Biology Principles of inheritance and variation class 12 Part 9 Notes
SEX- DETERMINATION :-
Establishment of sex through differential development in an individual at an early stage of life is called sex determination.
Different species use very different strategies for this purpose.
Sex of human being and insects like grass hopper, firefly, drosophila etc is Nave determiner genetically.
Some organisms like turtles rely entirely on environment factors such as temperature for sex determination.
Chromosomal basis of Sex determination :-
This was discovered by Henking (1891).
He traced a specific nuclear structure all through spermatogenesis in few insects (firefly).
Henking also observed that only 50% of the sperms received this structure. This structure was termed X-body by him But he could not explain its significance.
Further investigation by other scientists led to the conclusion that x body was actually a chromosomes it was given name X- chromosomes.
Stevens (1902)discovered Y-chromosomes X&Y chromosomes named as Sex chromosomes by Wilson and Stevens (1905).
Chromosomes basis of sex determination is of following types.
Male Heterogamety
In this type male individuals produces two different types of gametes. Thus the sperm determines the sex of offspring.
It involves two types of sex determining mechanism XO type and XY type.
i) XY type
In a large number of f other insects like drosophila and mammals including human beings the male contain two type of sex chromosomes (X and Y ).
While females possess two similar type of sex chromosomes (XX).
Both male and female have some number of chrormosomes.
In males, Y chromosomes is often shorter than the x chromosomes
ii) XO type
It is observed in large number of insects.example: Grasshopper
Number of chromosomes are different in male and female individual.
It is clear that all the eggs (ova) bear an additional X- chromosomes besides the autosomes while only 50% of the sperm bear X- chromosomes.
In grasshopper, eggs fertilized by (A+O) type sperm become males.