Establishment of sex through differential development in an early stage of life is called sex determination. Sex of human beings and insects like grasshopper, firefly, Drosophila etc. is determined genetically.
- Chromosomal basis of sex determination:- The foundation of this type was laid down by Henking (18999). He traced a specific nuclear structure all through spermatogenesis in a few insects. Henking also observed only 50% of the sperm received this structure. This structure was termed ‘X’ body by him, but he could not explain its significance. Further investigations by other scientists led to the conclusion that the X body was actually a chromosome, therefore it was given the name X-chromosome. Stevens (1902) discovered Y chromosome. X and Y chromosomes named as sex chromosomes by Wilson and Stevens (1905).
Chromosomal basis of sex-determination is of the following types:-
(a) Male heterogamety:– In this type male individual produces two different types of gametes. Thus, the sperm determines the sex of the offspring. It involves two types of sex determining mechanism; XO type and XY type.
(i) XO type (XX-XO type):- It is observed in large number of insects e.g. grasshopper. Number of chromosomes are different in male and female individuals.
It is clear that, eggs (ova) bear an additional X-chromosome besides the autosomes while only 50% of the sperms bear X-chromosomes.
(ii) XX type (XX-XY) type:- in a number of other insects like drosophila and mammals including human beings, the males contain two types of sex chromosomes (X and Y) while females possess two similar type of sex chromosomes (XX). Both male and females have same number of chromosomes. In males, Y-chromosome is often shorter than the X-chromosome.
Sex determination in Humans:– Human beings have 22 pairs of autosomes and one pair of sex chromosomes. Females are homogametic. Male individual produces two types of sperms, 50% of the total sperm possess the X-chromosome and the rest 50% has Y-chromosome besides the autosome. There ia an equal probability of fertilization of the ovum (22 + X) with the sperm carrying either X or Y chromosome. Thus, genetic makeup of the sperm determines the sex of the child. It is also clear that in each pregnancy there is always 50% or ½ probability of either a male or a female child.
Note:- In human beings, Y-chromosome carries a gene Sry (sex determining region) which codes for a product called testis-determining factor (TDF). TDF is required for the development of male sex and its absence leads to the development of female sex.
(b) Female heterogamety:– Female individual produces two different types of gametes. Thus, the egg determines the sex of the offspring. It involves two types of sex-determining mechanisms ZW type and ZO type.
(i) ZW type (ZW-ZZ type):- In birds, both the sexes possess two sex chromosomes. Unlike human beings, the females contain heteromorphic sex chromosomes (heterogametic) while the males have homomorphic sex chromosomes.
Different symbols in birds are used to distinguish the female heterogametic in birds (ZW) from male heterogametic sex (XY) in Drosophila and man.
(ii) ZO type (ZO-ZZ type):- In butterflies, sex-determination is exactly opposite the condition found in grasshoppers. Here females produce two types of eggs (A+ Z and A + O type).
Different systems based on chromosomal mechanism of sex-determination can be summarized as:-
Type | System | Gametes | Zygotes | ||
Sperms | Eggs | Males | Females | ||
Male heterogametic | XO O e.g., Grasshopper, Dioscorea | A+X (50%) A+O (50%) | A+X (100%) | AA+ XO | AA+XX |
XYmale e.g., Drosophila, Homo sapiens (Humans), Melandrium | A+X(50%) A+Y(50%) | A+X (100%) | AA+ XY | AA+XX | |
Female heterogametic | ZW female e.g., Birds | A+Z(100%) | A+Z(50%) A+W (50%) | AA+ ZZ | AA+ZW |
ZO female e.g., Butterflies, Moth | A+Z(100%) | A+Z(50%) A+O (50%) | AA+ ZZ | AA+ZO | |
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