Question

For each of the following, identify what characteristics of a species male and female karyotype would indicate that mechanism of sex-determination. In each case, list a characteristic that would be specific to only that sex-determination system if possible. If no such characteristic is possible, specify which other sex-determination system would show the same karyotype characteristic.

(A) Genic

(B) XY sex chromosomes:

(C) ZW sex chromosomes

(D) Haplo-diploid genomes

(E) XO sex chromosomes

(F) Environmental

Answer 1

1- Genic Balance or X/A Balance Theory Of Sex Determination:-

Guven by C.B. Bridge.​​​​​​ in Drosophila, Y chromosomes plays no role in sex determination rather the ratio of X chromosome and set of autosomes (X/A) determine the sex of that particular fly.

Chromosome Constitution	X/A ratio	Sex Index
AA+XXX	3/2=1.50	Super Female
AA+XX	2/2=1.00	Normal Female
AA+XXY	2/3=0.67	Intersex
AA+XY	1/2=0.50	Normal male(Fertile)
AA+XO	1/2=0.50	Sterile male
AAA+XY	1/3=0.33	Super male

From the above table it is clear that X/A ratio greater than 1 expresses super Female was, 1.0 femaleness, below 1.0 and above 0.5intersex, 0.5 maleness, and less than 0.5 supermaleness.

2-X/Y Sex chromosome:-

It is one of the concept of chromosomal mechanism of sex determination. Here both the X and Y chromosome are participate on determining the sex of the individual.

Found in most plants, insects and mammals (including human).

In XY type of sex determination, male is heteromorphic to Sex chromosome i.e XY while female is isomorphic having XX chromosome.

These different combinations of sex determining genes also leads to show different distinguishing male and female phenotypes.

3-ZW Sex Chromosome or ZW-ZZ Type :-

It is another type of sex determination which involved only sex chromosome (no autosomes involved).

Mostly found in birds, fishes, silkworm(insects), and in some reptiles.

Unlike XY Type, here the female is heteromorphic containing ZW type of combination while male is isomorphic contains ZZ type of sex chromosome.

4- Haplo-diplo genomes :-

In this type of sex determination, one individual carries haploid sets(n) of chromosome while other contains deploid set chromosome (2n). The unfertilized egg containing halpoid sets of chromosome develop parthenogenetically into male while female can be developed either from fertilized egg containing deploid sets of chromosome or from haploid sets of chromosome.

Deploid female are fertile while haploid female are sterile and are known as drone(protection and nourishment).

Example are honey bees, wasps, ants etc.

5-XO Sex Chromosome or XX-XO Type sex determination:-

Like XX-XY type, it also involved only sex chromosome however, here the female have both the sex chromosome i.e XX and male has only one sex chromosome i.e XO. Male lacks one X chromosome and no Y chromosome is involved here.

Examples are roundworms, Dioscorea(plant), some insects like grasshopper and cockroach.

6- Environmental Mechanism of sex determination:-

​​​​​​Observed by F. Baltzer in Bonnelia viridia(marine worm). In this organism the sex is undifferentiated in larval stage. When the larva which settled in down and grow up in mud it turns into mature female while those which are settled down near proboscis of female it becomes parasite develop as male.

It has been demonstrated that female secrets certain hormones which induces sex in larva.