Sex determination
X- Body
There are two types of sex determination mechanisms:
1.) Either with or without X chromosome. Eg; male grasshoppers having XO
chromosome.
2.) Some gametes with X chromosome and some with Y chromosome.
Eg; Male Heterogamety- Humans (xy)
Female Heterogamety- Birds (ZW)
Sex determination results in the development of individuals with characteristics that allow
them to be identified as males, females, or in some cases, hermaphrodites.
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1 Discovery!
Henking was the scientist to first discover the X body but he
could not explain its significance. (1891)
He found that 50% of the sperms received this nuclear
structure, while 50% did not.
INDEX
I.) Sex determination in:
1.) Humans
2.) Insects- Grasshoppers, Bees and Drosophila
3.) Animals- Birds
II.) Sex determination and Environment
1.)
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SEx determination in Humans
In humans,
✗ Mechanism of sex determination is XY type.
✗ Humans have 23 pairs of chromosomes , out of which 22 are exactly the
same in both male and female ; these are called autosomes.
✗ The 23rd pair is the pair of sex chromosomes which is different in male
and female-
female : a pair of X chromosomes ;
male : an X and a Y chromosome. - (two types of gametes are
produced )
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✗ So, during Spermatogenesis- 50% of the total sperm carries the X chromosome and
the rest 50% carries the Y chromosome.
✗ In females, only one type of ovum is produced which carries the X chromosome.
✗ If the ovum is fertilised by the sperm with X chromosome
-> girl
If the ovum is fertilised by the sperm carrying the Y chromosome
-> boy
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Mechanism:
After fertilization, there is a
period of time when the sex of
the foetus is indicepherable.
Although afterwards it is easy
to determine, using ultrasound,
it is not used in India. Due to
high cases of female foeticide.
As the female ovum is blamed,
which is not the case.
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Insects: Grasshoppers (Orthoptera)
The mechanism of sex determination - XO type.
✗ Here, females have a pair of X chromosome.(XX)
✗ Males, however, only have the X chromosome and it’s
counterpart is missing. (XO)
✗ So - fertilisation with the X bearing sperm- female grasshopper
fertilisation with the sperm not bearing a chromosome -
male grasshopper.
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Insects: Fruit Fly (drosophila melanogaster)
Sex is primarily determined by the X:A ratio, or the ratio of the number of X chromosomes to
the number of sets of autosomes. It was discovered by Cline and Meyer in 1996.
Female determining characteristics are present in the x body while male determining
characteristics are encoded on the autosomes.
Thus, XX, XXY, and XXYY flies are females, while XY and XO flies are males.
Dosage compensation: As flies are not able to have more than two copies of XX, they equalize
the genes.Unlike in mammals, all of the Drosophila X chromosomes remain active, and flies
adjust the levels of X-linked gene products by doubling expression from the X chromosome in
males.
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BEES:haplodiploid sex
determination
An unfertilized egg develops into a male
(drone) bee.
The males have half the number of
chromosomes (haploid - 16
chromosomes) as they are produced
asexually. They produce sperm via
mitosis.
The female are created via fertilization
and thus have 36 chromosomes, and are
diploid.
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In birds, sex is determined by Z and W Animals: Birds
chromosomes, and the females are
heterogametic.
Genetically male chickens can be
converted to females if eggs are injected
with estrogen at the sensitive stage of
development.Within the vertebrates, a
similar system of sex determination has
been identified in reptiles, as well as in
some fishes and amphibians.
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Animals: Birds
Investigations led to the discovery of the
DMRT1 gene on the chicken Z
chromosome. This lead to the discovery
of the heterogamety of female chickens.
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Sex detremination and Environment
Environment can play a big role in determination of sex. Among vertebrates,
temperature plays an important role. For example in crocodiles and turtles, the egg
incubation temperature determines the sex. In the snapping turtle, incubation of eggs
below 22°C or above 28°C gives rise to females, while incubation at intermediate
temperatures produces predominantly males.
The critical temperature-dependent component appears to be synthesis of the enzyme
aromatase, which converts androgens, such as testosterone, into estrogens. At higher
temperatures, increased aromatase activity produces more estrogens, which biases the
sex ratio toward more females.
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Thanks!
Any questions?
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