Q. When an animal has both the characters of male and female, it is called
AMUAMU 2006
Solution:
Genic balance theory of sex determination was devised to explain the mechanics of sex determination in male Drosophila melanogaster.
$X O$ or $X Y$ chromosome segregation was interpreted as a means of tipping the balance between maleness and femaleness, whereas more deep-seated processes were involved in the actual process of sex determination. Bridges experimentally produced various combinations of X-chromosomes and autosomes in Drosophila and deduced from comparisons that one $X$ chromosome $(X)$ and two sets of autosomes (A) produced a normal male. Normal males had a ratio of $\underline{X}$ chromosome to sets of autosomes of $0.5$, note that one set of autorsomes consists of three chromosomes, one each of chromosomes II, III and IV. (The X chromosome of Drosophila is chromosome-I). This combination of one $X$ and two A's resulted in a normal, diploid male; the combination of two $X$ chromosomes and two sets of autosomes $(2 X +2 A$, ratio of $2: 2=1)$
produced a normal diploid female (Table ).
Flies produced experimentally with $4 X / 3 A$ were also metafemales.
Those with $4 X / 4 A$ and also those with $3 X / 3 A$, both with an $X / A$ ratio
$=1$, were females.
The combinations $3 X / 4 A = 0.75$ and $2 X / 3 A =0.67$ were intermediate in
characteristics between males and females and were called 'intersexes'. Combinations of $2 X / 4 A =0.5$ were males and those of $1 X / 3 A = 0.33$ were metamales.
Table : Ratio of $X$-chromosomes to autosomes and corresponding sex type in Drosophila melanogaster
$X$-Chromosomes $(X)$ and sets of Autosomes $(A)$
Ratio $X/A$
Sex
$1 X\,\,2 A$
0.5
Male
$2 X\,\, 2 A$
1.0
Female
$3 X\,\,2 A$
1.5
Meta female
$4 X\,\,3 A$
1.33
Meta female
$4 X\,\,4 A$
1.0
Tetraploid female
$3 X\,\,3 A$
1.0
Tetraploid female
$3 X\,\,4 A$
0.75
Intersex
$2 X\,\,3 A$
0.67
Intersex
$2 X\,\,4 A$
0.5
Tetraploid male
$1 X\,\,3 A$
0.33
Metamale
| $X$-Chromosomes $(X)$ and sets of Autosomes $(A)$ | Ratio $X/A$ | Sex |
|---|---|---|
| $1 X\,\,2 A$ | 0.5 | Male |
| $2 X\,\, 2 A$ | 1.0 | Female |
| $3 X\,\,2 A$ | 1.5 | Meta female |
| $4 X\,\,3 A$ | 1.33 | Meta female |
| $4 X\,\,4 A$ | 1.0 | Tetraploid female |
| $3 X\,\,3 A$ | 1.0 | Tetraploid female |
| $3 X\,\,4 A$ | 0.75 | Intersex |
| $2 X\,\,3 A$ | 0.67 | Intersex |
| $2 X\,\,4 A$ | 0.5 | Tetraploid male |
| $1 X\,\,3 A$ | 0.33 | Metamale |