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Sex determination is a biological term that used to determine the sex of the organisms. We use sex-determination system that helps in determining the development of sexual characters in an organism. Most of the organisms that use sexual reproduction to create their offspring have two sexes. For example, sex determination in humans is between two sexes one male and another female.
In some species like bees, sex is not completely determined by sexual reproduction, sex is decided by the presence of the number of chromosomes they receive.
Sex determination
Sex determination is the method that helps us in the separation of one gender from another in a species. The sex of an individual is determined at the time of fertilisation when male and female gametes fuse. In some species, it is established by environmental factors.
Sex- is a mixing of genetic material with the help of meiosis and the fusion of gametes.
Types of sex determination
Genetic or Allosomic sex determination
When the determination of sex of an organism is determined by sex chromosome then it is considered as genetic or allosomic sex determination.
In some organisms, females have similar types of chromosomes(XX) on the other hand male have dissimilar types of chromosome(XY).
It clarifies that this type of fertilisation show male heterogamy because in this male organisms are responsible for the determination of sex of progeny.
On the other hand in organisms in which male contains similar types of chromosomes (ZZ)while the female contains the dissimilar type of chromosome(ZW).
Then,this type of fertilisation show female heterogamy because in this female organisms are responsible for determination of sex of progeny.
Non-allosomic sex determination
In non allosomic sex determination,sex is determination by autosomes which contains genes for the determination of sex of organisms.
In bacteria,plasmid which conatins f-factor plays a significant role in sex determination of bacteria.
The strains of Chlamydomonas are determined by autosomes which contain genes for their strain determination.
In Maize plant autosomes have two types of genes,one type of gene for determination of male inflorescence and other for determination of female inflorescence.
Environmental sex determination
It is purely a non-genetic sex determination is based on environmental factors.
Bonelli Worm is modified into a female organism when its larva swims isolated in the brackish water, when the larva
comes in contact with a female bonelia
then it gets modified into the male organism. At low temperatures, generally female mosquitoes develop from the eggs but at high temperatures, the males develop.
Haplodiploidy
In this type of sex determination, a determination is done by the set of the genome that plays a significant role; Mites, wasps, honey bees, etc, do not contain allosomes in these organisms one sex is haploid and the other sex is diploid. Male organisms are developed from eggs without fertilisation and they are haploid. The process of development of an organism without fertilisation is known as Parthenogenesis.
while females have developed from the fertilised eggs so females are diploid.
Genetic sex determination
In most of the organisms, sex is determined genetically. Sex of an organism usually under the genetic control of specific chromosomes called sex chromosomes or allosomes.
There are five genetic methods of sex determination:
(1) XX-XY method:XX-female, XY- male e.g, mammals, some insects like Drosophila.
(ii) XX-X0 method: XX-female, X0- male e.g,
roundworm, insects.
(iii) ZW-ZZ method:ZW-female, ZZ- male e.g, birds, reptiles, fishes.
(iv) Z0-ZZmethod: Z0-female, ZZ- male e.g.moths, butterfly.
(v) Haploid diploid method.
XX-XY type
In this type, the female is homogametic (isomorphic) possessing two similar sex chromosomes XX and the male are heteromorphic or heterogametic possessing one X-chromosome similar to that of female and a shorter and morphologically different Y-chromosome (humans); however morphology may vary in different organisms.
XX-XO type
In this type, the female has two homomorphic sex chromosomes and produces similar eggs (homogametic) while the male has one chromosome only and produces two types of sperms (heterogametic): (XX) gymnosperms with X and androsperms without X.
Male. Female
Parents (AA-X0) ( AA+XX)
Gamete (A+X)(A+0) (A+X)(A+X)
Offspring (AA+ XX), (AA +XX), (AA+ X0), (AA+ X0)
ZW – ZZ type
In this type, the male has two similar sex chromosomes (ZZ) and is homogametic, and the female has two similar
sex chromosomes (Z, W) and is heterogametic. This mechanism operates in certain insects and invertebrates (fishes, reptiles and birds).
Male. Female
Parents (AA+ZZ) ( AA+ZW)
Gamete (A+Z)(A+Z) (A+Z)(A+W)
Offspring (AA+ZZ), (AA +ZZ), (AA+ ZW), (AA+ ZW)
Z0-ZZ type
In this type, the female is heterogametic while the male is homogametic.
This type of sex determination occurs in certain butterflies and moths.
Male. Female
Parents (AA+ZZ) ( AA+Z0)
Gamete (A+Z)(A+Z) (A+Z)(A+0)
Offspring (AA+ZZ), (AA +ZZ), (AA+ Z0), (AA+ Z0)
Sex determination in humans
- The sex-determining mechanism in the case of humans is XX-XY type. Out of 23 pairs of chromosomes present, 22 pairs in both males and females; these are the autosomes.
- A pair of X-chromosomes are present in the female whereas the presence of an X and Y chromosome are determinants of the male characteristics. The genetic makeup of the sperm determines the sex of the child.
- The X chromosome was discovered by Henking (1891).
- The Y chromosome was discovered by Stevens (1902).
Conclusion
Sex determination shows that most people carry genes for both male and female characteristic,as illustrated by the fact that those with androgen-insesitivity syndrome have capacity to produce female characteristic,even though they have male chromosome.Indeed the gene for most male and female secondary sex characteristic are present not one the sex chromosomes both on autosomes.For crop improvement,the crop to be used in a breeding program,the determination of sex in seedling stage will reduce time as well as input cost.Hundred of theories and mechanism have discovered regarding to the sex-determination but non of the are satisfactory.Several things in the field of sex determination is yet to be discovered.
