A sex-determination system is biological system that
determines the development of sexual characteristics in
an organism.

primary & secondary sexual characteristics
XX/XY sex-determination system
– sex is determined by the X & Y chromosomes
 In humans: presence of SrY (sex-determining region in
the Y chromosome) gene is the determining factor to
become a male & develop testes as gonads
- genotype determines the gonadal differentiation
whether testes (XY) or ovaries (XX)
- Y chromosome is dominant to X in a 1:1 ratio
- XXY results to male phenotype but with breast
development & small testes (Klinefelter
syndrome)
Environment as Sex-determinating Factor
 In marine spoon worm, Bonellia viridis, larvae become
males if they make physical contact with a female
(secretes bonellin), and females if they end up on the
bare sea floor.
Parthenogenesis
 In lizards (desert) and few species of fishes, reptiles,
insects – unfertilized eggs undergo embryonic
development (parthenogenesis) thus all are female.
Sex Change
 In tropical clown fish, the dominant individual in a
group becomes female while the other ones are male
Protandry - all young fishes (fry & fingerlings) are born
males but will become female when mature.
Protogyny: Where the organism is born as a female,
and then changes sex to a male such as
wrass coral fish
Lyonization
- the process of random inactivation of one of the
X chromosomes in the cells of females to compensate for the presence of the double X
gene dose.
- Barr body (highly condensed inactivated
X chromosome)

Other Sex-Determining Systems:
In fruitflies (Drosophila melanogaster): Sex is
determined by the ratio of X chromosomes to
autosomes (X:A <1 male X:A≥1 female)
e.g. XY : 2 autosomes (1:2 or o.5) – male
XX : 2 autosomes (2:2 or 1) – female
ZZ-ZW Sex Determination System
 In birds, some reptiles & some insects: reverses
to the XY system with females(ZW) and males (ZZ)
Haplodiploidy Sex Determination System
 In Hymenoptera social insects ants, bees & wasps:
unfertilized eggs haploid will become a male whereas
fertilized egg, thus, diploid will become female.
Temperature-dependent Sex Determination System
 In some reptiles: turtles, alligators, tuatara - sex is
determined by the temperature at which the egg is
incubated during a temperature-sensitive sexdetermination period
@ high temp above 30oC will become a female
@ low temp below 30oC will become a male


XX/XO Sex-Determining System
 In the nematode (C. elegans): male
with one sex chromosome (X0);
hermaphrodite with a pair of
chromosomes (XX); no female
 In grasshoppers & crickets,
cockroaches: female with two copies of
the sex chromosome (XX) but males
have only one (X0)
Hermaphroditism – presence of both male
& female reproductive organs
* snails, slugs & earthworms

Sexual or Asexual reproduction
* boa constrictor & komodo dragon –
depend on whether a mate is available.
Primary Sexual Characteristics
Primary sexual characteristics are those characteristics other
than the gonads that are directly required for reproduction.
Male primary somatic sexual characteristics are the penis
and the scrotum, all of which allow a male to make and
deliver sperm. Female primary sexual characteristics are the
vagina, uterus, fallopian tubes, clitoris, cervix, and the ability
to bear children.
Secondary Sexual Characteristics
Secondary sexual characteristics are those sexually
dimorphic characteristics that are not directly involved in
reproduction. For males, secondary characteristics include
facial and chest hair, increased body hair, pelvic build (lack
of rounded hips), upper body muscular build, and the ability
to generate muscle mass at a faster rate than the female. For
females, secondary sex characteristics include relative lack
of body hair, thicker hair on the head (in some cases),
rounded hips/figure, a decreased ability to generate muscle
mass at a fast rate, decreased upper body strength, breasts,
ability to nurse children, a menstrual cycle, and increased
body fat composition. There are, of course, exceptions to any
efforts to use a list of features to classify people into the
conventionally defined sex categories. For instance, not all
women succeed in breast-feeding their infants, even if they
otherwise fit the conventional definition of female.
Source@https://www.sciencedirect.com/topics/medicine-anddentistry/primary-sexual-characteristics