles Warblers in a high quality territory produce
87% females but in a poor territory they produce 77% males.
• Female offspring stay and help raise the next brood with their
mother while males disperse away and reduce competition.
Gabriella Skollar & Rebecca Lewis / Wikimedia
Changing sex
Group-living fish
• Clown fish live in groups with a large
dominant female and a number of
smaller males.
– If the female is removed, the largest
male becomes female.
• Bluehead wrasse have one male with a
group of females.
– Removal of the male causes the
largest female to grow twice its size
and change its colouration.
• This sex change is influenced by aromatase, an enzyme that
controls the androgen:oestrogen ratio.
Changing sex
Size and parasites
• The sea anemone Metridium senile changes
from male to female as it gets larger.
– Bigger females may be necessary due to
the higher investment by females in
producing gametes for reproduction.
• A nematode (Gasteromermis sp.) parasitises the aquatic larvae of
a type of mayfly, Baetis bicaudatus.
– Females become sterile. Some parasitised males become
‘intersex’ while others become completely female.
– All the parasitised animals (males and females) behave like
females when adult and crawl into the water as if to lay eggs.
– This helps the parasite return to infect the next generation.
Stan Shebs / Wikimedia