Sex determination in animals
Sexually reproducing animals can be divided into monoecious or hermaphrodite or bisexual and dioecious or unisexual. Majority of animals are dioecious. The species split up into two sections from which the word sex is derived . it comes from Latin word sexus meaning sections or separation. The male section has testis or male gonad and female section has ovaries or female gonad. This kind of separation of sexes is known as gonachorium. The male and female animal differ in primary sex organ, secondary sex organ and tertiary sex characters. The phenomenon of molecular, morphological, physiological and behavioral differentiation between male and female sexes is sexual dimorphism.
The sexual dimorphism has been a biological riddle for the thinkers and Biologists of all time. People always tried to know those factors which determine the male and female sexes of species. Thousands of mistaken hypothesis and wild guesses were proposed before 1900 in vain attempts to find out solution to the problem of determination of sex. Geneticists have come up with different mechanisms of determination of sex.
Genetically controlled sex determining mechanism
a) Sex chromosome mechanism or Heterogamesis
Henking 1891 noted half of sperms contain an extra chromosome called by him as x body. McClung in 1902 suggested X body involved in determination of sex. Miss Stevens and Wilson studied oogenesis and spermatogenesis of insects and it was realized that X body was X chromosome. In male, chromosome homologous to X was smaller and was called as Y chromosome. There are two types of chromosomes.
i. Autosomes – they have no relation with sex and called as autosomes(A).
ii. Sex chromosomes- they are responsible for determination of sex.
Types of sex chromosomal mechanism
A) Heterogametic males
B) Heterogametic females
Heterogametic males
i) XX – XO type
All eggs produced are same having one X chromosome but about half of the sperms contain X chromosome and other half is without X chromosome. The sperm which fertilizes the egg determines the sex of new born. Example. Certain insects.
ii) XX – XY type
All eggs produced are same having one X chromosome but about half of the sperms contain X chromosome and other half is having Y chromosome. The Y containing sperm if happens to fertilize the egg, male baby is born otherwise female baby is born. Example. Man, Drosophila etc.
Heterogametic females
i) ZO – ZZ type
Eggs produced are not same here. Albout half of the eggs produced contain Z chromosome and other half are without Z. All the sperms are same having one Z chromosome. The egg containing Z if fertilized denotes the maleness. Example. Moths, butterflies and domestic chicken.
ii) ZW- ZZ type
All the sperms produced are of same type having one Z chromosome. Eggs are of different type. About half of them contain Z chromosome and other half contain W chromosome. The egg with Z chromosome is for male and egg with W chromosome is for female. Example insects, vertebrates like fishes, birds reptiles etc.
Genic Balance Mechanism
The experiments of Wilson 1909, Bridges 1921, Goldschmidt 1934 revealed that most organism have inherent potentialities for both sexes. There seems to exist delicate Genic balance. The first Genic balance mechanism was first studied by C B Bridges in 1921 in Drosophila.
Y chromosome is found essential for fertility an nothing to do with sex determination. The sex of an individual depends upon ratio of X chromosome to autosomes. The value of haploid set of autosomes is equal to 1 and value of X is one and one half ( 1 ½ ).
In female AAXX, the ratio is 2:3 which goes for female and in male AAXY the ratio is 2: 1 ½ which goes for the male.
